\input texinfo   @c -*-texinfo-*-
@input texiplus

@c %**start of header

@c oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
@c                                                                            o
@c                           GNAT DOCUMENTATION                               o
@c                                                                            o
@c                              G N A T _ RM                                  o
@c                                                                            o
@c                            $Revision$
@c                                                                            o
@c          Copyright (C) 1992-2001 Ada Core Technologies, Inc.               o
@c                                                                            o
@c  GNAT is free software;  you can  redistribute it  and/or modify it under  o
@c  terms of the  GNU General Public License as published  by the Free Soft-  o
@c  ware  Foundation;  either version 2,  or (at your option) any later ver-  o
@c  sion.  GNAT is distributed in the hope that it will be useful, but WITH-  o
@c  OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY  o
@c  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License  o
@c  for  more details.  You should have  received  a copy of the GNU General  o
@c  Public License  distributed with GNAT;  see file COPYING.  If not, write  o
@c  to  the Free Software Foundation,  59 Temple Place - Suite 330,  Boston,  o
@c  MA 02111-1307, USA.                                                       o
@c                                                                            o
@c  GNAT is maintained by Ada Core Technologies Inc (http://www.gnat.com).    o
@c                                                                            o
@c oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo

@setfilename gnat_rm.info
@settitle GNAT Reference Manual
@setchapternewpage odd
@syncodeindex fn cp

@titlepage


@title GNAT Reference Manual
@subtitle GNAT, The GNU Ada 95 Compiler
@subtitle Version 3.15w
@subtitle Document revision level $Revision$
@subtitle Date: $Date$
@author Ada Core Technologies, Inc.

@page
@vskip 0pt plus 1filll


Copyright @copyright{} 1995-2001, Ada Core Technologies

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with the Invariant Sections being ``GNU Free Documentation License'', with the
Front-Cover Texts being ``GNAT Reference Manual'', and with no Back-Cover Texts.
A copy of the license is included in the section entitled ``GNU
Free Documentation License''.

Silicon Graphics and IRIS are registered trademarks
and IRIX is a trademark of Silicon Graphics, Inc.

IBM PC is a trademark of International
Business Machines Corporation.

UNIX is a registered trademark of AT&T
Bell Laboratories. 
DIGITAL 

VADS is a registered trademark of Rational Software Inc.

The following are trademarks of Digital Equipment Corporation:
DEC, DEC Ada, DECthreads, Digital, OpenVMS, and VAX@.

@end titlepage
@ifinfo
@node Top, About This Guide, (dir), (dir)
@top GNAT Reference Manual


GNAT Reference Manual

GNAT, The GNU Ada 95 Compiler

Version 3.14a

Date: $Date$

Ada Core Technologies, Inc.


Copyright @copyright{} 1995-2001, Ada Core Technologies

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with the Invariant Sections being "GNU Free Documentation License", with the
Front-Cover Texts being "GNAT Reference Manual", and with no Back-Cover Texts.
A copy of the license is included in the section entitled "GNU
Free Documentation License".


Silicon Graphics and IRIS are registered trademarks
and IRIX is a trademark of Silicon Graphics, Inc.

IBM PC is a trademark of International
Business Machines Corporation.

UNIX is a registered trademark of AT&T
Bell Laboratories. 
DIGITAL 

VADS is a registered trademark of Rational Software Inc.

The following are trademarks of Digital Equipment Corporation:
DEC, DEC Ada, DECthreads, Digital, OpenVMS, and VAX@.

@menu
* About This Guide::            
* Implementation Defined Pragmas::  
* Implementation Defined Attributes::  
* Implementation Advice::       
* Implementation Defined Characteristics::  
* Intrinsic Subprograms::
* Representation Clauses and Pragmas::
* Standard Library Routines::   
* The Implementation of Standard I/O::  
* The GNAT Library::
* Interfacing to Other Languages::  
* Machine Code Insertions::
* GNAT Implementation of Tasking::
* Code generation for array aggregates::
* Specialized Needs Annexes::   
* Compatibility Guide::
* GNU Free Documentation License::
* Index::                       

 --- The Detailed Node Listing ---

About This Guide

* What This Reference Manual Contains::  
* Related Information::         

The Implementation of Standard I/O

* Standard I/O Packages::       
* FORM Strings::                
* Direct_IO::                   
* Sequential_IO::               
* Text_IO::                     
* Wide_Text_IO::                
* Stream_IO::                   
* Shared Files::                
* Open Modes::                  
* Operations on C Streams::     
* Interfacing to C Streams::    

The GNAT Library

* Ada.Characters.Wide_Latin_1 (a-cwila1.ads)::
* Ada.Command_Line.Remove (a-colire.ads)::
* Ada.Direct_IO.C_Streams (a-diocst.ads)::
* Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads)::
* Ada.Sequential_IO.C_Streams (a-siocst.ads)::
* Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads)::
* Ada.Strings.Unbounded.Text_IO (a-suteio.ads)::
* Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads)::
* Ada.Text_IO.C_Streams (a-tiocst.ads)::
* Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads)::
* GNAT.AWK (g-awk.ads)::
* GNAT.Bubble_Sort_A (g-busora.ads)::
* GNAT.Bubble_Sort_G (g-busorg.ads)::
* GNAT.Calendar (g-calend.ads)::
* GNAT.Calendar.Time_IO (g-catiio.ads)::
* GNAT.Case_Util (g-casuti.ads)::
* GNAT.CGI (g-cgi.ads)::
* GNAT.CGI.Cookie (g-cgicoo.ads)::
* GNAT.CGI.Debug (g-cgideb.ads)::
* GNAT.Command_Line (g-comlin.ads)::
* GNAT.CRC32 (g-crc32.ads)::
* GNAT.Current_Exception (g-curexc.ads)::
* GNAT.Debug_Pools (g-debpoo.ads)::
* GNAT.Debug_Utilities (g-debuti.ads)::
* GNAT.Directory_Operations (g-dirope.ads)::
* GNAT.Dynamic_Tables (g-dyntab.ads)::
* GNAT.Exception_Traces (g-exctra.ads)::
* GNAT.Expect (g-expect.ads)::
* GNAT.Float_Control (g-flocon.ads)::
* GNAT.Heap_Sort_A (g-hesora.ads)::
* GNAT.Heap_Sort_G (g-hesorg.ads)::
* GNAT.HTable (g-htable.ads)::
* GNAT.IO (g-io.ads)::
* GNAT.IO_Aux (g-io_aux.ads)::
* GNAT.Lock_Files (g-locfil.ads)::
* GNAT.Most_Recent_Exception (g-moreex.ads)::
* GNAT.OS_Lib (g-os_lib.ads)::
* GNAT.Regexp (g-regexp.ads)::
* GNAT.Registry (g-regist.ads)::
* GNAT.Regpat (g-regpat.ads)::
* GNAT.Sockets (g-socket.ads)::
* GNAT.Source_Info (g-souinf.ads)::
* GNAT.Spell_Checker (g-speche.ads)::
* GNAT.Spitbol.Patterns (g-spipat.ads)::
* GNAT.Spitbol (g-spitbo.ads)::
* GNAT.Spitbol.Table_Boolean (g-sptabo.ads)::
* GNAT.Spitbol.Table_Integer (g-sptain.ads)::
* GNAT.Spitbol.Table_VString (g-sptavs.ads)::
* GNAT.Table (g-table.ads)::
* GNAT.Task_Lock (g-tasloc.ads)::
* GNAT.Threads (g-thread.ads)::
* GNAT.Traceback (g-traceb.ads)::
* GNAT.Traceback.Symbolic (g-trasym.ads)::
* Interfaces.C.Extensions (i-cexten.ads)::
* Interfaces.C.Streams (i-cstrea.ads)::
* Interfaces.CPP (i-cpp.ads)::
* Interfaces.Os2lib (i-os2lib.ads)::
* Interfaces.Os2lib.Errors (i-os2err.ads)::
* Interfaces.Os2lib.Synchronization (i-os2syn.ads)::
* Interfaces.Os2lib.Threads (i-os2thr.ads)::
* Interfaces.Packed_Decimal (i-pacdec.ads)::
* Interfaces.VxWorks (i-vxwork.ads)::
* System.Address_Image (s-addima.ads)::
* System.Assertions (s-assert.ads)::
* System.Partition_Interface (s-parint.ads)::
* System.Task_Info (s-tasinf.ads)::
* System.Wch_Cnv (s-wchcnv.ads)::
* System.Wch_Con (s-wchcon.ads)::

Text_IO

* Text_IO Stream Pointer Positioning::  
* Text_IO Reading and Writing Non-Regular Files::  
* Get_Immediate::               
* Treating Text_IO Files as Streams::
* Text_IO Extensions::
* Text_IO Facilities for Unbounded Strings::

Wide_Text_IO

* Wide_Text_IO Stream Pointer Positioning::  
* Wide_Text_IO Reading and Writing Non-Regular Files::  

Interfacing to Other Languages

* Interfacing to C::
* Interfacing to C++::          
* Interfacing to COBOL::        
* Interfacing to Fortran::      
* Interfacing to non-GNAT Ada code::

GNAT Implementation of Tasking

* Mapping Ada Tasks onto the Underlying Kernel Threads::
* Ensuring Compliance with the Real-Time Annex::
@end menu

@end ifinfo

@node About This Guide
@unnumbered About This Guide

@noindent
This manual contains useful information in writing programs using the
GNAT compiler. It includes information on implementation dependent
characteristics of GNAT, including all the information required by Annex
M of the standard.

Ada 95 is designed to be highly portable,and guarantees that, for most
programs, Ada 95 compilers behave in exactly the same manner on
different machines. However, since Ada 95 is designed to be used in a
wide variety of applications, it also contains a number of system
dependent features to be used in interfacing to the external world. 

@c Maybe put the following in platform-specific section
@ignore
@cindex ProDev Ada
This reference manual discusses how these features are implemented for
use in ProDev Ada running on the IRIX 5.3 or greater operating systems.
@end ignore

@cindex Implementation-dependent features
@cindex Portability
Note: Any program that makes use of implementation-dependent features
may be non-portable.  You should follow good programming practice and
isolate and clearly document any sections of your program that make use
of these features in a non-portable manner.

@menu
* What This Reference Manual Contains::  
* Conventions::
* Related Information::         
@end menu

@node What This Reference Manual Contains
@unnumberedsec What This Reference Manual Contains

This reference manual contains the following chapters:

@itemize @bullet
@item
@ref{Implementation Defined Pragmas} lists GNAT implementation-dependent
pragmas, which can be used to extend and enhance the functionality of the
compiler.

@item
@ref{Implementation Defined Attributes} lists GNAT
implementation-dependent attributes which can be used to extend and
enhance the functionality of the compiler.

@item
@ref{Implementation Advice} provides information on generally
desirable behavior which are not requirements that all compilers must
follow since it cannot be provided on all systems, or which may be
undesirable on some systems.

@item
@ref{Implementation Defined Characteristics} provides a guide to
minimizing implementation dependent features.

@item
@ref{Intrinsic Subprograms} describes the intrinsic subprograms
implemented by GNAT, and how they can be imported into user
application programs.

@item
@ref{Representation Clauses and Pragmas} describes in detail the
way that GNAT represents data, and in particular the exact set
of representation clauses and pragmas that is accepted.

@item
@ref{Standard Library Routines} provides a listing of packages and a
brief description of the functionality that is provided by Ada's
extensive set of standard library routines as implemented by GNAT@.

@item
@ref{The Implementation of Standard I/O} details how the GNAT
implementation of the input-output facilities.

@item
@ref{Interfacing to Other Languages} describes how programs
written in Ada using GNAT can be interfaced to other programming
languages.

@item
@ref{Specialized Needs Annexes} describes the GNAT implementation of all
of the special needs annexes.

@item
@ref{Compatibility Guide} includes sections on compatibility of GNAT with
other Ada 83 and Ada 95 compilation systems, to assist in porting code
from other environments.
@end itemize

@cindex Ada 95 ISO/ANSI Standard
This reference manual assumes that you are familiar with Ada 95
language, as described in the International Standard
ANSI/ISO/IEC-8652:1995, Jan 1995.

@node Conventions
@unnumberedsec Conventions
@cindex Conventions, typographical
@cindex Typographical conventions

@noindent
Following are examples of the typographical and graphic conventions used
in this guide:

@itemize @bullet
@item
@code{Functions}, @code{utility program names}, @code{standard names},
and @code{classes}.

@item
@samp{Option flags}

@item
@file{File Names}, @file{button names}, and @file{field names}.

@item
@var{Variables}.

@item
@emph{Emphasis}.

@item
[optional information or parameters]

@item
Examples are described by text
@smallexample
and then shown this way.
@end smallexample
@end itemize

@noindent
Commands that are entered by the user are preceded in this manual by the
characters "$ " (dollar sign followed by space). If your system uses this
sequence as a prompt, then the commands will appear exactly as you see them
in the manual. If your system uses some other prompt, then the command will
appear with the $ replaced by whatever prompt character you are using.

@node Related Information
@unnumberedsec Related Information
See the following documents for further information on GNAT

@itemize @bullet
@item
@cite{GNAT User's Guide}, which provides information on how to use
the GNAT compiler system.

@item
@cite{Ada 95 Reference Manual}, which contains all reference
material for the Ada 95 programming language.

@item
@cite{Ada 95 Annotated Reference Manual}, which is an annotated version
of the standard reference manual cited above. The annotations describe
detailed aspects of the design decision, and in particular contain useful
sections on Ada 83 compatibility.

@item
@cite{DEC Ada, Technical Overview and Comparison on DIGITAL Platforms},
which contains specific information on compatibility between GNAT and
DEC Ada 83 systems.

@item
@cite{DEC Ada, Language Reference Manual, part number AA-PYZAB-TK} which
describes in detail the pragmas and attributes provided by the DEC Ada 83
compiler system.

@end itemize

@node Implementation Defined Pragmas
@chapter Implementation Defined Pragmas

@noindent
Ada 95 defines a set of pragmas that can be used to supply additional
information to the compiler. These language defined pragmas are
implemented in GNAT and work as described in the Ada 95 Reference
Manual.

In addition, Ada 95 allows implementations to define additional pragmas
whose meaning is defined by the implementation. GNAT provides a number
of these implementation-dependent pragmas which can be used to extend
and enhance the functionality of the compiler. This section of the GNAT
Reference Manual describes these additional pragmas.

Note that any program using these pragmas may not be portable to other
compilers (although GNAT implements this set of pragmas on all
platforms). Therefore if portability to other compilers is an important
consideration, the use of these pragmas should be minimized.

@table @code

@findex Abort_Defer
@cindex Deferring aborts
@item pragma Abort_Defer
@noindent
Syntax:

@smallexample
pragma Abort_Defer;
@end smallexample

@noindent
This pragma must appear at the start of the statement sequence of a
handled sequence of statements (right after the @code{begin}). It has
the effect of deferring aborts for the sequence of statements (but not
for the declarations or handlers, if any, associated with this statement
sequence).

@item pragma Ada_83
@findex Ada_83
@noindent
Syntax:

@smallexample
pragma Ada_83;
@end smallexample

@noindent
A configuration pragma that establishes Ada 83 mode for the unit to
which it applies, regardless of the mode set by the command line
switches. In Ada 83 mode, GNAT attempts to be as compatible with
the syntax and semantics of Ada 83, as defined in the original Ada
83 Reference Manual as possible. In particular, the new Ada 95
keywords are not recognized, optional package bodies are allowed,
and generics may name types with unknown discriminants without using
the (<>) notation. In addition, some but not all of the additional
restrictions of Ada 83 are enforced.

Ada 83 mode is intended for two purposes. Firstly, it allows existing
legacy Ada 83 code to be compiled and adapted to GNAT with less effort.
Secondly, it aids in keeping code backwards compatible with Ada 83. 
However, there is no guarantee that code that is processed correctly
by GNAT in Ada 83 mode will in fact compile and execute with an Ada
83 compiler, since GNAT does not enforce all the additional checks
required by Ada 83.

@findex Ada_95
@item pragma Ada_95
@noindent
Syntax:

@smallexample
pragma Ada_95;
@end smallexample

@noindent
A configuration pragma that establishes Ada 95 mode for the unit to which
it applies, regardless of the mode set by the command line switches.
This mode is set automatically for the @code{Ada} and @code{System}
packages and their children, so you need not specify it in these
contexts. This pragma is useful when writing a reusable component that
itself uses Ada 95 features, but which is intended to be usable from
either Ada 83 or Ada 95 programs.

@findex Annotate
@item pragma Annotate
@noindent
Syntax:

@smallexample
pragma Annotate (IDENTIFIER @{, ARG@});

ARG ::= NAME | EXPRESSION
@end smallexample

@noindent
This pragma is used to annotate programs. @var{identifier} identifies
the type of annotation. GNAT verifies this is an identifier, but does
not otherwise analyze it. The @var{arg} argument
can be either a string literal or an
expression. String literals are assumed to be of type
@code{Standard.String}. Names of entities are simply analyzed as entity
names. All other expressions are analyzed as expressions, and must be
unambiguous.

The analyzed pragma is retained in the tree, but not otherwise processed
by any part of the GNAT compiler. This pragma is intended for use by
external tools, including ASIS@.

@findex Assert
@item pragma Assert
@noindent
Syntax:

@smallexample
pragma Assert (
  boolean_EXPRESSION
  [, static_string_EXPRESSION])
@end smallexample

@noindent
The effect of this pragma depends on whether the corresponding command
line switch is set to activate assertions. The pragma expands into code
equivalent to the following:

@smallexample
if assertions-enabled then
   if not boolean_EXPRESSION then
      System.Assertions.Raise_Assert_Failure
        (string_EXPRESSION); 
   end if;
end if;
@end smallexample

@noindent
The string argument, if given, is the message that will be associated
with the exception occurrence if the exception is raised. If no second
argument is given, the default message is @samp{@var{file}:@var{nnn}},
where @var{file} is the name of the source file containing the assert,
and @var{nnn} is the line number of the assert. A pragma is not a
statement, so if a statement sequence contains nothing but a pragma
assert, then a null statement is required in addition, as in:

@smallexample
...
if J > 3 then
   pragma Assert (K > 3, "Bad value for K");
   null;
end if;
@end smallexample

@noindent
Note that, as with the if statement to which it is equivalent, the
type of the expression is either Standard.Boolean, or any type derived
from this standard type.

If assertions are disabled (switch @code{-gnata} not used), then there
is no effect (and in particular, any side effects from the expression
are suppressed). More precisely it is not quite true that the pragma
has no effect, since the expression is analyzed, and may cause types
to be frozen if they are mentioned here for the first time.

If assertions are enabled, then the given expression is tested, and if
it is @code{False} then System.Assertions.Raise_Assert_Failure is called
which results in the raising of Assert_Failure with the given message.

If the boolean expression has side effects, these side effects will turn
on and off with the setting of the assertions mode, resulting in
assertions that have an effect on the program. You should generally 
avoid side effects in the expression arguments of this pragma. However,
the expressions are analyzed for semantic correctness whether or not
assertions are enabled, so turning assertions on and off cannot affect
the legality of a program.

@cindex OpenVMS
@findex Ast_Entry
@item pragma Ast_Entry
@noindent
Syntax:

@smallexample
pragma AST_Entry (entry_IDENTIFIER);
@end smallexample

@noindent
This pragma is implemented only in the OpenVMS implementation of GNAT@. The
argument is the simple name of a single entry; at most one @code{AST_Entry}
pragma is allowed for any given entry. This pragma must be used in 
conjunction with the @code{AST_Entry} attribute, and is only allowed after
the entry declaration and in the same task type specification or single task
as the entry to which it applies. This pragma specifies that the given entry
may be used to handle an OpenVMS asynchronous system trap (@code{AST})
resulting from an OpenVMS system service call. The pragma does not affect
normal use of the entry. For further details on this pragma, see the 
DEC Ada Language Reference Manual, section 9.12a.

@cindex Passing by copy
@findex C_Pass_By_Copy
@item pragma C_Pass_By_Copy
@noindent
Syntax:

@smallexample
pragma C_Pass_By_Copy
  ([Max_Size =>] static_integer_EXPRESSION);
@end smallexample

@noindent
Normally the default mechanism for passing C convention records to C
convention subprograms is to pass them by reference, as suggested by RM
B.3(69). Use the configuration pragma @code{C_Pass_By_Copy} to change
this default, by requiring that record formal parameters be passed by
copy if all of the following conditions are met:

@itemize @bullet
@item
The size of the record type does not exceed@*@var{static_integer_expression}.
@item
The record type has @code{Convention C}.
@item
The formal parameter has this record type, and the subprogram has a
foreign (non-Ada) convention.
@end itemize

@noindent
If these conditions are met the argument is passed by copy, i.e.@: in a
manner consistent with what C expects if the corresponding formal in the
C prototype is a struct (rather than a pointer to a struct).

You can also pass records by copy by specifying the convention
@code{C_Pass_By_Copy} for the record type, or by using the extended
@code{Import} and @code{Export} pragmas, which allow specification of
passing mechanisms on a parameter by parameter basis.

@findex Comment
@item pragma Comment
@noindent
Syntax:

@smallexample
pragma Comment (static_string_EXPRESSION);
@end smallexample

@noindent
This is almost identical in effect to pragma Ident. It allows the
placement of a comment into the object file and hence into the
executable file if the operating system permits such usage. The
difference is that Comment, unlike Ident, has no limit on the
length of the string argument, and no limitations on placement
of the pragma (it can be placed anywhere in the main source unit).

@findex Common_Object
@item pragma Common_Object
@noindent
Syntax:

@smallexample
pragma Common_Object (
     [Internal =>] LOCAL_NAME,
  [, [External =>] EXTERNAL_SYMBOL]
  [, [Size     =>] EXTERNAL_SYMBOL] )

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
@end smallexample

@noindent
This pragma enables the shared use of variables stored in overlaid
linker areas corresponding to the use of @code{COMMON}
in Fortran.  The single
object @var{local_name} is assigned to the area designated by
the @var{External} argument.
You may define a record to correspond to a series
of fields. The @var{size} argument
is syntax checked in GNAT, but otherwise ignored.

@code{Common_Object} is not supported on all platforms. If no
support is available, then the code generator will issue a message
indicating that the necessary attribute for implementation of this
pragma is not available.

@findex Complex_Representation
@item pragma Complex_Representation
@noindent
Syntax:

@smallexample
pragma Complex_Representation
        ([Entity =>] LOCAL_NAME);
@end smallexample

@noindent
The @var{Entity} argument must be the name of a record type which has
two fields of the same floating-point type. The effect of this pragma is
to force gcc to use the special internal complex representation form for
this record, which may be more efficient. Note that this may result in
the code for this type not conforming to standard ABI (application
binary interface) requirements for the handling of record types. For
example, in some environments, there is a requirement for passing
records by pointer, and the use of this pragma may result in passing
this type in floating-point registers.

@cindex Alignments of components
@findex Component_Alignment
@item pragma Component_Alignment
@noindent
Syntax:

@smallexample
pragma Component_Alignment (
     [Form =>] ALIGNMENT_CHOICE
  [, [Name =>] type_LOCAL_NAME]);

ALIGNMENT_CHOICE ::=
  Component_Size
| Component_Size_4
| Storage_Unit
| Default
@end smallexample

@noindent
Specifies the alignment of components in array or record types.
The meaning of the @var{Form} argument is as follows:

@table @code
@findex Component_Size
@item Component_Size
Aligns scalar components and subcomponents of the array or record type
on boundaries appropriate to their inherent size (naturally
aligned). For example, 1-byte components are aligned on byte boundaries,
2-byte integer components are aligned on 2-byte boundaries, 4-byte
integer components are aligned on 4-byte boundaries and so on. These
alignment rules correspond to the normal rules for C compilers on all
machines except the VAX@.

@findex Component_Size_4
@item Component_Size_4
Naturally aligns components with a size of four or fewer
bytes. Components that are larger than 4 bytes are placed on the next
4-byte boundary.

@findex Storage_Unit
@item Storage_Unit
Specifies that array or record components are byte aligned, i.e.@:
aligned on boundaries determined by the value of the constant
@code{System.Storage_Unit}.

@cindex OpenVMS
@item Default
Specifies that array or record components are aligned on default
boundaries, appropriate to the underlying hardware or operating system or
both. For OpenVMS VAX systems, the @code{Default} choice is the same as
the @code{Storage_Unit} choice (byte alignment). For all other systems,
the @code{Default} choice is the same as @code{Component_Size} (natural
alignment).
@end table

If the @code{Name} parameter is present, @var{type_local_name} must
refer to a local record or array type, and the specified alignment
choice applies to the specified type. The use of
@code{Component_Alignment} together with a pragma @code{Pack} causes the
@code{Component_Alignment} pragma to be ignored. The use of
@code{Component_Alignment} together with a record representation clause
is only effective for fields not specified by the representation clause.

If the @code{Name} parameter is absent, the pragma can be used as either
a configuration pragma, in which case it applies to one or more units in
accordance with the normal rules for configuration pragmas, or it can be
used within a declarative part, in which case it applies to types that
are declared within this declarative part, or within any nested scope
within this declarative part. In either case it specifies the alignment
to be applied to any record or array type which has otherwise standard
representation.

If the alignment for a record or array type is not specified (using
pragma @code{Pack}, pragma @code{Component_Alignment}, or a record rep
clause), the GNAT uses the default alignment as described previously.
  
@findex CPP_Class
@cindex Interfacing with C++
@item pragma CPP_Class
@noindent
Syntax:

@smallexample
pragma CPP_Class ([Entity =>] LOCAL_NAME);
@end smallexample

@noindent
The argument denotes an entity in the current declarative region
that is declared as a tagged or untagged record type. It indicates that
the type corresponds to an externally declared C++ class type, and is to
be laid out the same way that C++ would lay out the type.

If (and only if) the type is tagged, at least one component in the
record must be of type @code{Interfaces.CPP.Vtable_Ptr}, corresponding
to the C++ Vtable (or Vtables in the case of multiple inheritance) used
for dispatching.

Types for which @code{CPP_Class} is specified do not have assignment or
equality operators defined (such operations can be imported or declared
as subprograms as required). Initialization is allowed only by
constructor functions (see pragma @code{CPP_Constructor}).

Pragma @code{CPP_Class} is intended primarily for automatic generation
using an automatic binding generator tool. Ada Core Technologies does
not currently supply such a
tool; See @ref{Interfacing to C++} for more details.

@cindex Interfacing with C++
@findex CPP_Constructor
@item pragma CPP_Constructor
@noindent
Syntax:

@smallexample
pragma CPP_Constructor ([Entity =>] LOCAL_NAME);
@end smallexample

@noindent
This pragma identifies an imported function (imported in the usual way
with pragma Import) as corresponding to a C++
constructor. The argument is a name that must have been
previously mentioned in a pragma
Import with @var{Convention CPP}, and must be of one of the following
forms:

@itemize @bullet
@item
@code{function @var{Fname} return @var{T}'Class}

@item
@code{function @var{Fname} (@dots{}) return @var{T}'Class}
@end itemize

@noindent
where @var{T} is a tagged type to which the pragma @code{CPP_Class} applies.

The first form is the default constructor, used when an object of type
@var{T} is created on the Ada side with no explicit constructor. Other
constructors (including the copy constructor, which is simply a special
case of the second form in which the one and only argument is of type
@var{T}), can only appear in two contexts:

@itemize @bullet
@item
On the right side of an initialization of an object of type @var{T}.
@item
In an extension aggregate for an object of a type derived from @var{T}.
@end itemize

Although the constructor is described as a function that returns a value
on the Ada side, it is typically a procedure with an extra implicit
argument (the object being initialized) at the implementation
level. GNAT issues the appropriate call, whatever it is, to get the
object properly initialized.

In the case of derived objects, you may use one of two possible forms
for declaring and creating an object:

@itemize @bullet
@item @code{New_Object : Derived_T}
@item @code{New_Object : Derived_T := (@var{constructor-function-call with} @dots{})}
@end itemize

In the first case the default constructor is called and extension fields
if any are initialized according to the default initialization
expressions in the Ada declaration. In the second case, the given
constructor is called and the extension aggregate indicates the explicit
values of the extension fields.

If no constructors are imported, it is impossible to create any objects
on the Ada side. If no default constructor is imported, only the
initialization forms using an explicit call to a constructor are
permitted.

Pragma @code{CPP_Constructor} is intended primarily for automatic generation
using an automatic binding generator tool. Ada Core Technologies does
not currently supply such a
tool; See @ref{Interfacing to C++} for more details.

@cindex Interfacing to C++
@findex CPP_Virtual
@item pragma CPP_Virtual
@noindent
Syntax:

@smallexample
pragma CPP_Virtual
     [Entity     =>] ENTITY,
  [, [Vtable_Ptr =>] vtable_ENTITY,]
  [, [Position   =>] static_integer_EXPRESSION])
@end smallexample

This pragma serves the same function as pragma @code{Import} in that
case of a virtual function imported from C++. The @var{Entity} argument
must be a
primitive subprogram of a tagged type to which pragma @code{CPP_Class}
applies. The @var{Vtable_Ptr} argument specifies
the Vtable_Ptr component which contains the
entry for this virtual function. The @var{Position} argument
is the sequential number
counting virtual functions for this Vtable starting at 1.

The @code{Vtable_Ptr} and @code{Position} arguments may be omitted if
there is one Vtable_Ptr present (single inheritance case) and all
virtual functions are imported.  In that case the compiler can deduce both
these values.

No @code{External_Name} or @code{Link_Name} arguments are required for a
virtual function, since it is always accessed indirectly via the
appropriate Vtable entry.

Pragma @code{CPP_Virtual} is intended primarily for automatic generation
using an automatic binding generator tool. Ada Core Technologies does
not currently supply such a
tool; See @ref{Interfacing to C++} for more details.

@cindex Interfacing with C++
@findex CPP_Vtable
@item pragma CPP_Vtable
@noindent
Syntax:

@smallexample
pragma CPP_Vtable (
  [Entity      =>] ENTITY,
  [Vtable_Ptr  =>] vtable_ENTITY,
  [Entry_Count =>] static_integer_EXPRESSION);
@end smallexample

@noindent
Given a record to which the pragma @code{CPP_Class} applies,
this pragma can be specified for each component of type
@code{CPP.Interfaces.Vtable_Ptr}.
@var{Entity} is the tagged type, @var{Vtable_Ptr}
is the record field of type @code{Vtable_Ptr}, and @var{Entry_Count} is
the number of virtual functions on the C++ side. Not all of these
functions need to be imported on the Ada side.

You may omit the @code{CPP_Vtable} pragma if there is only one
@code{Vtable_Ptr} component in the record and all virtual functions are
imported on the Ada side (the default value for the entry count in this
case is simply the total number of virtual functions).

Pragma @code{CPP_Vtable} is intended primarily for automatic generation
using an automatic binding generator tool. Ada Core Technologies does
not currently supply such a
tool; See @ref{Interfacing to C++} for more details.

@findex Debug
@item pragma Debug
@noindent
Syntax:

@smallexample
pragma Debug (PROCEDURE_CALL_STATEMENT);
@end smallexample

@noindent
If assertions are not enabled on the command line, this pragma has no
effect. If asserts are enabled, the semantics of the pragma is exactly
equivalent to the procedure call. Pragmas are permitted in sequences of
declarations, so you can use pragma @code{Debug} to intersperse calls to
debug procedures in the middle of declarations.

@cindex Elaboration control
@findex Elaboration_Checks
@item pragma Elaboration_Checks
@noindent
Syntax:

@smallexample
pragma Elaboration_Checks (RM | Static);
@end smallexample

@noindent
This is a configuration pragma that provides control over the
elaboration model used by the compilation affected by the
pragma. If the parameter is RM, then the dynamic elaboration
model described in the Ada Reference Manual is used, as though
the @code{-gnatE} switch had been specified on the command
line. If the parameter is Static, then the default GNAT static
model is used. This configuration pragma overrides the setting
of the command line. For full details on the elaboration models
used by the GNAT compiler, see section "Elaboration Order
Handling in GNAT" in the GNAT Users Guide.

@cindex Elimination of unused subprograms
@findex Eliminate
@item pragma Eliminate
@noindent
Syntax:

@smallexample
pragma Eliminate (
    [Unit_Name =>] IDENTIFIER |
                   SELECTED_COMPONENT);

pragma Eliminate (
    [Unit_Name       =>]  IDENTIFIER |
                          SELECTED_COMPONENT
    [Entity          =>]  IDENTIFIER |
                          SELECTED_COMPONENT |
                          STRING_LITERAL]
   [,[Parameter_Types =>] PARAMETER_TYPES]
   [,[Result_Type     =>] result_SUBTYPE_NAME]]);

PARAMETER_TYPES ::= (SUBTYPE_NAME @{, SUBTYPE_NAME@})
SUBTYPE_NAME    ::= STRING_LITERAL
@end smallexample

@noindent
This pragma indicates that the given entity is not used outside the
compilation unit it is defined in. The entity may be either a subprogram 
or a variable.

If the entity to be eliminated is a library level subprogram, then
the first form of pragma @code{Eliminate} is used with only a single argument.
In this form, the @code{Unit_Name} argument specifies the name of the
library  level unit to be eliminated.

In all other cases, both @code{Unit_Name} and @code{Entity} arguments
are required. item is an entity of a library package, then the first
argument specifies the unit name, and the second argument specifies
the particular entity. If the second argument is in string form, it must
correspond to the internal manner in which GNAT stores entity names (see
compilation unit Namet in the compiler sources for details).
The third and fourth parameters are optionally used to distinguish
between overloaded subprograms, in a manner similar to that used for 
the extended @code{Import} and @code{Export} pragmas, except that the
subtype names are always given as string literals, again corresponding
to the internal manner in which GNAT stores entity names.

The effect of the pragma is to allow the compiler to eliminate
the code or data associated with the named entity.  Any reference to 
an eliminated entity outside the compilation unit it is defined in,
causes a compile time or link time error.

The intention of pragma Eliminate is to allow a program to be compiled
in a system independent manner, with unused entities eliminated, without
the requirement of modifying the source text. Normally the required set
of Eliminate pragmas is constructed automatically using the gnatelim tool.
Elimination of unused entities local to a compilation unit is automatic,
without requiring the use of pragma Eliminate.

Note that the reason this pragma takes string literals where names might
be expected is that a pragma Eliminate can appear in a context where the
relevant names are not visible.

@cindex OpenVMS
@findex Export_Exception
@item pragma Export_Exception
@noindent
Syntax:

@smallexample
pragma Export_Exception (
     [Internal =>] LOCAL_NAME,
  [, [External =>] EXTERNAL_SYMBOL,]
  [, [Form     =>] Ada | VMS]
  [, [Code     =>] static_integer_EXPRESSION]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
@end smallexample

@noindent
This pragma is implemented only in the OpenVMS implementation of GNAT@. It
causes the specified exception to be propagated outside of the Ada program,
so that it can be handled by programs written in other OpenVMS languages.
This pragma establishes an external name for an Ada exception and makes the
name available to the OpenVMS Linker as a global symbol. For further details
on this pragma, see the
DEC Ada Language Reference Manual, section 13.9a3.2.

@cindex Argument passing mechanisms
@findex Export_Function
@item pragma Export_Function @dots{}

@noindent
Syntax:

@smallexample
pragma Export_Function (
     [Internal         =>] LOCAL_NAME,      
  [, [External         =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types  =>] PARAMETER_TYPES]
  [, [Result_Type      =>] result_SUBTYPE_MARK]
  [, [Mechanism        =>] MECHANISM]
  [, [Result_Mechanism =>] MECHANISM_NAME]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| SUBTYPE_MARK @{, SUBTYPE_MARK@}

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::=
  Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]

CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca
@end smallexample

Use this pragma to make a function externally callable and optionally
provide information on mechanisms to be used for passing parameter and
result values. We recommend, for the purposes of improving portability,
this pragma always be used in conjunction with a separate pragma
@code{Export}, which must precede the pragma @code{Export_Function}.
GNAT does not require a separate pragma @code{Export}, but if none is
present, @code{Convention Ada} is assumed, which is usually
not what is wanted, so it is usually appropriate to use this
pragma in conjunction with a @code{Export} or @code{Convention}
pragma that specifies the desired foreign convention.
Pragma @code{Export_Function}
(and @code{Export}, if present) must appear in the same declarative
region as the function to which they apply.

@var{internal_name} must uniquely designate the function to which the
pragma applies. If more than one function name exists of this name in
the declarative part you must use the @code{Parameter_Types} and
@code{Result_Type} parameters is mandatory to achieve the required
unique designation. @var{subtype_ mark}s in these parameters must
exactly match the subtypes in the corresponding function specification,
using positional notation to match parameters with subtype marks.
@cindex OpenVMS
@cindex Passing by descriptor
Passing by descriptor is supported only on the OpenVMS ports of GNAT@.

@findex Export_Object
@item pragma Export_Object @dots{}
@noindent
Syntax:

@smallexample
pragma Export_Object
      [Internal =>] LOCAL_NAME,
   [, [External =>] EXTERNAL_SYMBOL]
   [, [Size     =>] EXTERNAL_SYMBOL]

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
@end smallexample

This pragma designates an object as exported, and apart from the
extended rules for external symbols, is identical in effect to the use of
the normal @code{Export} pragma applied to an object. You may use a
separate Export pragma (and you probably should from the point of view
of portability), but it is not required.  @var{Size} is syntax checked,
but otherwise ignored by GNAT@.

@findex Export_Procedure
@item pragma Export_Procedure @dots{}
@noindent
Syntax:

@smallexample
pragma Export_Procedure (
     [Internal        =>] LOCAL_NAME
  [, [External        =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types =>] PARAMETER_TYPES]
  [, [Mechanism       =>] MECHANISM]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| SUBTYPE_MARK @{, SUBTYPE_MARK@}

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::=
  Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]

CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca
@end smallexample

@noindent
This pragma is identical to @code{Export_Function} except that it
applies to a procedure rather than a function and the parameters
@code{Result_Type} and @code{Result_Mechanism} are not permitted.
GNAT does not require a separate pragma @code{Export}, but if none is
present, @code{Convention Ada} is assumed, which is usually
not what is wanted, so it is usually appropriate to use this
pragma in conjunction with a @code{Export} or @code{Convention}
pragma that specifies the desired foreign convention.

@findex Export_Valued_Procedure
@item pragma Export_Valued_Procedure
@noindent
Syntax:

@smallexample
pragma Export_Valued_Procedure (
     [Internal        =>] LOCAL_NAME
  [, [External        =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types =>] PARAMETER_TYPES]
  [, [Mechanism       =>] MECHANISM]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| SUBTYPE_MARK @{, SUBTYPE_MARK@}

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::=
  Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]

CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca
@end smallexample

This pragma is identical to @code{Export_Procedure} except that the
first parameter of @var{local_name}, which must be present, must be of
mode @code{OUT}, and externally the subprogram is treated as a function
with this parameter as the result of the function. GNAT provides for
this capability to allow the use of @code{OUT} and @code{IN OUT}
parameters in interfacing to external functions (which are not permitted
in Ada functions).
GNAT does not require a separate pragma @code{Export}, but if none is
present, @code{Convention Ada} is assumed, which is almost certainly
not what is wanted since the whole point of this pragma is to interface
with foreign language functions, so it is usually appropriate to use this
pragma in conjunction with a @code{Export} or @code{Convention}
pragma that specifies the desired foreign convention.

@cindex @code{system}, extending
@cindex Dec Ada 83
@findex Extend_System
@item pragma Extend_System
@noindent
Syntax:

@smallexample
pragma Extend_System ([Name =>] IDENTIFIER);
@end smallexample

@noindent
This pragma is used to provide backwards compatibility with other
implementations that extend the facilities of package @code{System}. In
GNAT, @code{System} contains only the definitions that are present in
the Ada 95 RM@. However, other implementations, notably the DEC Ada 83
implementation, provide many extensions to package @code{System}.

For each such implementation accommodated by this pragma, GNAT provides a
package @code{Aux_@var{xxx}}, e.g.@: @code{Aux_DEC} for the DEC Ada 83
implementation, which provides the required additional definitions. You
can use this package in two ways.  You can @code{with} it in the normal
way and access entities either by selection or using a @code{use}
clause. In this case no special processing is required.

However, if existing code contains references such as
@code{System.@var{xxx}} where @var{xxx} is an entity in the extended
definitions provided in package @code{System}, you may use this pragma
to extend visibility in @code{System} in a non-standard way that
provides greater compatibility with the existing code. Pragma
@code{Extend_System} is a configuration pragma whose single argument is
the name of the package containing the extended definition
(e.g.@: @code{Aux_DEC} for the DEC Ada case). A unit compiled under
control of this pragma will be processed using special visibility
processing that looks in package @code{System.Aux_@var{xxx}} where
@code{Aux_@var{xxx}} is the pragma argument for any entity referenced in
package @code{System}, but not found in package @code{System}.

You can use this pragma either to access a predefined @code{System}
extension supplied with the compiler, for example @code{Aux_DEC} or
you can construct your own extension unit following the above
definition. Note that such a package is a child of @code{System}
and thus is considered part of the implementation. To compile
it you will have to use the appropriate switch for compiling
system units. See the GNAT User's Guide for details.

@findex External
@item pragma External
@noindent
Syntax:

@smallexample
pragma External (
  [   Convention    =>] convention_IDENTIFIER,
  [   Entity        =>] local_NAME
  [, [External_Name =>] static_string_EXPRESSION ]
  [, [Link_Name     =>] static_string_EXPRESSION ]);
@end smallexample

@noindent
This pragma is identical in syntax and semantics to pragma
@code{Export} as defined in the Ada Reference Manual. It is
provided for compatibility with some Ada 83 compilers that
used this pragma for exactly the same purposes as pragma
@code{Export} before the latter was standardized.

@cindex Dec Ada 83 casing compatibility
@cindex External Names, casing
@cindex Casing of External names
@findex External_Name_Casing
@item pragma External_Name_Casing
@noindent
Syntax:

@smallexample
pragma External_Name_Casing (
  Uppercase | Lowercase
  [, Uppercase | Lowercase | As_Is]);
@end smallexample

@noindent
This pragma provides control over the casing of external names associated
with Import and Export pragmas. There are two cases to consider:

@table @asis
@item Implicit external names
Implicit external names are derived from identifiers. The most common case
arises when a standard Ada 95 Import or Export pragma is used with only two
arguments, as in:

@smallexample
   pragma Import (C, C_Routine);
@end smallexample

@noindent
Since Ada is a case insensitive language, the spelling of the identifier in
the Ada source program does not provide any information on the desired
casing of the external name, and so a convention is needed. In GNAT the
default treatment is that such names are converted to all lower case
letters. This corresponds to the normal C style in many environments.
The first argument of pragma @code{External_Name_Casing} can be used to
control this treatment. If @code{Uppercase} is specified, then the name
will be forced to all uppercase letters. If @code{Lowercase} is specified,
then the normal default of all lower case letters will be used.

This same implicit treatment is also used in the case of extended DEC Ada 83
compatible Import and Export pragmas where an external name is explicitly
specified using an identifier rather than a string.

@item Explicit external names
Explicit external names are given as string literals. The most common case
arises when a standard Ada 95 Import or Export pragma is used with three
arguments, as in:

@smallexample
pragma Import (C, C_Routine, "C_routine");
@end smallexample

@noindent
In this case, the string literal normally provides the exact casing required
for the external name. The second argument of pragma 
@code{External_Name_Casing} may be used to modify this behavior. 
If @code{Uppercase} is specified, then the name
will be forced to all uppercase letters. If @code{Lowercase} is specified,
then the name will be forced to all lowercase letters. A specification of
@code{As_Is} provides the normal default behavior in which the casing is
taken from the string provided.
@end table

@noindent
This pragma may appear anywhere that a pragma is valid. in particular, it
can be used as a configuration pragma in the @code{gnat.adc} file, in which
case it applies to all subsequent compilations, or it can be used as a program
unit pragma, in which case it only applies to the current unit, or it can
be used more locally to control individual Import/Export pragmas.

It is primarily intended for use with @code{OpenVMS} systems, where many
compilers convert all symbols to upper case by default. For interfacing to
such compilers (e.g.@: the DEC C compiler), it may be convenient to use
the pragma:

@smallexample
pragma External_Name_Casing (Uppercase, Uppercase);
@end smallexample

@noindent
to enforce the upper casing of all external symbols. 

@findex Finalize_Storage_Only
@item pragma Finalize_Storage_Only
@noindent
Syntax:

@smallexample
pragma Finalize_Storage_Only (first_subtype_LOCAL_NAME);
@end smallexample

@noindent
This pragma allows the compiler not to emit a Finalize call for objects
defined at the library level. This is mostly useful for types where
finalization is only used to deal with storage reclamation since in most
environments it is not necessary to reclaim memory just before terminating
execution, hence the name.

@cindex OpenVMS
@findex Float_Representation
@item pragma Float_Representation
@noindent
Syntax:

@smallexample
pragma Float_Representation (FLOAT_REP);

FLOAT_REP ::= VAX_Float | IEEE_Float
@end smallexample

@noindent
This pragma is implemented only in the OpenVMS implementation of GNAT@.
It allows control over the internal representation chosen for the predefined
floating point types declared in the packages @code{Standard} and
@code{System}. For further details on this pragma, see the
DEC Ada Language Reference Manual, section 3.5.7a. Note that to use this
pragma, the standard runtime libraries must be recompiled. See the
description of the @code{GNAT LIBRARY} command in the OpenVMS version
of the GNAT Users Guide for details on the use of this command.

@findex Ident
@item pragma Ident
@noindent
Syntax:

@smallexample
pragma Ident (static_string_EXPRESSION);
@end smallexample

@noindent
This pragma provides a string identification in the generated object file,
if the system supports the concept of this kind of identification string.
The maximum permitted length of the string literal is 31 characters.
This pragma is allowed only in the outermost declarative part or
declarative items of a compilation unit.
@cindex OpenVMS
On OpenVMS systems, the effect of the pragma is identical to the effect of
the DEC Ada 83 pragma of the same name. 

@cindex OpenVMS
@findex Import_Exception
@item pragma Import_Exception
@noindent
Syntax:

@smallexample
pragma Import_Exception (
     [Internal =>] LOCAL_NAME,
  [, [External =>] EXTERNAL_SYMBOL,]
  [, [Form     =>] Ada | VMS]
  [, [Code     =>] static_integer_EXPRESSION]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
@end smallexample

@noindent
This pragma is implemented only in the OpenVMS implementation of GNAT@.
It allows OpenVMS conditions (for example, from OpenVMS system services or
other OpenVMS languages) to be propagated to Ada programs as Ada exceptions.
The pragma specifies that the exception associated with an exception
declaration in an Ada program be defined externally (in non-Ada code).
For further details on this pragma, see the
DEC Ada Language Reference Manual, section 13.9a.3.1.

@findex Import_Function
@item pragma Import_Function @dots{}
@noindent
Syntax:

@smallexample
pragma Import_Function (
     [Internal                 =>] LOCAL_NAME,
  [, [External                 =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types          =>] PARAMETER_TYPES]
  [, [Result_Type              =>] SUBTYPE_MARK]
  [, [Mechanism                =>] MECHANISM]
  [, [Result_Mechanism         =>] MECHANISM_NAME]
  [, [First_Optional_Parameter =>] IDENTIFIER]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| SUBTYPE_MARK @{, SUBTYPE_MARK@}

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::=
  Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]

CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca
@end smallexample

This pragma is used in conjunction with a pragma @code{Import} to
specify additional information for an imported function. The pragma
@code{Import} (or equivalent pragma @code{Interface}) must precede the
@code{Import_Function} pragma and both must appear in the same
declarative part as the function specification.

The @var{Internal_Name} argument must uniquely designate
the function to which the
pragma applies. If more than one function name exists of this name in
the declarative part you must use the @code{Parameter_Types} and
@var{Result_Type} parameters to achieve the required unique
designation. Subtype marks in these parameters must exactly match the
subtypes in the corresponding function specification, using positional
notation to match parameters with subtype marks.

You may optionally use the @var{Mechanism} and @var{Result_Mechanism}
parameters to specify passing mechanisms for the
parameters and result. If you specify a single mechanism name, it
applies to all parameters.  Otherwise you may specify a mechanism on a
parameter by parameter basis using either positional or named
notation. If the mechanism is not specified, the default mechanism
is used.

@cindex OpenVMS
@cindex Passing by descriptor
Passing by descriptor is supported only on the to OpenVMS ports of GNAT@.

@code{First_Optional_Parameter} applies only to OpenVMS ports of GNAT@.
It specifies that the designated parameter and all following parameters
are optional, meaning that they are not passed at the generated code
level (this is distinct from the notion of optional parameters in Ada
where the parameters are passed anyway with the designated optional
parameters). All optional parameters must be of mode @code{IN} and have
default parameter values that are either known at compile time
expressions, or uses of the @code{'Null_Parameter} attribute.

@findex Import_Object
@item pragma Import_Object
@noindent
Syntax:

@smallexample
pragma Import_Object
     [Internal =>] LOCAL_NAME,
  [, [External =>] EXTERNAL_SYMBOL],
  [, [Size     =>] EXTERNAL_SYMBOL])

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
@end smallexample

@noindent
This pragma designates an object as imported, and apart from the
extended rules for external symbols, is identical in effect to the use of
the normal @code{Import} pragma applied to an object.  Unlike the
subprogram case, you need not use a separate @code{Import} pragma,
although you may do so (and probably should do so from a portability
point of view). @var{size} is syntax checked, but otherwise ignored by
GNAT@.

@findex Import_Procedure
@item pragma Import_Procedure
@noindent
Syntax:

@smallexample
pragma Import_Procedure (
     [Internal                 =>] LOCAL_NAME,
  [, [External                 =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types          =>] PARAMETER_TYPES]
  [, [Mechanism                =>] MECHANISM]
  [, [First_Optional_Parameter =>] IDENTIFIER]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| SUBTYPE_MARK @{, SUBTYPE_MARK@}

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::=
  Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]

CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca
@end smallexample

@noindent
This pragma is identical to @code{Import_Function} except that it
applies to a procedure rather than a function and the parameters
@code{Result_Type} and @code{Result_Mechanism} are not permitted.

@findex Import_Valued_Procedure
@item pragma Import_Valued_Procedure @dots{}
@noindent
Syntax:

@smallexample
pragma Import_Valued_Procedure (
     [Internal                 =>] LOCAL_NAME,
  [, [External                 =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types          =>] PARAMETER_TYPES]
  [, [Mechanism                =>] MECHANISM]
  [, [First_Optional_Parameter =>] IDENTIFIER]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| SUBTYPE_MARK @{, SUBTYPE_MARK@}

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::=
  Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]

CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca
@end smallexample

@noindent
This pragma is identical to @code{Import_Procedure} except that the
first parameter of @var{local_name}, which must be present, must be of
mode @code{OUT}, and externally the subprogram is treated as a function
with this parameter as the result of the function. The purpose of this
capability is to allow the use of @code{OUT} and @code{IN OUT}
parameters in interfacing to external functions (which are not permitted
in Ada functions).  You may optionally use the @code{Mechanism}
parameters to specify passing mechanisms for the parameters.
If you specify a single mechanism name, it applies to all parameters.
Otherwise you may specify a mechanism on a parameter by parameter
basis using either positional or named notation. If the mechanism is not
specified, the default mechanism is used.

Note that it is important to use this pragma in conjunction with a separate
pragma Import that specifies the desired convention, since otherwise the
default convention is Ada, which is almost certainly not what is required.

@findex Initialize_Scalars
@cindex debugging with Initialize_Scalars
@item pragma Initialize_Scalars
@noindent
Syntax:

@smallexample
pragma Initialize_Scalars;
@end smallexample

@noindent
This pragma is similar to @code{Normalize_Scalars} conceptually but has 
two important differences. First, there is no requirement for the pragma
to be used uniformly in all units of a partition, in particular, it is fine
to use this just for some or all of the application units of a partition,
without needing to recompile the run-time library.

In the case where some units are compiled with the pragma, and some without,
then a declaration of a variable where the type is defined in package
Standard or is locally declared will always be subject to initialization,
as will any declaration of a scalar variable. For composite variables,
whether the variable is initialized may also depend on whether the package
in which the type of the variable is declared is compiled with the pragma.

The other important difference is that there is control over the value used
for initializing scalar objects. At bind time, you can select whether to
initialize with invalid values (like Normalize_Scalars), or with high or
low values, or with a specified bit pattern. See the users guide for binder
options for specifying these cases.

This means that you can compile a program, and then without having to
recompile the program, you can run it with different values being used
for initializing otherwise uninitialized values, to test if your program
behavior depends on the choice. Of course the behavior should not change,
and if it does, then most likely you have an erroneous reference to an
uninitialized value.

Note that pragma @code{Initialize_Scalars} is particularly useful in
conjunction with the enhanced validity checking that is now provided
in @code{GNAT}, which checks for invalid values under more conditions.
Using this feature (see description of the @code{-gnatv} flag in the
users guide) in conjunction with pragma @code{Initialize_Scalars}
provides a powerful new tool to assist in the detection of problems
caused by uninitialized variables.

@findex Inline_Always
@item pragma Inline_Always
@noindent
Syntax:

@smallexample
pragma Inline_Always (NAME [, NAME]);
@end smallexample

@noindent
Similar to pragma  @code{Inline} except that inlining is not subject to
the use of option @code{-gnatn} for inter-unit inlining.

@findex Inline_Generic
@item pragma Inline_Generic
@noindent
Syntax:

@smallexample
pragma Inline_Generic (generic_package_NAME)
@end smallexample

@noindent
This is implemented for compatibility with DEC Ada 83 and is recognized,
but otherwise ignored, by GNAT@. All generic instantiations are inlined
by default when using GNAT@.

@findex Interface
@item pragma Interface
@noindent
Syntax:

@smallexample
pragma Interface (
     [Convention    =>] convention_identifier,
     [Entity =>] local_name
  [, [External_Name =>] static_string_expression],
  [, [Link_Name     =>] static_string_expression]);
@end smallexample

@noindent
This pragma is identical in syntax and semantics to
the standard Ada 95 pragma @code{Import}. It is provided for compatibility
with Ada 83. The definition is upwards compatible both with pragma
@code{Interface} as defined in the Ada 83 Reference Manual, and also
with some extended implementations of this pragma in certain Ada 83
implementations.

@findex Interface_Name
@item pragma Interface_Name
@noindent
Syntax:

@smallexample
pragma Interface_Name ( 
     [Entity        =>] LOCAL_NAME
  [, [External_Name =>] static_string_EXPRESSION]
  [, [Link_Name     =>] static_string_EXPRESSION]);
@end smallexample

@noindent
This pragma provides an alternative way of specifying the interface name
for an interfaced subprogram, and is provided for compatibility with Ada
83 compilers that use the pragma for this purpose. You must provide at
least one of @var{External_Name} or @var{Link_Name}.

@findex License
@item pragma License
@cindex License checking
@noindent
Syntax:

@smallexample
pragma License (Unrestricted | GPL | Modified_GPL | Restricted);
@end smallexample

@noindent
This pragma is provided to allow automated checking for appropriate license
conditions with respect to the standard and modified GPL@. A pragma License,
which is a configuration pragma that typically appears at the start of a
source file or in a separate @file{gnat.adc} file, specifies the licensing
conditions of a unit as follows:

@itemize @bullet
@item Unrestricted
This is used for a unit that can be freely used with no license restrictions.
Examples of such units are public domain units, and units from the Ada
Reference Manual.

@item GPL
This is used for a unit that is licensed under the unmodified GPL, and which
therefore cannot be @code{with}'ed by a restricted unit.

@item Modified_GPL
This is used for a unit licensed under the GNAT modified GPL that includes
a special exception paragraph that specifically permits the inclusion of
the unit in programs without requiring the entire program to be released
under the GPL@. This is the license used for the GNAT run-time which ensures
that the run-time can be used freely in any program without GPL concerns.

@item Restricted
This is used for a unit that is restricted in that it is not permitted to
depend on units that are licensed under the GPL@. Typical examples are
proprietary code that is to be released under more restrictive license
conditions. Note that restricted units are permitted to @code{with} units
which are licensed under the modified GPL (this is the whole point of the
modified GPL).

@end itemize

@noindent
Normally a unit with no @code{License} pragma is considered to have an
unknown license, and no checking is done. However, standard GNAT headers
are recognized, and license information is derived from them as follows.

@itemize @bullet

A GNAT license header starts with a line containing 78 hyphens. The following
comment text is searched for the appearence of any of the following strings.

If the string "GNU General Public License" is found, then the unit is assumed
to have GPL license, unless the string "As a special exception" follows, in
which case the license is assumed to be modified GPL@.

If one of the strings
"This specification is adapated from the Ada Semantic Interface" or
"This specification is derived from the Ada Reference Manual" is found
then the unit is assumed to be unrestricted.
@end itemize

@noindent
These default actions means that a program with a restricted license pragma
will automatically get warnings if a GPL unit is inappropriately
@code{with}'ed. For example, the program:

@smallexample
with Sem_Ch3;
with GNAT.Sockets;
procedure Secret_Stuff is
...
end Secret_Stuff
@end smallexample

@noindent
if compiled with pragma @code{License} (@code{Restricted}) in a
@file{gnat.adc} file will generate the warning:

@smallexample
1. with Sem_Ch3;
        |
   >>> license of withed unit "Sem_Ch3" is incompatible

2. with GNAT.Sockets;
3. procedure Secret_Stuff is
@end smallexample
@noindent
Here we get a warning on @code{Sem_Ch3} since it is part of the GNAT
compiler and is licensed under the
GPL, but no warning for @code{GNAT.Sockets} which is part of the GNAT 
run time, and is therefore licensed under the modified GPL@.

@findex Link_With
@item pragma Link_With
@noindent
Syntax:

@smallexample
pragma Link_With (static_string_EXPRESSION @{,static_string_EXPRESSION@});
@end smallexample

@noindent
This pragma is provided for compatibility with certain Ada 83 compilers.
It has exactly the same effect as pragma @code{Linker_Options} except
that spaces occurring within one of the string expressions are treated
as separators. For example, in the following case:

@smallexample
pragma Link_With ("-labc -ldef");
@end smallexample

@noindent
results in passing the strings @code{-labc} and @code{-ldef} as two
separate arguments to the linker.

@findex Linker_Alias
@item pragma Linker_Alias
@noindent
Syntax:

@smallexample
pragma Linker_Alias (
  [Entity =>] LOCAL_NAME
  [Alias  =>] static_string_EXPRESSION);
@end smallexample

@noindent
This pragma establishes a linker alias for the given named entity. For
further details on the exact effect, consult the GCC manual.

@findex Linker_Section
@item pragma Linker_Section
@noindent
Syntax:

@smallexample
pragma Linker_Section (
  [Entity  =>] LOCAL_NAME
  [Section =>] static_string_EXPRESSION);
@end smallexample

@noindent
This pragma specifies the name of the linker section for the given entity.
For further details on the exact effect, consult the GCC manual.

@findex No_Run_Time
@item pragma No_Run_Time
@noindent
Syntax:

@smallexample
pragma No_Run_Time;
@end smallexample

@noindent
This is a configuration pragma that makes sure the user code does not
use nor need anything from the GNAT run time. This is mostly useful in
context where code certification is required. Please consult the High
Integrity product documentation for additional information.

@findex Normalize_Scalars
@item pragma Normalize_Scalars
@noindent
Syntax:

@smallexample
pragma Normalize_Scalars;
@end smallexample

@noindent
This is a language defined pragma which is fully implemented in GNAT@. The
effect is to cause all scalar objects that are not otherwise initialized
to be initialized. The initial values are implementation dependent and
are as follows:

@table @code
@item Standard.Character
@noindent
Objects whose root type is Standard.Character are initialized to
Character'Last. This will be out of range of the subtype only if
the subtype range excludes this value.

@item Standard.Wide_Character
@noindent
Objects whose root type is Standard.Wide_Character are initialized to
Wide_Character'Last. This will be out of range of the subtype only if
the subtype range excludes this value.

@item Integer types
@noindent
Objects of an integer type are initialized to base_type'First, where
base_type is the base type of the object type. This will be out of range
of the subtype only if the subtype range excludes this value. For example,
if you declare the subtype:

@smallexample
subtype Ityp is integer range 1 .. 10;
@end smallexample

@noindent
then objects of type x will be initialized to Integer'First, a negative
number that is certainly outside the range of subtype @code{Ityp}.

@item Real types
Objects of all real types (fixed and floating) are initialized to
base_type'First, where base_Type is the base type of the object type.
This will be out of range of the subtype only if the subtype range
excludes this value.

@item Modular types
Objects of a modular type are initialized to typ'Last. This will be out
of range of the subtype only if the subtype excludes this value.

@item Enumeration types
Objects of an enumeration type are initialized to all one-bits, i.e.@: to
the value 2 ** typ'Size - 1. This will be out of range of the enumeration
subtype in all cases except where the subtype contains exactly
2**8, 2**16, or 2**32.

@end table

@cindex OpenVMS
@findex Long_Float
@item pragma Long_Float
@noindent
Syntax:

@smallexample
pragma Long_Float (FLOAT_FORMAT);

FLOAT_FORMAT ::= D_Float | G_Float
@end smallexample

@noindent
This pragma is implemented only in the OpenVMS implementation of GNAT@.
It allows control over the internal representation chosen for the predefined
type @code{Long_Float} and for floating point type representations with
@code{digits} specified in the range 7 .. 15.
For further details on this pragma, see the
DEC Ada Language Reference Manual, section 3.5.7b. Note that to use this
pragma, the standard runtime libraries must be recompiled. See the
description of the @code{GNAT LIBRARY} command in the OpenVMS version
of the GNAT Users Guide for details on the use of this command.

@findex Machine_Attribute
@item pragma Machine_Attribute @dots{}
@noindent
Syntax:

@smallexample
pragma Machine_Attribute (
  [Attribute_Name =>] string_EXPRESSION,
  [Entity         =>] LOCAL_NAME);
@end smallexample

Machine dependent attributes can be specified for types and/or
declarations. Currently only subprogram entities are supported.  This
pragma is semantically equivalent to @code{__attribute__((
@var{string_expression}))} in GNU C, where @code{string_expression}> is
recognized by the GNU C macros @code{VALID_MACHINE_TYPE_ATTRIBUTE} and
@code{VALID_MACHINE_DECL_ATTRIBUTE} which are defined in the
configuration header file @file{tm.h} for each machine.  See the GCC
manual for further information.

@cindex OpenVMS
@findex Main_Storage
@item pragma Main_Storage
@noindent
Syntax:

@smallexample
pragma Main_Storage
  (MAIN_STORAGE_OPTION [, MAIN_STORAGE_OPTION]);

MAIN_STORAGE_OPTION ::=
  [WORKING_STORAGE =>] static_SIMPLE_EXPRESSION
| [TOP_GUARD       =>] static_SIMPLE_EXPRESSION

@end smallexample

@noindent
This pragma is provided for compatibility with OpenVMS Vax Systems. It has
no effect in GNAT, other than being syntax checked. Note that the pragma
also has no effect in DEC Ada 83 for OpenVMS Alpha Systems.

@findex No_Return
@item pragma No_Return
@noindent
Syntax:

@smallexample
pragma No_Return (procedure_LOCAL_NAME);
@end smallexample

@noindent
@var{procedure_local_NAME} must refer to one or more procedure
declarations in the current declarative part. A procedure to which this
pragma is applied may not contain any explicit @code{return} statements,
and also may not contain any implicit return statements from falling off
the end of a statement sequence. One use of this pragma is to identify
procedures whose only purpose is to raise an exception.

Another use of this pragma is to suppress incorrect warnings about
missing returns in functions, where the last statement of a function
statement sequence is a call to such a procedure.

@findex Passive
@item pragma Passive
@noindent
Syntax:

@smallexample
pragma Passive ([Semaphore | No]);
@end smallexample

@noindent
Syntax checked, but otherwise ignored by GNAT@. This is recognized for
compatibility with DEC Ada 83 implementations, where it is used within a
task definition to request that a task be made passive.  If the argument
@code{Semaphore} is present, or no argument is omitted, then DEC Ada 83
treats the pragma as an assertion that the containing task is passive
and that optimization of context switch with this task is permitted and
desired.  If the argument @code{No} is present, the task must not be
optimized. GNAT does not attempt to optimize any tasks in this manner
(since protected objects are available in place of passive tasks).

@findex Polling 
@item pragma Polling
@noindent
Syntax:

@smallexample
pragma Polling (ON | OFF);
@end smallexample

@noindent
This pragma controls the generation of polling code. This is normally off.
If @code{pragma Polling (ON)} is used then periodic calls are generated to
the routine Ada.Exceptions.Poll. This routine is a separate unit in the
runtime library, and can be found in file a-excpol.adb.

Pragma polling can appear as a configuration pragma (for example it can be
placed in the gnat.adc file) to enable polling globally, or it can be used
in the statement or declaration sequence to control polling more locally.

A call to the polling routine is generated at the start of every loop and
at the start of every subprogram call. This guarantees that the Poll
routine is called frequently, and places an upper bound (determined by
the complexity of the code) on the period between two Poll calls.

The primary purpose of the polling interface is to enable asynchronous 
aborts on targets that cannot otherwise support it (for example Windows
NT), but it may be used for any other purpose requiring periodic polling.
The standard version is null, and can be replaced by a user program. This
will require re-compilation of the Ada.Exceptions package that can be found
in files a-except.ads/adb.

A standard alternative unit (called 4wexcpol.adb in the standard GNAT
distribution) is used to enable the asynchronous abort capability on
targets that do not normally support the capability. The version of Poll
in this file makes a call to the appropriate runtime routine to test for
an abort condition.

Note that polling can also be enabled by use of the -gnatP switch. See
the GNAT User's Guide for details.

@findex Propagate_Exceptions
@cindex Zero Cost Exceptions
@item pragma Propagate_Exceptions
@noindent
Syntax:

@smallexample
pragma Propagate_Exceptions (subprogram_LOCAL_NAME);
@end smallexample

@noindent
This pragma indicates that the given entity, which is the name of an
imported foreign-language subprogram may receive an Ada exception, 
and that the exception should be propagated. It is relevant only if
zero cost exception handling is in use, and is thus never needed if
the alternative longjmp/setjmp implementation of exceptions is used
(although it is harmless to use it in such cases).

The implementation of fast exceptions always properly propagates
exceptions through Ada code, as described in the Ada Reference Manual.
However, this manual is silent about the propagation of exceptions
through foreign code. For example, consider the
situation where @code{P1} calls
@code{P2}, and @code{P2} calls @code{P3}, where
@code{P1} and @code{P3} are in Ada, but @code{P2} is in C@.
@code{P3} raises an Ada exception. The question is whether or not
it will be propagated through @code{P2} and can be handled in 
@code{P1}.

For the longjmp/setjmp implementation of exceptions, the answer is
always yes. For some targets on which zero cost exception handling
is implemented, the answer is also always yes. However, there are
some targets, notably in the current version all x86 architecture
targets, in which the answer is that such propagation does not
happen automatically. If such propagation is required on these
targets, it is mandatory to use @code{Propagate_Exceptions} to 
name all foreign language routines through which Ada exceptions
may be propagated.

@findex Psect_Object
@item pragma Psect_Object
@noindent
Syntax:

@smallexample
pragma Psect_Object
     [Internal =>] LOCAL_NAME,
  [, [External =>] EXTERNAL_SYMBOL]
  [, [Size     =>] EXTERNAL_SYMBOL]

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
@end smallexample

@noindent
This pragma is identical in effect to pragma @code{Common_Object}.

@findex Pure_Function
@item pragma Pure_Function
@noindent
Syntax:

@smallexample
pragma Pure_Function ([Entity =>] function_LOCAL_NAME);
@end smallexample

This pragma appears in the same declarative part as a function
declaration (or a set of function declarations if more than one
overloaded declaration exists, in which case the pragma applies
to all entities). If specifies that the function @code{Entity} is
to be considered pure for the purposes of code generation. This means
that the compiler can assume that there are no side effects, and
in particular that two calls with identical arguments produce the
same result. It also means that the function can be used in an
address clause.

Note that, quite deliberately, there are no static checks to try
to ensure that this promise is met, so @var{Pure_Function} can be used
with functions that are conceptually pure, even if they do modify
global variables. For example, a square root function that is
instrumented to count the number of times it is called is still
conceptually pure, and can still be optimized, even though it
modifies a global variable (the count). Memo functions are another
example (where a table of previous calls is kept and consulted to
avoid re-computation).

@findex Pure
Note: Most functions in a @code{Pure} package are automatically pure, and
there is no need to use pragma @code{Pure_Function} for such functions. An
exception is any function that has at least one formal of type
@code{System.Address} or a type derived from it. Such functions are not
considered pure by default, since the compiler assumes that the
@code{Address} parameter may be functioning as a pointer and that the
referenced data may change even if the address value does not. The use
of pragma Pure_Function for such a function will override this default
assumption, and cause the compiler to treat such a function as pure.

Note: If pragma @code{Pure_Function} is applied to a renamed function, it
applies to the underlying renamed function. This can be used to
disambiguate cases of overloading where some but not all functions
in a set of overloaded functions are to be designated as pure.

@findex Ravenscar
@item pragma Ravenscar
@noindent
Syntax:

@smallexample
pragma Ravenscar
@end smallexample

@noindent
A configuration pragma that establishes the following set of restrictions:

@table @code
@item No_Abort_Statements
[RM D.7] There are no abort_statements, and there are 
no calls to Task_Identification.Abort_Task.

@item No_Select_Statements
There are no select_statements.

@item No_Task_Hierarchy
[RM D.7] All (non-environment) tasks depend 
directly on the environment task of the partition.  

@item No_Task_Allocators
[RM D.7] There are no allocators for task types
or types containing task subcomponents.

@item No_Dynamic_Priorities
[RM D.7] There are no semantic dependencies on the package Dynamic_Priorities.

@item No_Terminate_Alternatives
[RM D.7] There are no selective_accepts with terminate_alternatives

@item No_Dynamic_Interrupts
There are no semantic dependencies on Ada.Interrupts.

@item No_Protected_Type_Allocators
There are no allocators for protected types or
types containing protected subcomponents.

@item No_Local_Protected_Objects
Protected objects and access types that designate 
such objects shall be declared only at library level.

@item No_Requeue
Requeue statements are not allowed.

@item No_Calendar
There are no semantic dependencies on the package Ada.Calendar.

@item No_Relative_Delay
There are no delay_relative_statements.

@item No_Task_Attributes
There are no semantic dependencies on the Ada.Task_Attributes package and
there are no references to the attributes Callable and Terminated [RM 9.9].

@item Static_Storage_Size
The expression for pragma Storage_Size is static.

@item Boolean_Entry_Barriers
Entry barrier condition expressions shall be boolean 
objects which are declared in the protected type 
which contains the entry.

@item Max_Asynchronous_Select_Nesting = 0
[RM D.7] Specifies the maximum dynamic nesting level of asynchronous_selects.
A value of zero prevents the use of any asynchronous_select.

@item Max_Task_Entries = 0
[RM D.7] Specifies the maximum number of entries
per task.  The bounds of every entry family
of a task unit shall be static, or shall be
defined by a discriminant of a subtype whose
corresponding bound is static.  A value of zero
indicates that no rendezvous are possible.  For
the Ravenscar pragma, the value of Max_Task_Entries is always
0 (zero).

@item Max_Protected_Entries = 1
[RM D.7] Specifies the maximum number of entries per 
protected type.  The bounds of every entry family of 
a protected unit shall be static, or shall be defined 
by a discriminant of a subtype whose corresponding 
bound is static.  For the Ravenscar pragma the value of 
Max_Protected_Entries is always 1.

@item Max_Select_Alternatives = 0
[RM D.7] Specifies the maximum number of alternatives in a selective_accept.
For the Ravenscar pragma the value if always 0.

@item No_Task_Termination
Tasks which terminate are erroneous.

@item No_Entry_Queue
No task can be queued on a protected entry. Note that this restrictions is
checked at run time. The violation of this restriction generates a
Program_Error exception.
@end table

@noindent
This set of restrictions corresponds to the definition of the "Ravenscar
Profile" for limited tasking, devised and published by the International
Workshop On Real Time Ada", 1997. 

The above set is a superset of the restrictions provided by pragma
@code{Restricted_Run_Time}, it includes six additional restrictions
(@code{Boolean_Entry_Barriers}, @code{No_Select_Statements},
@code{No_Calendar}, @code{Static_Storage_Size},
@code{No_Relative_Delay} and @code{No_Task_Termination}). This means
that pragma Ravenscar, like the pragma Restricted_Run_Time, automatically
causes the use of a simplified, more efficient version of the tasking
run-time system.

@findex Restricted_Run_Time
@item pragma Restricted_Run_Time
@noindent
Syntax:

@smallexample
pragma Restricted_Run_Time
@end smallexample

@noindent
A configuration pragma that establishes the following set of restrictions:

@itemize @bullet
@item No_Abort_Statements
@item No_Asynchronous_Control
@item No_Entry_Queue
@item No_Task_Hierarchy
@item No_Task_Allocators
@item No_Dynamic_Priorities
@item No_Terminate_Alternatives
@item No_Dynamic_Interrupts
@item No_Protected_Type_Allocators
@item No_Local_Protected_Objects
@item No_Requeue
@item No_Task_Attributes
@item Max_Asynchronous_Select_Nesting =  0
@item Max_Task_Entries =  0
@item Max_Protected_Entries = 1
@item Max_Select_Alternatives = 0
@end itemize

@noindent
This set of restrictions causes the automatic selection of a simplified
version of the run time that provides improved performance for the
limited set of tasking functionality permitted by this set of restrictions.

@findex Share_Generic
@item pragma Share_Generic
@noindent
Syntax:

@smallexample
pragma Share_Generic (NAME @{, NAME@});
@end smallexample

@noindent
This pragma is recognized for compatibility with other Ada compilers
but is ignored by GNAT@. GNAT does not provide the capability for
sharing of generic code. All generic instantiations result in making
an inlined copy of the template with appropriate substitutions.

@findex Source_File_Name
@item pragma Source_File_Name
@noindent
Syntax:

@smallexample
pragma Source_File_Name (
  [Unit_Name   =>] unit_NAME,
  Spec_File_Name =>  STRING_LITERAL);

pragma Source_File_Name (
  [Unit_Name   =>] unit_NAME,
  Body_File_Name =>  STRING_LITERAL);
@end smallexample

@noindent
Use this to override the normal naming convention. It is a configuration
pragma, and so has the usual applicability of configuration pragmas
(i.e.@: it applies to either an entire partition, or to all units in a
compilation, or to a single unit, depending on how it is used.
@var{unit_name} is mapped to @var{file_name_literal}. The identifier for
the second argument is required, and indicates whether this is the file
name for the spec or for the body.

Another form of the @code{Source_File_Name} pragma allows
the specification of patterns defining alternative file naming schemes
to apply to all files. 

@smallexample
pragma Source_File_Name
  (Spec_File_Name => STRING_LITERAL
   [,Casing => CASING_SPEC]
   [,Dot_Replacement => STRING_LITERAL]);

pragma Source_File_Name
  (Body_File_Name => STRING_LITERAL
   [,Casing => CASING_SPEC]
   [,Dot_Replacement => STRING_LITERAL]);

pragma Source_File_Name
  (Subunit_File_Name => STRING_LITERAL
   [,Casing => CASING_SPEC]
   [,Dot_Replacement => STRING_LITERAL]);

CASING_SPEC ::= Lowercase | Uppercase | Mixedcase
@end smallexample

@noindent
The first argument is a pattern that contains a single asterisk indicating
the point at which the unit name is to be inserted in the pattern string
to form the file name. The second argument is optional. If present it
specifies the casing of the unit name in the resulting file name string.
The default is lower case. Finally the third argument allows for systematic
replacement of any dots in the unit name by the specified string literal.

For more details on the use of the @code{Source_File_Name} pragma,
see the sections "Using Other File Names", and "Alternative File
Naming Schemes" in the GNAT User's Guide.

@findex Source_Reference
@item pragma Source_Reference
@noindent
Syntax:

@smallexample
pragma Source_Reference (INTEGER_LITERAL,
                         STRING_LITERAL);
@end smallexample

@noindent
This pragma must appear as the first line of a source file.
@var{integer_literal} is the logical line number of the line following
the pragma line (for use in error messages and debugging
information). @var{string_literal} is a static string constant that
specifies the file name to be used in error messages and debugging
information. This is most notably used for the output of @code{gnatchop}
with the @samp{-r} switch, to make sure that the original unchopped
source file is the one referred to.

The second argument must be a string literal, it cannot be a static
string expression other than a string literal. This is because its value
is needed for error messages issued by all phases of the compiler.

@findex Stream_Convert
@item pragma Stream_Convert
@noindent
Syntax:

@smallexample
pragma Stream_Convert (
  [Entity =>] type_LOCAL_NAME,
  [Read   =>] function_NAME,
  [Write  =>] function NAME);
@end smallexample

@noindent
This pragma provides an efficient way of providing stream functions for
types defined in packages. Not only is it simpler to use than declaring
the necessary functions with attribute representation clauses, but more
significantly, it allows the declaration to made in such a way that the
stream packages are not loaded unless they are needed. The use of
the Stream_Convert pragma adds no overhead at all, unless the stream
attributes are actually used on the designated type.

The first argument specifies the type for which stream functions are
provided. The second parameter provides a function used to read values
of this type. It must name a function whose argument type may be any
subtype, and whose returned type must be the type given as the first
argument to the pragma.

The meaning of the @var{Read}
parameter is that if a stream attribute directly
or indirectly specifies reading of the type given as the first parameter,
then a value of the type given as the argument to the Read function is
read from the stream, and then the Read function is used to convert this
to the required target type.

Similarly the @var{Write} parameter specifies how to treat write attributes
that directly or indirectly apply to the type given as the first parameter.
It must have an input parameter of the type specified by the first parameter,
and the return type must be the same as the input type of the Read function.
The effect is to first call the Write function to convert to the given stream
type, and then write the result type to the stream.

The Read and Write functions must not be overloaded subprograms. If necessary
renamings can be supplied to meet this requirement.
The usage of this attribute is best illustrated by a simple example, taken
from the GNAT implementation of package Ada.Strings.Unbounded:

@smallexample
function To_Unbounded (S : String)
           return Unbounded_String
  renames To_Unbounded_String;

pragma Stream_Convert
  (Unbounded_String, To_Unbounded, To_String);
@end smallexample

@noindent
The specifications of the referenced functions, as given in the Ada 95
Reference Manual are:

@smallexample
function To_Unbounded_String (Source : String)
  return Unbounded_String;

function To_String (Source : Unbounded_String)
  return String;
@end smallexample

@noindent
The effect is that if the value of an unbounded string is written to a
stream, then the representation of the item in the stream is in the same
format used for @code{Standard.String}, and this same representation is
expected when a value of this type is read from the stream.

@findex Style_Checks
@item pragma Style_Checks
@noindent
Syntax:

@smallexample
pragma Style_Checks (string_LITERAL | ALL_CHECKS |
                     On | Off [, LOCAL_NAME]);
@end smallexample

@noindent
This pragma is used in conjunction with compiler switches to control the
built in style checking provided by GNAT@. The compiler switches, if set
provide an initial setting for the switches, and this pragma may be used
to modify these settings, or the settings may be provided entirely by
the use of the pragma. This pragma can be used anywhere that a pragma
is legal, including use as a configuration pragma (including use in
the @file{gnat.adc} file).

The form with a string literal specifies which style options are to be
activated. These are additive, so they apply in addition to any previously
set style check options. The codes for the options are the same as those
used in the @code{-gnaty} switch on the @code{gcc} or @code{gnatmake}
line. For example the following two methods can be used to enable
layout checking:

@smallexample
pragma Style_Checks ("l");
gcc -c -gnatyl ...
@end smallexample

@noindent
The form ALL_CHECKS activates all standard checks (its use is equivalent
to the use of the @code{gnaty} switch with no options. See GNAT User's
Guide for details.

The forms with @code{Off} and @code{On}
can be used to temporarily disable style checks
as shown in the following example:

@smallexample
@iftex
@leftskip=0cm
@end iftex
pragma Style_Checks ("k"); -- requires keywords in lower case
pragma Style_Checks (Off); -- turn off style checks
NULL;                      -- this will not generate an error message
pragma Style_Checks (On);  -- turn style checks back on
NULL;                      -- this will generate an error message
@end smallexample

@noindent
Finally the two argument form is allowed only if the first argument is
@code{On} or @code{Off}. The effect is to turn of semantic style checks
for the specified entity, as shown in the following example:

@smallexample
@iftex
@leftskip=0cm
@end iftex
pragma Style_Checks ("r"); -- require consistency of identifier casing
Arg : Integer;
Rf1 : Integer := ARG;      -- incorrect, wrong case
pragma Style_Checks (Off, Arg);
Rf2 : Integer := ARG;      -- OK, no error
@end smallexample

@findex Subtitle
@item pragma Subtitle
@noindent
Syntax:

@smallexample
pragma Subtitle ([Subtitle =>] STRING_LITERAL);
@end smallexample

@noindent
This pragma is recognized for compatibility with other Ada compilers
but is ignored by GNAT@.

@findex Suppress_All
@item pragma Suppress_All
@noindent
Syntax:

@smallexample
pragma Suppress_All;
@end smallexample

@noindent
This pragma can only appear immediately following a compilation
unit. The effect is to apply @code{Suppress (All_Checks)} to the unit
which it follows. This pragma is implemented for compatibility with DEC
Ada 83 usage. The use of pragma @code{Suppress (All_Checks)} as a normal
configuration pragma is the preferred usage in GNAT@.

@findex Suppress_Initialization
@cindex Suppressing initialization
@cindex Initialization, suppression of
@item pragma Suppress_Initialization
@noindent
Syntax:

@smallexample
pragma Suppress_Initialization ([Entity =>] type_Name);
@end smallexample

@noindent
This pragma suppresses any implicit or explicit initialization
associated with the given type name for all variables of this type.

@findex Task_Info
@item pragma Task_Info
@noindent
Syntax

@smallexample
pragma Task_Info (EXPRESSION);
@end smallexample

@noindent
This pragma appears within a task definition (like pragma
@code{Priority}) and applies to the task in which it appears. The
argument must be of type @code{System.Task_Info.Task_Info_Type}.
The @code{Task_Info} pragma provides system dependent control over
aspect of tasking implementation, for example, the ability to map
tasks to specific processors. For details on the facilities available
for the version of GNAT that you are using, see the documentation
in the specification of package System.Task_Info in the runtime
library.

@findex Task_Name
@item pragma Task_Name
@noindent
Syntax

@smallexample
pragma Task_Name (string_EXPRESSION);
@end smallexample

@noindent
This pragma appears within a task definition (like pragma
@code{Priority}) and applies to the task in which it appears. The
argument must be of type String, and provides a name to be used for
the task instance when the task is created. Note that this expression
is not required to be static, and in particular, it can contain
references to task discriminants. This facility can be used to
provide different names for different tasks as they are created,
as illustrated in the example below.

The task name is recorded internally in the run-time structures
and is accessible to tools like the debugger. In addition the
routine @code{Ada.Task_Identification.Image} will return this
string, with a unique task address appended.

@smallexample
--  Example of the use of pragma Task_Name

with Ada.Task_Identification;
use Ada.Task_Identification;
with Text_IO; use Text_IO;
procedure t3 is

   type Astring is access String;

   task type Task_Typ (Name : access String) is
      pragma Task_Name (Name.all);
   end Task_Typ;
   
   task body Task_Typ is
      Nam : constant String := Image (Current_Task);
   begin
      Put_Line ("-->" & Nam (1 .. 14) & "<--");
   end Task_Typ;
   
   type Ptr_Task is access Task_Typ;
   Task_Var : Ptr_Task;

begin
   Task_Var :=
     new Task_Typ (new String'("This is task 1"));
   Task_Var :=
     new Task_Typ (new String'("This is task 2"));
end;
@end smallexample

@findex Task_Storage
@item pragma Task_Storage
Syntax:

@smallexample
pragma Task_Storage
  [Task_Type =>] LOCAL_NAME,
  [Top_Guard =>] static_integer_EXPRESSION);
@end smallexample

This pragma specifies the length of the guard area for tasks.  The guard
area is an additional storage area allocated to a task.  A value of zero
means that either no guard area is created or a minimal guard area is
created, depending on the target. This pragma can appear anywhere a
@code{Storage_Size} attribute definition clause is allowed for a task
type.

@findex Time_Slice
@item pragma Time_Slice
@noindent
Syntax:

@smallexample
pragma Time_Slice (static_duration_EXPRESSION);
@end smallexample

@noindent
For implementations of GNAT on operating systems where it is possible
to supply a time slice value, this pragma may be used for this purpose.
It is ignored if it is used in a system that does not allow this control,
or if it appears in other than the main program unit.
@cindex OpenVMS
Note that the effect of this pragma is identical to the effect of the
DEC Ada 83 pragma of the same name when operating under OpenVMS systems.

@findex Title
@item pragma Title
@noindent
Syntax:

@smallexample
pragma Title (TITLING_OPTION [, TITLING OPTION]);

TITLING_OPTION ::=
  [Title    =>] STRING_LITERAL,
| [Subtitle =>] STRING_LITERAL
@end smallexample

@noindent
Syntax checked but otherwise ignored by GNAT@. This is a listing control
pragma used in DEC Ada 83 implementations to provide a title and/or
subtitle for the program listing. The program listing generated by GNAT
does not have titles or subtitles.

Unlike other pragmas, the full flexibility of named notation is allowed
for this pragma, i.e.@: the parameters may be given in any order if named
notation is used, and named and positional notation can be mixed
following the normal rules for procedure calls in Ada.

@cindex Unions in C
@findex Unchecked_Union
@item pragma Unchecked_Union
@noindent
Syntax:

@smallexample
pragma Unchecked_Union (first_subtype_LOCAL_NAME)
@end smallexample

@noindent
This pragma is used to declare that the specified type should be represented
in a manner
equivalent to a C union type, and is intended only for use in
interfacing with C code that uses union types. In Ada terms, the named
type must obey the following rules:

@itemize @bullet
@item
It is a non-tagged non-limited record type.
@item
It has a single discrete discriminant with a default value.
@item
The component list consists of a single variant part.
@item
Each variant has a component list with a single component.
@item
No nested variants are allowed.
@item
No component has an explicit default value.
@item
No component has a non-static constraint.
@end itemize

In addition, given a type that meets the above requirements, the
following restrictions apply to its use throughout the program:

@itemize @bullet
@item
The discriminant name can be mentioned only in an aggregate.
@item
No subtypes may be created of this type.
@item
The type may not be constrained by giving a discriminant value.
@item
The type cannot be passed as the actual for a generic formal with a
discriminant.
@end itemize

Equality and inequality operations on @code{unchecked_unions} are not
available, since there is no discriminant to compare and the compiler
does not even know how many bits to compare. It is implementation
dependent whether this is detected at compile time as an illegality or
whether it is undetected and considered to be an erroneous construct. In
GNAT, a direct comparison is illegal, but GNAT does not attempt to catch
the composite case (where two composites are compared that contain an
unchecked union component), so such comparisons are simply considered
erroneous.

The layout of the resulting type corresponds exactly to a C union, where
each branch of the union corresponds to a single variant in the Ada
record. The semantics of the Ada program is not changed in any way by
the pragma, i.e.@: provided the above restrictions are followed, and no
erroneous incorrect references to fields or erroneous comparisons occur,
the semantics is exactly as described by the Ada reference manual.
Pragma @code{Suppress (Discriminant_Check)} applies implicitly to the
type and the default convention is C

@findex Unimplemented_Unit
@item pragma Unimplemented_Unit
@noindent
Syntax:

@smallexample
pragma Unimplemented_Unit;
@end smallexample

@noindent
If this pragma occurs in a unit that is processed by the compiler, GNAT
aborts with the message @samp{@var{xxx} not implemented}, where
@var{xxx} is the name of the current compilation unit.  This pragma is
intended to allow the compiler to handle unimplemented library units in
a clean manner.

The abort only happens if code is being generated.  Thus you can use
specs of unimplemented packages in syntax or semantic checking mode.

@findex Unreserve_All_Interrupts
@item pragma Unreserve_All_Interrupts
@noindent
Syntax:

@smallexample
pragma Unreserve_All_Interrupts;
@end smallexample

@noindent
Normally certain interrupts are reserved to the implementation. Any attempt
to attach an interrupt causes Program_Error to be raised, as described in
RM C.3.2(22). A typical example is the @code{SIGINT} interrupt used in
many systems for an @code{Ctrl-C} interrupt. Normally this interrupt is
reserved to the implementation, so that @code{Ctrl-C} can be used to
interrupt execution.

If the pragma Unreserve_All_Interrupts appears anywhere in any unit in
a program, then all such interrupts are unreserved. This allows the
program to handle these interrupts, but disables their standard
functions. For example, if this pragma is used, then pressing
@code{Ctrl-C} will not automatically interrupt execution. However,
a program can then handle the @code{SIGINT} interrupt as it chooses.

For a full list of the interrupts handled in a specific implementation,
see the source code for the specification of Ada.Interrupts.Names in
file a-intnam.ads. This is a target dependent file that contains the
list of interrupts recognized for a given target. The documentation in
this file also specifies what interrupts are affected by the use of
the Unreserve_All_Interrupts pragma.

@findex Unsuppress
@item pragma Unsuppress
@noindent
Syntax:

@smallexample
pragma Unsuppress (IDENTIFIER [, [On =>] NAME]);
@end smallexample

@noindent
This pragma undoes the effect of a previous pragma @code{Suppress}. If
there is no corresponding pragma @code{Suppress} in effect, it has no
effect. The range of the effect is the same as for pragma
@code{Suppress}. The meaning of the arguments is identical to that used
in pragma @code{Suppress}.

One important application is to ensure that checks are on in cases where
code depends on the checks for its correct functioning, so that the code
will compile correctly even if the compiler switches are set to suppress
checks.

@cindex @code{Size}, VADS compatibility
@findex Use_VADS_Size
@item pragma Use_VADS_Size
@noindent
Syntax:

@smallexample
pragma Use_VADS_Size;
@end smallexample

@noindent
This is a configuration pragma. In a unit to which it applies, any use
of the 'Size attribute is automatically interpreted as a use of the
'VADS_Size attribute. Note that this may result in incorrect semantic
processing of valid Ada 95 programs. This is intended to aid in the
handling of legacy code which depends on the interpretation of Size
as implemented in the VADS compiler. See description of the VADS_Size
attribute for further details.

@findex Validity_Checks
@item pragma Validity_Checks
@noindent
Syntax:

@smallexample
pragma Validity_Checks (string_LITERAL | ALL_CHECKS | On | Off);
@end smallexample

@noindent
This pragma is used in conjunction with compiler switches to control the
built in validity checking provided by GNAT@. The compiler switches, if set
provide an initial setting for the switches, and this pragma may be used
to modify these settings, or the settings may be provided entirely by
the use of the pragma. This pragma can be used anywhere that a pragma
is legal, including use as a configuration pragma (including use in
the @file{gnat.adc} file).

The form with a string literal specifies which validity options are to be
activated. The validity checks are first set to include only the default
reference manual settings, and then a string of letters in the string
specifies the exact set of options required. The form of this string
is exactly as described for the @code{-gnatVx} compiler switch (see the
GNAT users guide for details). For example the following two methods
can be used to enable validity checking for mode @code{in} and
@code{in out} subprogram parameters:

@smallexample
pragma Validity_Checks ("im");
gcc -c -gnatVim ...
@end smallexample

@noindent
The form ALL_CHECKS activates all standard checks (its use is equivalent
to the use of the @code{gnatva} switch.

The forms with @code{Off} and @code{On}
can be used to temporarily disable validity checks
as shown in the following example:

@smallexample
@iftex
@leftskip=0cm
@end iftex
pragma Validity_Checks ("c"); -- validity checks for copies
pragma Validity_Checks (Off); -- turn off validity checks
A := B;                       -- B will not be validity checked
pragma Validity_Checks (On);  -- turn validity checks back on
A := C;                       -- C will be validity checked
@end smallexample

@findex Volatile
@item pragma Volatile
@noindent
Syntax:

@smallexample
pragma Volatile (local_NAME)
@end smallexample

@noindent
This pragma is defined by the Ada 95 Reference Manual, and the GNAT
implementation is fully conformant with this definition. The reason it
is mentioned in this section is that a pragma of the same name was supplied
in some Ada 83 compilers, including DEC Ada 83. The Ada 95 implementation
of pragma Volatile is upwards compatible with the implementation in
Dec Ada 83.

@findex Warnings
@item pragma Warnings
@noindent
Syntax:

@smallexample
pragma Warnings (On | Off [, LOCAL_NAME]);
@end smallexample

@noindent
Normally warnings are enabled, with the output being controlled by
the command line switch. Warnings (@code{Off}) turns off generation of
warnings until a Warnings (@code{On}) is encountered or the end of the
current unit. If generation of warnings is turned off using this
pragma, then no warning messages are output, regardless of the
setting of the command line switches.

The form with a single argument is a configuration pragma.

If the @var{local_name} parameter is present, warnings are suppressed for
the specified entity. This suppression is effective from the point where
it occurs till the end of the extended scope of the variable (similar to
the scope of @code{Suppress}).

@findex Weak_External
@item pragma Weak_External
@noindent
Syntax:

@smallexample
pragma Weak_External ([Entity =>] LOCAL_NAME);
@end smallexample

@noindent
This pragma specifies that the given entity should be marked as a weak
external (one that does not have to be resolved) for the linker. For
further details, consult the GCC manual.
@end table

@node Implementation Defined Attributes
@chapter Implementation Defined Attributes
Ada 95 defines (throughout the Ada 95 reference manual,
summarized in annex K),
a set of attributes that provide useful additional functionality in all
areas of the language. These language defined attributes are implemented
in GNAT and work as described in the Ada 95 Reference Manual.

In addition, Ada 95 allows implementations to define additional
attributes whose meaning is defined by the implementation. GNAT provides
a number of these implementation-dependent attributes which can be used
to extend and enhance the functionality of the compiler. This section of
the GNAT reference manual describes these additional attributes.

Note that any program using these attributes may not be portable to
other compilers (although GNAT implements this set of attributes on all
platforms). Therefore if portability to other compilers is an important
consideration, you should minimize the use of these attributes.

@table @code
@findex Abort_Signal
@item Abort_Signal
@noindent
@code{Standard'Abort_Signal} (@code{Standard} is the only allowed
prefix) provides the entity for the special exception used to signal
task abort or asynchronous transfer of control. Normally this attribute
should only be used in the tasking runtime (it is highly peculiar, and
completely outside the normal semantics of Ada, for a user program to
intercept the abort exception).

@cindex Size of @code{Address}
@findex Address_Size
@item Address_Size
@noindent
@code{Standard'Address_Size} (@code{Standard} is the only allowed
prefix) is a static constant giving the number of bits in an
@code{Address}. It is used primarily for constructing the definition of
@code{Memory_Size} in package @code{Standard}, but may be freely used in user
programs and has the advantage of being static, while a direct
reference to System.Address'Size is non-static because Address
is a private type.

@findex Asm_Input
@item Asm_Input
@noindent
The @code{Asm_Input} attribute denotes a function that takes two
parameters.  The first is a string, the second is an expression of the
type designated by the prefix. The first (string) argument is required
to be a static expression, and is the constraint for the parameter,
(e.g.@: what kind of register is required). The second argument is the
value to be used as the input argument. The possible values for the
constant are the same as those used in the RTL, and are dependent on
the configuration file used to built the GCC back end.
@ref{Machine Code Insertions}

@findex Asm_Output
@item Asm_Output
@noindent
The @code{Asm_Output} attribute denotes a function that takes two
parameters.  The first is a string, the second is the name of a variable
of the type designated by the attribute prefix. The first (string)
argument is required to be a static expression and designates the
constraint for the parameter (e.g.@: what kind of register is
required). The second argument is the variable to be updated with the
result. The possible values for constraint are the same as those used in
the RTL, and are dependent on the configuration file used to build the
GCC back end.  If there are no output operands, then this argument may
either be omitted, or explicitly given as @code{No_Output_Operands}.
@ref{Machine Code Insertions}

@cindex OpenVMS
@findex AST_Entry
@item AST_Entry
@noindent
This attribute is implemented only in OpenVMS versions of GNAT@. Applied to
the name of an entry, it yields a value of the predefined type AST_Handler
(declared in the predefined package System, as extended by the use of
pragma Extend_System (Aux_DEC)). This value enables the given entry to
be called when an AST occurs. For further details, refer to the DEC Ada
Language Reference Manual, section 9.12a.

@findex Bit
@item Bit
@code{@var{obj}'Bit}, where @var{obj} is any object, yields the bit
offset within the storage unit (byte) that contains the first bit of
storage allocated for the object. The value of this attribute is of the
type @code{Universal_Integer}, and is always a non-negative number not
exceeding the value of @code{System.Storage_Unit}.

For an object that is a variable or a constant allocated in a register,
the value is zero. (The use of this attribute does not force the
allocation of a variable to memory).

For an object that is a formal parameter, this attribute applies
to either the matching actual parameter or to a copy of the
matching actual parameter.

For an access object the value is zero. Note that
@code{@var{obj}.all'Bit} is subject to an @code{Access_Check} for the
designated object. Similarly for a record component
@code{@var{X}.@var{C}'Bit} is subject to a discriminant check and
@code{@var{X}(@var{I}).Bit} and @code{@var{X}(@var{I1}..@var{I2})'Bit}
are subject to index checks.

This attribute is designed to be compatible with the DEC Ada 83 definition
and implementation of the @code{Bit} attribute.

@findex Bit_Position
@item Bit_Position
@noindent
@code{@var{R.C}'Bit}, where @var{R} is a record object and C is one
of the fields of the record type, yields the bit
offset within the record contains the first bit of
storage allocated for the object. The value of this attribute is of the
type @code{Universal_Integer}. The value depends only on the field
@var{C} and is independent of the alignment of
the containing record @var{R}.

@findex Code_Address
@cindex Subprogram address
@cindex Address of subprogram code
@item Code_Address
@noindent
The @code{'Address}
attribute may be applied to subprograms in Ada 95, but the
intended effect from the Ada 95 reference manual seems to be to provide
an address value which can be used to call the subprogram by means of
an address clause as in the following example:

@smallexample
procedure K is ...

procedure L;
for L'Address use K'Address;
pragma Import (Ada, L);
@end smallexample

@noindent
A call to L is then expected to result in a call to K@. In Ada 83, where
there were no access-to-subprogram values, this was a common work around
for getting the effect of an indirect call.
GNAT implements the above use of Address and the technique illustrated
by the example code works correctly.

However, for some purposes, it is useful to have the address of the start
of the generated code for the subprogram. On some architectures, this is
not necessarily the same as the Address value described above. For example,
the Address value may reference a subprogram descriptor rather than the
subprogram itself.

The @code{'Code_Address} attribute, which can only be applied to 
subprogram entities, always returns the address of the start of the 
generated code of the specified subprogram, which may or may not be
the same value as is returned by the corresponding @code{'Address}
attribute.

@cindex Big endian
@cindex Little endian
@findex Default_Bit_Order
@item Default_Bit_Order
@noindent
@code{Standard'Default_Bit_Order} (@code{Standard} is the only
permissible prefix), provides the value @code{System.Default_Bit_Order}
as a @code{Pos} value (0 for @code{High_Order_First}, 1 for
@code{Low_Order_First}). This is used to construct the definition of
@code{Default_Bit_Order} in package @code{System}.

@findex Elaborated
@item Elaborated
@noindent
The prefix of the @code{'Elaborated} attribute must be a unit name. The
value is a Boolean which indicates whether or not the given unit has been
elaborated. This attribute is primarily intended for internal use by the
generated code for dynamic elaboration checking, but it can also be used
in user programs. The value will always be True once elaboration of all
units has been completed.

@findex Elab_Body
@item Elab_Body
@noindent
This attribute can only be applied to a program unit name. It returns
the entity for the corresponding elaboration procedure for elaborating
the body of the referenced unit. This is used in the main generated
elaboration procedure by the binder and is not normally used in any
other context.  However, there may be specialized situations in which it
is useful to be able to call this elaboration procedure from Ada code,
e.g.@: if it is necessary to do selective re-elaboration to fix some
error.

@findex Elab_Spec
@item Elab_Spec
@noindent
This attribute can only be applied to a program unit name. It returns
the entity for the corresponding elaboration procedure for elaborating
the specification of the referenced unit. This is used in the main
generated elaboration procedure by the binder and is not normally used
in any other context.  However, there may be specialized situations in
which it is useful to be able to call this elaboration procedure from
Ada code, e.g.@: if it is necessary to do selective re-elaboration to fix
some error.

@cindex Ada 83 attributes
@findex Emax
@item Emax
@noindent
The @code{Emax} attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@cindex Representation of enums
@findex Enum_Rep
@item Enum_Rep
@noindent
For every enumeration subtype @var{S}, @code{@var{S}'Enum_Rep} denotes a
function with the following specification:

@smallexample
function @var{S}'Enum_Rep (Arg : @var{S}'Base)
  return Universal_Integer;
@end smallexample

@noindent
It is also allowable to apply Enum_Rep directly to an object of an
enumeration type or to a non-overloaded enumeration
literal. In this case @code{@var{S}'Enum_Rep} is equivalent to 
@code{@var{typ}'Enum_Rep(@var{S})} where @var{typ} is the type of the
enumeration literal or object.

The function returns the representation value for the given enumeration
value. This will be equal to value of the @code{Pos} attribute in the
absence of an enumeration representation clause. This is a static
attribute (i.e.@: the result is static if the argument is static).

@var{S}'Enum_Rep can also be used with integer types and objects, in which
case it simply returns the integer value. The reason for this is to allow
it to be used for (<>) discrete formal arguments in a generic unit that
can be instantiated with either enumeration types or integer types. Note
that if Enum_Rep is used on a modular type whose upper bound exceeds the
upper bound of the largest signed integer type, and the argument is a
variable, so that the universal integer calculation is done at run-time,
then the call to @code{Enum_Rep} may raise @code{Constraint_Error}.

@cindex Ada 83 attributes
@findex Epsilon
@item Epsilon
@noindent
The @code{Epsilon} attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@findex Fixed_Value
@item Fixed_Value
@noindent
For every fixed-point type @var{S}, @code{@var{S}'Fixed_Value} denotes a
function with the following specification:

@smallexample
function @var{S}'Fixed_Value (Arg : Universal_Integer)
  return @var{S};
@end smallexample

@noindent
The value returned is the fixed-point value @var{V} such that

@smallexample
@var{V} = Arg * @var{S}'Small
@end smallexample

@noindent
The effect is thus equivalent to first converting the argument to the
integer type used to represent @var{S}, and then doing an unchecked
conversion to the fixed-point type. This attribute is primarily intended
for use in implementation of the input-output functions for fixed-point
values.

@cindex Discriminants, testing for
@findex Has_Discriminants
@item Has_Discriminants
@noindent
The prefix of the @code{Has_Discriminants} attribute is a type. The result
is a Boolean value which is True if the type has discriminants, and False
otherwise. The intended use of this attribute is in conjunction with generic
definitions. If the attribute is applied to a generic private type, it
indicates whether or not the corresponding actual type has discriminants.

@findex Img
@item Img
@noindent
The @code{Img} attribute differs from @code{Image} in that it may be
applied to objects as well as types, in which case it gives the
@code{Image} for the subtype of the object. This is convenient for
debugging:

@smallexample
Put_Line ("X = " & X'Img);
@end smallexample

@noindent
has the same meaning as the more verbose:

@smallexample
Put_Line ("X = " & @var{type}'Image (X));
@end smallexample

where @var{type} is the subtype of the object X@.

@findex Integer_Value
@item Integer_Value
@noindent
For every integer type @var{S}, @code{@var{S}'Integer_Value} denotes a
function with the following specification:

@smallexample
function @var{S}'Integer_Value (Arg : Universal_Fixed)
  return @var{S};
@end smallexample

@noindent
The value returned is the integer value @var{V}, such that

@smallexample
Arg = @var{V} * @var{type}'Small
@end smallexample

@noindent
The effect is thus equivalent to first doing an unchecked convert from
the fixed-point type to its corresponding implementation type, and then
converting the result to the target integer type. This attribute is
primarily intended for use in implementation of the standard
input-output functions for fixed-point values.

@cindex Ada 83 attributes
@findex Large
@item Large
@noindent
The @code{Large} attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@findex Machine_Size
@item Machine_Size
@noindent
This attribute is identical to the @code{Object_Size} attribute. It is
provided for compatibility with the DEC Ada 83 attribute of this name.
   
@cindex Ada 83 attributes
@findex Mantissa
@item Mantissa
@noindent
The @code{Mantissa} attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@cindex Interrupt priority, maximum
@findex Max_Interrupt_Priority
@item Max_Interrupt_Priority
@noindent
@code{Standard'Max_Interrupt_Priority} (@code{Standard} is the only
permissible prefix), provides the value
@code{System.Max_Interrupt_Priority} and is intended primarily for
constructing this definition in package @code{System}.

@cindex Priority, maximum
@findex Max_Priority
@item Max_Priority
@noindent
@code{Standard'Max_Priority} (@code{Standard} is the only permissible
prefix) provides the value @code{System.Max_Priority} and is intended
primarily for constructing this definition in package @code{System}.

@cindex Alignment, maximum
@findex Maximum_Alignment
@item Maximum_Alignment
@noindent
@code{Standard'Maximum_Alignment} (@code{Standard} is the only
permissible prefix) provides the maximum useful alignment value for the
target. This is a static value that can be used to specify the alignment
for an object, guaranteeing that it is properly aligned in all
cases. This is useful when an external object is imported and its
alignment requirements are unknown.

@cindex Return values, passing mechanism
@cindex Parameters, passing mechanism
@findex Mechanism_Code
@item Mechanism_Code
@noindent
@code{@var{function}'Mechanism_Code} yields an integer code for the
mechanism used for the result of function, and
@code{@var{subprogram}'Mechanism_Code (@var{n})} yields the mechanism
used for formal parameter number @var{n} (a static integer value with 1
meaning the first parameter) of @var{subprogram}. The code returned is:

@table @asis
@item 1
by copy (value)
@item 2
by reference
@item 3
by descriptor (default descriptor class)
@item 4
by descriptor (UBS: unaligned bit string)
@item 5
by descriptor (UBSB: aligned bit string with arbitrary bounds)
@item 6
by descriptor (UBA: unaligned bit array)
@item 7
by descriptor (S: string, also scalar access type parameter)
@item 8
by descriptor (SB: string with arbitrary bounds)
@item 9
by descriptor (A: contiguous array)
@item 10
by descriptor (NCA: non-contiguous array)
@end table

@cindex OpenVMS
Values from 3-10 are only relevant to Digital OpenVMS implementations.

@cindex Zero address, passing
@findex Null_Parameter
@item Null_Parameter
@noindent
A reference @code{@var{T}'Null_Parameter} denotes an imaginary object of
type or subtype @var{T} allocated at machine address zero. The attribute
is allowed only as the default expression of a formal parameter, or as
an actual expression of a subprogram call. In either case, the
subprogram must be imported.

The identity of the object is represented by the address zero in the
argument list, independent of the passing mechanism (explicit or
default).

This capability is needed to specify that a zero address should be
passed for a record or other composite object passed by reference.
There is no way of indicating this without the @code{Null_Parameter}
attribute.

@cindex Size, used for objects
@findex Object_Size
@item Object_Size
@noindent
The size of an object is not necessarily the same as the size of the type
of an object. This is because by default object sizes are increased to be
a multiple of the alignment of the object. For example, 
@code{Natural'Size} is
31, but by default objects of type @code{Natural} will have a size of 32 bits.
Similarly, a record containing an integer and a character:

@smallexample
type Rec is record
   I : Integer;
   C : Character;
end record;
@end smallexample

@noindent
will have a size of 40 (that is @code{Rec'Size} will be 40. The 
alignment will be 4, because of the
integer field, and so the default size of record objects for this type
will be 64 (8 bytes).

The @code{@var{type}'Object_Size} attribute
has been added to GNAT to allow the
default object size of a type to be easily determined. For example,
@code{Natural'Object_Size} is 32, and
@code{Rec'Object_Size} (for the record type in the above example) will be
64. Note also that, unlike the situation with the
@code{Size} attribute as defined in the Ada RM, the 
@code{Object_Size} attribute can be specified individually
for different subtypes. For example:

@smallexample
type R is new Integer;
subtype R1 is R range 1 .. 10;
subtype R2 is R range 1 .. 10;
for R2'Object_Size use 8;
@end smallexample

@noindent
In this example, @code{R'Object_Size} and @code{R1'Object_Size} are both
32 since the default object size for a subtype is the same as the object size
for the parent subtype. This means that objects of type @code{R}
or @code{R1} will
by default be 32 bits (four bytes). But objects of type
@code{R2} will be only
8 bits (one byte), since @code{R2'Object_Size} has been set to 8.

@cindex Parameters, when passed by reference
@findex Passed_By_Reference
@item Passed_By_Reference
@noindent
@code{@var{type}'Passed_By_Reference} for any subtype @var{type} returns
a value of type @code{Boolean} value that is @code{True} if the type is
normally passed by reference and @code{False} if the type is normally
passed by copy in calls. For scalar types, the result is always @code{False}
and is static. For non-scalar types, the result is non-static.

@findex Range_Length
@item Range_Length
@noindent
@code{@var{type}'Range_Length} for any discrete type @var{type} yields
the number of values represented by the subtype (zero for a null
range). The result is static for static subtypes. @code{Range_Length}
applied to the index subtype of a one dimensional array always gives the
same result as @code{Range} applied to the array itself.

@cindex Ada 83 attributes
@findex Safe_Emax
@item Safe_Emax
@noindent
The @code{Safe_Emax} attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@cindex Ada 83 attributes
@findex Safe_Large
@item Safe_Large
@noindent
The @code{Safe_Large} attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@cindex Ada 83 attributes
@findex Safe_Large
@item Safe_Large
@noindent
The @code{Safe_Large} attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@cindex Ada 83 attributes
@findex Small
@item Small
@noindent
The @code{Small} attribute is defined in Ada 95 only for fixed-point types.
GNAT also allows this attribute to be applied to floating-point types
for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute when applied to floating-point types.

@findex Storage_Unit
@item Storage_Unit
@noindent
@code{Standard'Storage_Unit} (@code{Standard} is the only permissible
prefix) provides the value @code{System.Storage_Unit} and is intended
primarily for constructing this definition in package @code{System}.

@findex Tick
@item Tick
@noindent
@code{Standard'Tick} (@code{Standard} is the only permissible prefix)
provides the value of @code{System.Tick} and is intended primarily for
constructing this definition in package @code{System}.

@findex To_Address
@item To_Address
@noindent
The @code{System'To_Address}
(@code{System} is the only permissible prefix)
denotes a function identical to 
@code{System.Storage_Elements.To_Address} except that
it is a static attribute. This means that if its argument is
a static expression, then the result of the attribute is a
static expression. The result is that such an expression can be
used in contexts (e.g.@: preelaborable packages) which require a
static expression and where the function call could not be used
(since the function call is always non-static, even if its 
argument is static).

@findex Type_Class
@item Type_Class
@noindent
@code{@var{type}'Type_Class} for any type or subtype @var{type} yields
the value of the type class for the full type of @var{type}. If
@var{type} is a generic formal type, the value is the value for the
corresponding actual subtype.  The value of this attribute is of type
@code{System.Aux_DEC.Type_Class}, which has the following definition:

@smallexample
  type Type_Class is
    (Type_Class_Enumeration,
     Type_Class_Integer,
     Type_Class_Fixed_Point,
     Type_Class_Floating_Point,
     Type_Class_Array,
     Type_Class_Record,
     Type_Class_Access,
     Type_Class_Task,
     Type_Class_Address);
@end smallexample

@noindent
Protected types yield the value @code{Type_Class_Task}, which thus
applies to all concurrent types. This attribute is designed to
be compatible with the DEC Ada 83 attribute of the same name.

@findex UET_Address
@item UET_Address
@noindent
The @code{UET_Address} attribute can only be used for a prefix which
denotes a library package. It yields the address of the unit exception
table when zero cost exception handling is used. This attribute is
intended only for use within the GNAT implementation. See the unit
@code{Ada.Exceptions} in files @file{a-except.ads,a-except.adb}
for details on how this attribute is used in the implementation.

@cindex Named numbers, representation of
@findex Universal_Literal_String
@item Universal_Literal_String
@noindent
The prefix of @code{Universal_Literal_String} must be a named
number. The static result is the string consisting of the characters of
the number as defined in the original source. This allows the user
program to access the actual text of named numbers without intermediate
conversions and without the need to enclose the strings in quotes (which
would preclude their use as numbers). This is used internally for the
construction of values of the floating-point attributes from the file
@file{ttypef.ads}, but may also be used by user programs.

@cindex @code{Access}, unrestricted
@findex Unrestricted_Access
@item Unrestricted_Access
@noindent
The @code{Unrestricted_Access} attribute is similar to @code{Access}
except that all accessibility and aliased view checks are omitted. This
is a user-beware attribute.  It is similar to
@code{Address}, for which it is a desirable replacement where the value
desired is an access type. In other words, its effect is identical to
first applying the @code{Address} attribute and then doing an unchecked
conversion to a desired access type. In GNAT, but not necessarily in
other implementations, the use of static chains for inner level
subprograms means that @code{Unrestricted_Access} applied to a
subprogram yields a value that can be called as long as the subprogram
is in scope (normal Ada 95 accessibility rules restrict this usage).

@cindex @code{Size}, VADS compatibility
@findex VADS_Size
@item VADS_Size
@noindent
The @code{'VADS_Size} attribute is intended to make it easier to port
legacy code which relies on the semantics of @code{'Size} as implemented
by the VADS Ada 83 compiler. GNAT makes a best effort at duplicating the
same semantic interpretation. In particular, @code{'VADS_Size} applied
to a predefined or other primitive type with no Size clause yields the
Object_Size (for example, @code{Natural'Size} is 32 rather than 31 on
typical machines). In addition @code{'VADS_Size} applied to an object
gives the result that would be obtained by applying the attribute to
the corresponding type.

@cindex @code{Size}, setting for not-first subtype
@findex Value_Size
@item Value_Size
@code{@var{type}'Value_Size} is the number of bits required to represent
a value of the given subtype. It is the same as @code{@var{type}'Size},
but, unlike @code{Size}, may be set for non-first subtypes.

@findex Wchar_T_Size
@item Wchar_T_Size
@code{Standard'Wchar_T_Size} (@code{Standard} is the only permissible
prefix) provides the size in bits of the C @code{wchar_t} type 
primarily for constructing the definition of this type in 
package @code{Interfaces.C}.

@findex Word_Size
@item Word_Size
@code{Standard'Word_Size} (@code{Standard} is the only permissible
prefix) provides the value @code{System.Word_Size} and is intended
primarily for constructing this definition in package @code{System}.
@end table
@node Implementation Advice
@chapter Implementation Advice
The main text of the Ada 95 Reference Manual describes the required
behavior of all Ada 95 compilers, and the GNAT compiler conforms to
these requirements.

In addition, there are sections throughout the Ada 95
reference manual headed
by the phrase ``implementation advice''. These sections are not normative,
i.e.@: they do not specify requirements that all compilers must
follow. Rather they provide advice on generally desirable behavior. You
may wonder why they are not requirements. The most typical answer is
that they describe behavior that seems generally desirable, but cannot
be provided on all systems, or which may be undesirable on some systems.

As far as practical, GNAT follows the implementation advice sections in
the Ada 95 Reference Manual. This chapter contains a table giving the
reference manual section number, paragraph number and several keywords
for each advice.  Each entry consists of the text of the advice followed
by the GNAT interpretation of this advice. Most often, this simply says
``followed'', which means that GNAT follows the advice. However, in a
number of cases, GNAT deliberately deviates from this advice, in which
case the text describes what GNAT does and why.

@table @strong
@cindex Error detection
@item 1.1.3(20): Error Detection
@sp 1
@cartouche
If an implementation detects the use of an unsupported Specialized Needs
Annex feature at run time, it should raise @code{Program_Error} if
feasible.
@end cartouche
Not relevant. All specialized needs annex features are either supported,
or diagnosed at compile time.

@cindex Child Units
@item 1.1.3(31): Child Units
@sp 1
@cartouche
If an implementation wishes to provide implementation-defined
extensions to the functionality of a language-defined library unit, it
should normally do so by adding children to the library unit.
@end cartouche
Followed.

@cindex Bounded errors
@item 1.1.5(12): Bounded Errors
@sp 1
@cartouche
If an implementation detects a bounded error or erroneous
execution, it should raise @code{Program_Error}.
@end cartouche
Followed in all cases in which the implementation detects a bounded
error or erroneous execution. Not all such situations are detected at
runtime.

@cindex Pragmas
@item 2.8(16): Pragmas
@sp 1
@cartouche
Normally, implementation-defined pragmas should have no semantic effect
for error-free programs; that is, if the implementation-defined pragmas
are removed from a working program, the program should still be legal,
and should still have the same semantics.
@end cartouche
The following implementation defined pragmas are exceptions to this
rule:

@table @code
@item Abort_Defer
Affects semantics
@item Ada_83
Affects legality
@item Assert
Affects semantics
@item CPP_Class
Affects semantics
@item CPP_Constructor
Affects semantics
@item CPP_Virtual
Affects semantics
@item CPP_Vtable
Affects semantics
@item Debug
Affects semantics
@item Interface_Name
Affects semantics
@item Machine_Attribute
Affects semantics
@item Unimplemented_Unit
Affects legality
@item Unchecked_Union
Affects semantics
@end table

In each of the above cases, it is essential to the purpose of the pragma
that this advice not be followed. For details see the separate section
on implementation defined pragmas.

@item 2.8(17-19): Pragmas
@sp 1
@cartouche
Normally, an implementation should not define pragmas that can
make an illegal program legal, except as follows:
@end cartouche
@sp 1
@cartouche
A pragma used to complete a declaration, such as a pragma @code{Import};
@end cartouche
@sp 1
@cartouche
A pragma used to configure the environment by adding, removing, or
replacing @code{library_items}.
@end cartouche
See response to paragraph 16 of this same section.

@cindex Character Sets
@cindex Alternative Character Sets
@item 3.5.2(5): Alternative Character Sets
@sp 1
@cartouche
If an implementation supports a mode with alternative interpretations
for @code{Character} and @code{Wide_Character}, the set of graphic
characters of @code{Character} should nevertheless remain a proper
subset of the set of graphic characters of @code{Wide_Character}. Any
character set ``localizations'' should be reflected in the results of
the subprograms defined in the language-defined package
@code{Characters.Handling} (see A.3) available in such a mode. In a mode with
an alternative interpretation of @code{Character}, the implementation should
also support a corresponding change in what is a legal
@code{identifier_letter}.
@end cartouche
Not all wide character modes follow this advice, in particular the JIS
and IEC modes reflect standard usage in Japan, and in these encoding,
the upper half of the Latin-1 set is not part of the wide-character
subset, since the most significant bit is used for wide character
encoding. However, this only applies to the external forms. Internally
there is no such restriction.

@cindex Integer types
@item 3.5.4(28): Integer Types

@sp 1
@cartouche
An implementation should support @code{Long_Integer} in addition to
@code{Integer} if the target machine supports 32-bit (or longer)
arithmetic. No other named integer subtypes are recommended for package
@code{Standard}. Instead, appropriate named integer subtypes should be
provided in the library package @code{Interfaces} (see B.2).
@end cartouche
@code{Long_Integer} is supported. Other standard integer types are supported
so this advice is not fully followed. These types
are supported for convenient interface to C, and so that all hardware
types of the machine are easily available.
@item 3.5.4(29): Integer Types

@sp 1
@cartouche
An implementation for a two's complement machine should support
modular types with a binary modulus up to @code{System.Max_Int*2+2}. An
implementation should support a non-binary modules up to @code{Integer'Last}.
@end cartouche
Followed.

@cindex Enumeration values
@item 3.5.5(8): Enumeration Values
@sp 1
@cartouche
For the evaluation of a call on @code{@var{S}'Pos} for an enumeration
subtype, if the value of the operand does not correspond to the internal
code for any enumeration literal of its type (perhaps due to an
un-initialized variable), then the implementation should raise
@code{Program_Error}. This is particularly important for enumeration
types with noncontiguous internal codes specified by an
enumeration_representation_clause.
@end cartouche
Followed.

@cindex Float types
@item 3.5.7(17): Float Types
@sp 1
@cartouche
An implementation should support @code{Long_Float} in addition to
@code{Float} if the target machine supports 11 or more digits of
precision. No other named floating point subtypes are recommended for
package @code{Standard}. Instead, appropriate named floating point subtypes
should be provided in the library package @code{Interfaces} (see B.2).
@end cartouche
@code{Short_Float} and @code{Long_Long_Float} are also provided. The
former provides improved compatibility with other implementations
supporting this type. The latter corresponds to the highest precision
floating-point type supported by the hardware. On most machines, this
will be the same as @code{Long_Float}, but on some machines, it will
correspond to the IEEE extended form. The notable case is all ia32
(x86) implementations, where @code{Long_Long_Float} corresponds to
the 80-bit extended precision format supported in hardware on this
processor. Note that the 128-bit format on SPARC is not supported,
since this is a software rather than a hardware format.

@cindex Multidimensional arrays
@cindex Arrays, multidimensional
@item 3.6.2(11): Multidimensional Arrays
@sp 1
@cartouche
An implementation should normally represent multidimensional arrays in
row-major order, consistent with the notation used for multidimensional
array aggregates (see 4.3.3). However, if a pragma @code{Convention}
(@code{Fortran}, ...) applies to a multidimensional array type, then
column-major order should be used instead (see B.5, ``Interfacing with
Fortran'').
@end cartouche
Followed.

@findex Duration'Small
@item 9.6(30-31): Duration'Small
@sp 1
@cartouche
Whenever possible in an implementation, the value of @code{Duration'Small}
should be no greater than 100 microseconds.
@end cartouche
Followed. (@code{Duration'Small} = 10**(-9)).

@sp 1
@cartouche
The time base for @code{delay_relative_statements} should be monotonic;
it need not be the same time base as used for @code{Calendar.Clock}.
@end cartouche
Followed.

@item 10.2.1(12): Consistent Representation
@sp 1
@cartouche
In an implementation, a type declared in a pre-elaborated package should
have the same representation in every elaboration of a given version of
the package, whether the elaborations occur in distinct executions of
the same program, or in executions of distinct programs or partitions
that include the given version.
@end cartouche
Followed, except in the case of tagged types. Tagged types involve
implicit pointers to a local copy of a dispatch table, and these pointers
have representations which thus depend on a particular elaboration of the
package. It is not easy to see how it would be possible to follow this
advice without severely impacting efficiency of execution.

@cindex Exception information
@item 11.4.1(19): Exception Information
@sp 1
@cartouche
@code{Exception_Message} by default and @code{Exception_Information}
should produce information useful for
debugging. @code{Exception_Message} should be short, about one
line. @code{Exception_Information} can be long. @code{Exception_Message}
should not include the
@code{Exception_Name}. @code{Exception_Information} should include both
the @code{Exception_Name} and the @code{Exception_Message}.
@end cartouche
Followed. For each exception that doesn't have a specified
@code{Exception_Message}, the compiler generates one containing the location
of the raise statement. This location has the form "file:line", where
file is the short file name (without path information) and line is the line
number in the file. Note that in the case of the Zero Cost Exception
mechanism, these messages become redundant with the Exception_Information that
contains a full backtrace of the calling sequence, so they are disabled.
To disable explicitly the generation of the source location message, use the
Pragma @code{Discard_Names}.

@cindex Suppression of checks
@cindex Checks, suppression of
@item 11.5(28): Suppression of Checks
@sp 1
@cartouche
The implementation should minimize the code executed for checks that
have been suppressed.
@end cartouche
Followed.

@cindex Representation clauses
@item 13.1 (21-24): Representation Clauses
@sp 1
@cartouche
The recommended level of support for all representation items is
qualified as follows:
@end cartouche
@sp 1
@cartouche
An implementation need not support representation items containing
non-static expressions, except that an implementation should support a
representation item for a given entity if each non-static expression in
the representation item is a name that statically denotes a constant
declared before the entity.
@end cartouche
Followed. GNAT does not support non-static expressions in representation
clauses unless they are constants declared before the entity. For
example:

@smallexample
X : typ;
for X'Address use To_address (16#2000#); 
@end smallexample

@noindent
will be rejected, since the To_Address expression is non-static. Instead
write: 

@smallexample
X_Address : constant Address : = 
To_Address    ((16#2000#); 
X : typ;
for X'Address use X_Address;
@end smallexample

@sp 1
@cartouche
An implementation need not support a specification for the @code{Size}
for a given composite subtype, nor the size or storage place for an
object (including a component) of a given composite subtype, unless the
constraints on the subtype and its composite subcomponents (if any) are
all static constraints.
@end cartouche
Followed. Size Clauses are not permitted on non-static components, as
described above.

@sp 1
@cartouche
An aliased component, or a component whose type is by-reference, should
always be allocated at an addressable location.
@end cartouche
Followed.

@cindex Packed types
@item 13.2(6-8): Packed Types
@sp 1
@cartouche
If a type is packed, then the implementation should try to minimize
storage allocated to objects of the type, possibly at the expense of
speed of accessing components, subject to reasonable complexity in
addressing calculations.
@end cartouche
@sp 1
@cartouche
The recommended level of support pragma @code{Pack} is:

For a packed record type, the components should be packed as tightly as
possible subject to the Sizes of the component subtypes, and subject to
any @code{record_representation_clause} that applies to the type; the
implementation may, but need not, reorder components or cross aligned
word boundaries to improve the packing. A component whose @code{Size} is
greater than the word size may be allocated an integral number of words.
@end cartouche
Followed. Tight packing of arrays is supported for all component sizes
up to 64-bits.

@sp 1
@cartouche
An implementation should support Address clauses for imported
subprograms.
@end cartouche
Followed.
@cindex @code{Address} clauses
@item 13.3(14-19): Address Clauses

@sp 1
@cartouche
For an array @var{X}, @code{@var{X}'Address} should point at the first
component of the array, and not at the array bounds.
@end cartouche
Followed.

@sp 1
@cartouche
The recommended level of support for the @code{Address} attribute is:

@code{@var{X}'Address} should produce a useful result if @var{X} is an
object that is aliased or of a by-reference type, or is an entity whose
@code{Address} has been specified.
@end cartouche
Followed.  A valid address will be produced even if none of those
conditions have been met.  If necessary, the object is forced into
memory to ensure the address is valid.

@sp 1
@cartouche
An implementation should support @code{Address} clauses for imported
subprograms.
@end cartouche
Followed.
            
@sp 1
@cartouche
Objects (including subcomponents) that are aliased or of a by-reference
type should be allocated on storage element boundaries.
@end cartouche
Followed.
          
@sp 1
@cartouche
If the @code{Address} of an object is specified, or it is imported or exported,
then the implementation should not perform optimizations based on
assumptions of no aliases.
@end cartouche
Followed.

@cindex @code{Alignment} clauses
@item 13.3(29-35): Alignment Clauses
@sp 1
@cartouche
The recommended level of support for the @code{Alignment} attribute for
subtypes is:

An implementation should support specified Alignments that are factors
and multiples of the number of storage elements per word, subject to the
following:
@end cartouche
Followed.
          
@sp 1
@cartouche
An implementation need not support specified @code{Alignment}s for
combinations of @code{Size}s and @code{Alignment}s that cannot be easily
loaded and stored by available machine instructions.
@end cartouche
Followed.
            
@sp 1
@cartouche
An implementation need not support specified @code{Alignment}s that are
greater than the maximum @code{Alignment} the implementation ever returns by
default.
@end cartouche
Followed.

@sp 1
@cartouche
The recommended level of support for the @code{Alignment} attribute for
objects is:

Same as above, for subtypes, but in addition:
@end cartouche
Followed.
           
@sp 1
@cartouche
For stand-alone library-level objects of statically constrained
subtypes, the implementation should support all @code{Alignment}s
supported by the target linker. For example, page alignment is likely to
be supported for such objects, but not for subtypes.
@end cartouche
Followed.

@cindex @code{Size} clauses
@item 13.3(42-43): Size Clauses
@sp 1
@cartouche
The recommended level of support for the @code{Size} attribute of
objects is:

A @code{Size} clause should be supported for an object if the specified
@code{Size} is at least as large as its subtype's @code{Size}, and
corresponds to a size in storage elements that is a multiple of the
object's @code{Alignment} (if the @code{Alignment} is nonzero).
@end cartouche
Followed.

@item 13.3(50-56): Size Clauses
@sp 1
@cartouche
If the @code{Size} of a subtype is specified, and allows for efficient
independent addressability (see 9.10) on the target architecture, then
the @code{Size} of the following objects of the subtype should equal the
@code{Size} of the subtype:

Aliased objects (including components).
@end cartouche
Followed.

@sp 1
@cartouche
@code{Size} clause on a composite subtype should not affect the
internal layout of components.
@end cartouche
Followed.

@sp 1
@cartouche
The recommended level of support for the @code{Size} attribute of subtypes is:
@end cartouche
@sp 1
@cartouche
The @code{Size} (if not specified) of a static discrete or fixed point
subtype should be the number of bits needed to represent each value
belonging to the subtype using an unbiased representation, leaving space
for a sign bit only if the subtype contains negative values. If such a
subtype is a first subtype, then an implementation should support a
specified @code{Size} for it that reflects this representation.
@end cartouche
Followed.

@sp 1
@cartouche
For a subtype implemented with levels of indirection, the @code{Size}
should include the size of the pointers, but not the size of what they
point at.
@end cartouche
Followed.

@cindex @code{Component_Size} clauses
@item 13.3(71-73): Component Size Clauses
@sp 1
@cartouche
The recommended level of support for the @code{Component_Size}
attribute is:
@end cartouche
@sp 1
@cartouche
An implementation need not support specified @code{Component_Sizes} that are
less than the @code{Size} of the component subtype.
@end cartouche
Followed.

@sp 1
@cartouche
An implementation should support specified @code{Component_Size}s that
are factors and multiples of the word size. For such
@code{Component_Size}s, the array should contain no gaps between
components. For other @code{Component_Size}s (if supported), the array
should contain no gaps between components when packing is also
specified; the implementation should forbid this combination in cases
where it cannot support a no-gaps representation.
@end cartouche
Followed.

@cindex Enumeration representation clauses
@cindex Representation clauses, enumeration
@item 13.4(9-10): Enumeration Representation Clauses
@sp 1
@cartouche
The recommended level of support for enumeration representation clauses
is:

An implementation need not support enumeration representation clauses
for boolean types, but should at minimum support the internal codes in
the range @code{System.Min_Int.System.Max_Int}.
@end cartouche
Followed.

@cindex Record representation clauses
@cindex Representation clauses, records
@item 13.5.1(17-22): Record Representation Clauses
@sp 1
@cartouche
The recommended level of support for
@*@code{record_representation_clauses} is:

An implementation should support storage places that can be extracted
with a load, mask, shift sequence of machine code, and set with a load,
shift, mask, store sequence, given the available machine instructions
and run-time model.
@end cartouche
Followed.

@sp 1
@cartouche
A storage place should be supported if its size is equal to the
@code{Size} of the component subtype, and it starts and ends on a
boundary that obeys the @code{Alignment} of the component subtype.
@end cartouche
Followed.
           
@sp 1
@cartouche
If the default bit ordering applies to the declaration of a given type,
then for a component whose subtype's @code{Size} is less than the word
size, any storage place that does not cross an aligned word boundary
should be supported.
@end cartouche
Followed.

@sp 1
@cartouche
An implementation may reserve a storage place for the tag field of a
tagged type, and disallow other components from overlapping that place.
@end cartouche
Followed. The storage place for the tag field is the beginning of the tagged
record, and its size is Address'Size. GNAT will reject an explicit component
clause for the tag field.

@sp 1
@cartouche
An implementation need not support a @code{component_clause} for a
component of an extension part if the storage place is not after the
storage places of all components of the parent type, whether or not
those storage places had been specified.
@end cartouche
Followed. The above advice on record representation clauses is followed,
and all mentioned features are implemented.

@cindex Storage place attributes
@item 13.5.2(5): Storage Place Attributes
@sp 1
@cartouche
If a component is represented using some form of pointer (such as an
offset) to the actual data of the component, and this data is contiguous
with the rest of the object, then the storage place attributes should
reflect the place of the actual data, not the pointer. If a component is
allocated discontinuously from the rest of the object, then a warning
should be generated upon reference to one of its storage place
attributes.
@end cartouche
Followed. There are no such components in GNAT@.

@cindex Bit ordering
@item 13.5.3(7-8): Bit Ordering
@sp 1
@cartouche
The recommended level of support for the non-default bit ordering is:
@end cartouche
@sp 1
@cartouche
If @code{Word_Size} = @code{Storage_Unit}, then the implementation
should support the non-default bit ordering in addition to the default
bit ordering.
@end cartouche
Followed. Word size does not equal storage size in this implementation.
Thus non-default bit ordering is not supported.

@cindex @code{Address}, as private type
@item 13.7(37): Address as Private
@sp 1
@cartouche
@code{Address} should be of a private type.
@end cartouche
Followed.

@cindex Operations, on @code{Address}
@cindex @code{Address}, operations of
@item 13.7.1(16): Address Operations
@sp 1
@cartouche
Operations in @code{System} and its children should reflect the target
environment semantics as closely as is reasonable. For example, on most
machines, it makes sense for address arithmetic to ``wrap around.''
Operations that do not make sense should raise @code{Program_Error}.
@end cartouche
Followed. Address arithmetic is modular arithmetic that wraps around. No
operation raises @code{Program_Error}, since all operations make sense.

@cindex Unchecked conversion
@item 13.9(14-17): Unchecked Conversion
@sp 1
@cartouche
The @code{Size} of an array object should not include its bounds; hence,
the bounds should not be part of the converted data.
@end cartouche
Followed. 

@sp 1
@cartouche
The implementation should not generate unnecessary run-time checks to
ensure that the representation of @var{S} is a representation of the
target type. It should take advantage of the permission to return by
reference when possible. Restrictions on unchecked conversions should be
avoided unless required by the target environment.
@end cartouche
Followed. There are no restrictions on unchecked conversion. A warning is
generated if the source and target types do not have the same size since
the semantics in this case may be target dependent.

@sp 1
@cartouche
The recommended level of support for unchecked conversions is:
@end cartouche
@sp 1
@cartouche
Unchecked conversions should be supported and should be reversible in
the cases where this clause defines the result. To enable meaningful use
of unchecked conversion, a contiguous representation should be used for
elementary subtypes, for statically constrained array subtypes whose
component subtype is one of the subtypes described in this paragraph,
and for record subtypes without discriminants whose component subtypes
are described in this paragraph.
@end cartouche
Followed. 

@cindex Heap usage, implicit
@item 13.11(23-25): Implicit Heap Usage
@sp 1
@cartouche
An implementation should document any cases in which it dynamically
allocates heap storage for a purpose other than the evaluation of an
allocator.
@end cartouche
Followed, the only other points at which heap storage is dynamically
allocated are as follows:

@itemize @bullet
@item
At initial elaboration time, to allocate dynamically sized global
objects.

@item
To allocate space for a task when a task is created.

@item
To extend the secondary stack dynamically when needed. The secondary
stack is used for returning variable length results.
@end itemize

@sp 1
@cartouche
A default (implementation-provided) storage pool for an access-to-
constant type should not have overhead to support de-allocation of
individual objects.
@end cartouche
Followed. 

@sp 1
@cartouche
A storage pool for an anonymous access type should be created at the
point of an allocator for the type, and be reclaimed when the designated
object becomes inaccessible.
@end cartouche
Followed. 

@cindex Unchecked deallocation
@item 13.11.2(17): Unchecked De-allocation
@sp 1
@cartouche
For a standard storage pool, @code{Free} should actually reclaim the
storage.
@end cartouche
Followed. 

@cindex Stream oriented attributes
@item 13.13.2(17): Stream Oriented Attributes
@sp 1
@cartouche
If a stream element is the same size as a storage element, then the
normal in-memory representation should be used by @code{Read} and
@code{Write} for scalar objects. Otherwise, @code{Read} and @code{Write}
should use the smallest number of stream elements needed to represent
all values in the base range of the scalar type.
@end cartouche
Followed. In particular, the interpretation chosen is that of AI-195,
which specifies that the size to be used is that of the first subtype.

@item A.1(52): Implementation Advice
@sp 1
@cartouche
If an implementation provides additional named predefined integer types,
then the names should end with @samp{Integer} as in
@samp{Long_Integer}. If an implementation provides additional named
predefined floating point types, then the names should end with
@samp{Float} as in @samp{Long_Float}.
@end cartouche
Followed. 

@findex Ada.Characters.Handling
@item A.3.2(49): @code{Ada.Characters.Handling}
@sp 1
@cartouche
If an implementation provides a localized definition of @code{Character}
or @code{Wide_Character}, then the effects of the subprograms in
@code{Characters.Handling} should reflect the localizations. See also
3.5.2.
@end cartouche
Followed. GNAT provides no such localized definitions. 

@cindex Bounded-length strings
@item A.4.4(106): Bounded-Length String Handling
@sp 1
@cartouche
Bounded string objects should not be implemented by implicit pointers
and dynamic allocation.
@end cartouche
Followed. No implicit pointers or dynamic allocation are used. 

@cindex Random number generation
@item A.5.2(46-47): Random Number Generation
@sp 1
@cartouche
Any storage associated with an object of type @code{Generator} should be
reclaimed on exit from the scope of the object.
@end cartouche
Followed. 

@sp 1
@cartouche
If the generator period is sufficiently long in relation to the number
of distinct initiator values, then each possible value of
@code{Initiator} passed to @code{Reset} should initiate a sequence of
random numbers that does not, in a practical sense, overlap the sequence
initiated by any other value. If this is not possible, then the mapping
between initiator values and generator states should be a rapidly
varying function of the initiator value.
@end cartouche
Followed. The generator period is sufficiently long for the first
condition here to hold true. 

@findex Get_Immediate
@item A.10.7(23): @code{Get_Immediate}
@sp 1
@cartouche
The @code{Get_Immediate} procedures should be implemented with
unbuffered input. For a device such as a keyboard, input should be
@dfn{available} if a key has already been typed, whereas for a disk
file, input should always be available except at end of file. For a file
associated with a keyboard-like device, any line-editing features of the
underlying operating system should be disabled during the execution of
@code{Get_Immediate}.
@end cartouche
Followed.

@findex Export
@item B.1(39-41): Pragma @code{Export}
@sp 1
@cartouche
If an implementation supports pragma @code{Export} to a given language,
then it should also allow the main subprogram to be written in that
language. It should support some mechanism for invoking the elaboration
of the Ada library units included in the system, and for invoking the
finalization of the environment task. On typical systems, the
recommended mechanism is to provide two subprograms whose link names are
@code{adainit} and @code{adafinal}. @code{adainit} should contain the
elaboration code for library units. @code{adafinal} should contain the
finalization code. These subprograms should have no effect the second
and subsequent time they are called.
@end cartouche
Followed. 

@sp 1
@cartouche
Automatic elaboration of pre-elaborated packages should be
provided when pragma Export is supported.
@end cartouche
Followed when the main program is in Ada. If the main program is in a
foreign language, then
@code{adainit} must be called to elaborate pre-elaborated
packages.

@sp 1
@cartouche
For each supported convention @var{L} other than @code{Intrinsic}, an
implementation should support @code{Import} and @code{Export} pragmas
for objects of @var{L}-compatible types and for subprograms, and pragma
@code{Convention} for @var{L}-eligible types and for subprograms,
presuming the other language has corresponding features. Pragma
@code{Convention} need not be supported for scalar types.
@end cartouche
Followed. 

@cindex Package @code{Interfaces}
@findex Interfaces
@item B.2(12-13): Package @code{Interfaces}
@sp 1
@cartouche
For each implementation-defined convention identifier, there should be a
child package of package Interfaces with the corresponding name. This
package should contain any declarations that would be useful for
interfacing to the language (implementation) represented by the
convention. Any declarations useful for interfacing to any language on
the given hardware architecture should be provided directly in
@code{Interfaces}.
@end cartouche
Followed. An additional package not defined
in the Ada 95 Reference Manual is @code{Interfaces.CPP}, used
for interfacing to C++.

@sp 1
@cartouche
An implementation supporting an interface to C, COBOL, or Fortran should
provide the corresponding package or packages described in the following
clauses.
@end cartouche
Followed. GNAT provides all the packages described in this section. 

@cindex C, interfacing with
@item B.3(63-71): Interfacing with C
@sp 1
@cartouche
An implementation should support the following interface correspondences
between Ada and C@.
@end cartouche
Followed.

@sp 1
@cartouche
An Ada procedure corresponds to a void-returning C function.
@end cartouche
Followed. 

@sp 1
@cartouche
An Ada function corresponds to a non-void C function.
@end cartouche
Followed. 

@sp 1
@cartouche
An Ada @code{in} scalar parameter is passed as a scalar argument to a C
function.
@end cartouche
Followed. 

@sp 1
@cartouche
An Ada @code{in} parameter of an access-to-object type with designated
type @var{T} is passed as a @code{@var{t}*} argument to a C function,
where @var{t} is the C type corresponding to the Ada type @var{T}.
@end cartouche
Followed. 

@sp 1
@cartouche
An Ada access @var{T} parameter, or an Ada @code{out} or @code{in out}
parameter of an elementary type @var{T}, is passed as a @code{@var{t}*}
argument to a C function, where @var{t} is the C type corresponding to
the Ada type @var{T}. In the case of an elementary @code{out} or
@code{in out} parameter, a pointer to a temporary copy is used to
preserve by-copy semantics.
@end cartouche
Followed. 

@sp 1
@cartouche
An Ada parameter of a record type @var{T}, of any mode, is passed as a
@code{@var{t}*} argument to a C function, where @var{t} is the C
structure corresponding to the Ada type @var{T}.
@end cartouche
Followed. This convention may be overridden by the use of the C_Pass_By_Copy
pragma, or Convention, or by explicitly specifying the mechanism for a given
call using an extended import or export pragma.

@sp 1
@cartouche
An Ada parameter of an array type with component type @var{T}, of any
mode, is passed as a @code{@var{t}*} argument to a C function, where
@var{t} is the C type corresponding to the Ada type @var{T}.
@end cartouche
Followed. 

@sp 1
@cartouche
An Ada parameter of an access-to-subprogram type is passed as a pointer
to a C function whose prototype corresponds to the designated
subprogram's specification.
@end cartouche
Followed. 

@cindex COBOL, interfacing with
@item B.4(95-98): Interfacing with COBOL
@sp 1
@cartouche
An Ada implementation should support the following interface
correspondences between Ada and COBOL@.
@end cartouche
Followed.

@sp 1
@cartouche
An Ada access @var{T} parameter is passed as a ``BY REFERENCE'' data item of
the COBOL type corresponding to @var{T}.
@end cartouche
Followed. 

@sp 1
@cartouche
An Ada in scalar parameter is passed as a ``BY CONTENT'' data item of
the corresponding COBOL type.
@end cartouche
Followed. 

@sp 1
@cartouche
Any other Ada parameter is passed as a ``BY REFERENCE'' data item of the
COBOL type corresponding to the Ada parameter type; for scalars, a local
copy is used if necessary to ensure by-copy semantics.
@end cartouche
Followed. 

@cindex Fortran, interfacing with
@item B.5(22-26): Interfacing with Fortran
@sp 1
@cartouche
An Ada implementation should support the following interface
correspondences between Ada and Fortran:
@end cartouche
Followed.

@sp 1
@cartouche
An Ada procedure corresponds to a Fortran subroutine.
@end cartouche
Followed.

@sp 1
@cartouche
An Ada function corresponds to a Fortran function.
@end cartouche
Followed.

@sp 1
@cartouche
An Ada parameter of an elementary, array, or record type @var{T} is
passed as a @var{T} argument to a Fortran procedure, where @var{T} is
the Fortran type corresponding to the Ada type @var{T}, and where the
INTENT attribute of the corresponding dummy argument matches the Ada
formal parameter mode; the Fortran implementation's parameter passing
conventions are used. For elementary types, a local copy is used if
necessary to ensure by-copy semantics.
@end cartouche
Followed.

@sp 1
@cartouche
An Ada parameter of an access-to-subprogram type is passed as a
reference to a Fortran procedure whose interface corresponds to the
designated subprogram's specification.
@end cartouche
Followed.

@cindex Machine operations
@item C.1(3-5): Access to Machine Operations
@sp 1
@cartouche
The machine code or intrinsic support should allow access to all
operations normally available to assembly language programmers for the
target environment, including privileged instructions, if any.
@end cartouche
Followed.

@sp 1
@cartouche
The interfacing pragmas (see Annex B) should support interface to
assembler; the default assembler should be associated with the
convention identifier @code{Assembler}.
@end cartouche
Followed. 

@sp 1
@cartouche
If an entity is exported to assembly language, then the implementation
should allocate it at an addressable location, and should ensure that it
is retained by the linking process, even if not otherwise referenced
from the Ada code. The implementation should assume that any call to a
machine code or assembler subprogram is allowed to read or update every
object that is specified as exported.
@end cartouche
Followed. 

@item C.1(10-16): Access to Machine Operations
@sp 1
@cartouche
The implementation should ensure that little or no overhead is
associated with calling intrinsic and machine-code subprograms.
@end cartouche
Followed for both intrinsics and machine-code subprograms.

@sp 1
@cartouche
It is recommended that intrinsic subprograms be provided for convenient
access to any machine operations that provide special capabilities or
efficiency and that are not otherwise available through the language
constructs.
@end cartouche
Followed. A full set of machine operation intrinsic subprograms is provided.

@sp 1
@cartouche
Atomic read-modify-write operations -- e.g.@:, test and set, compare and
swap, decrement and test, enqueue/dequeue.
@end cartouche
Followed on any target supporting such operations.

@sp 1
@cartouche
Standard numeric functions -- e.g.@:, sin, log.
@end cartouche
Followed on any target supporting such operations.

@sp 1
@cartouche
String manipulation operations -- e.g.@:, translate and test.
@end cartouche
Followed on any target supporting such operations.

@sp 1
@cartouche
Vector operations -- e.g.@:, compare vector against thresholds.
@end cartouche
Followed on any target supporting such operations.

@sp 1
@cartouche
Direct operations on I/O ports.
@end cartouche
Followed on any target supporting such operations.

@cindex Interrupt support
@item C.3(28): Interrupt Support
@sp 1
@cartouche
If the @code{Ceiling_Locking} policy is not in effect, the
implementation should provide means for the application to specify which
interrupts are to be blocked during protected actions, if the underlying
system allows for a finer-grain control of interrupt blocking.
@end cartouche
Followed. The underlying system does not allow for finer-grain control
of interrupt blocking.

@cindex Protected procedure handlers
@item C.3.1(20-21): Protected Procedure Handlers
@sp 1
@cartouche
Whenever possible, the implementation should allow interrupt handlers to
be called directly by the hardware.
@end cartouche
@c SGI info:
@ignore
This is never possible under IRIX, so this is followed by default. 
@end ignore
Followed on any target where the underlying operating system permits
such direct calls.

@sp 1
@cartouche
Whenever practical, violations of any
implementation-defined restrictions should be detected before run time.
@end cartouche
Followed. Compile time warnings are given when possible. 

@cindex Package @code{Interrupts}
@findex Interrupts
@item C.3.2(25): Package @code{Interrupts}

@sp 1
@cartouche
If implementation-defined forms of interrupt handler procedures are
supported, such as protected procedures with parameters, then for each
such form of a handler, a type analogous to @code{Parameterless_Handler}
should be specified in a child package of @code{Interrupts}, with the
same operations as in the predefined package Interrupts.
@end cartouche
Followed. 

@cindex Pre-elaboration requirements
@item C.4(14): Pre-elaboration Requirements
@sp 1
@cartouche
It is recommended that pre-elaborated packages be implemented in such a
way that there should be little or no code executed at run time for the
elaboration of entities not already covered by the Implementation
Requirements.
@end cartouche
Followed. Executable code is generated in some cases, e.g.@: loops
to initialize large arrays.

@item C.5(8): Pragma @code{Discard_Names}

@sp 1
@cartouche
If the pragma applies to an entity, then the implementation should
reduce the amount of storage used for storing names associated with that
entity.
@end cartouche
Followed.

@cindex Package @code{Task_Attributes}
@findex Task_Attributes
@item C.7.2(30): The Package Task_Attributes
@sp 1
@cartouche
Some implementations are targeted to domains in which memory use at run
time must be completely deterministic. For such implementations, it is
recommended that the storage for task attributes will be pre-allocated
statically and not from the heap. This can be accomplished by either
placing restrictions on the number and the size of the task's
attributes, or by using the pre-allocated storage for the first @var{N}
attribute objects, and the heap for the others. In the latter case,
@var{N} should be documented.
@end cartouche
Not followed. This implementation is not targeted to such a domain. 

@cindex Locking Policies
@item D.3(17): Locking Policies

@sp 1
@cartouche
The implementation should use names that end with @samp{_Locking} for
locking policies defined by the implementation.
@end cartouche
Followed. A single implementation-defined locking policy is defined,
whose name (@code{Inheritance_Locking}) follows this suggestion.

@cindex Entry queuing policies
@item D.4(16): Entry Queuing Policies
@sp 1
@cartouche
Names that end with @samp{_Queuing} should be used
for all implementation-defined queuing policies.
@end cartouche
Followed. No such implementation-defined queueing policies exist. 

@cindex Preemptive abort
@item D.6(9-10): Preemptive Abort
@sp 1
@cartouche
Even though the @code{abort_statement} is included in the list of
potentially blocking operations (see 9.5.1), it is recommended that this
statement be implemented in a way that never requires the task executing
the @code{abort_statement} to block.
@end cartouche
Followed. 

@sp 1
@cartouche
On a multi-processor, the delay associated with aborting a task on
another processor should be bounded; the implementation should use
periodic polling, if necessary, to achieve this.
@end cartouche
Followed. 

@cindex Tasking restrictions
@item D.7(21): Tasking Restrictions
@sp 1
@cartouche
When feasible, the implementation should take advantage of the specified
restrictions to produce a more efficient implementation.
@end cartouche
GNAT currently takes advantage of these restrictions by providing an optimized
run time when the Ravenscar profile and the GNAT restricted run time set
of restrictions are specified. See pragma @code{Ravenscar} and pragma
@code{Restricted_Run_Time} for more details.

@cindex Time, monotonic
@item D.8(47-49): Monotonic Time
@sp 1
@cartouche
When appropriate, implementations should provide configuration
mechanisms to change the value of @code{Tick}.
@end cartouche
Such configuration mechanisms are not appropriate to this implementation
and are thus not supported.

@sp 1
@cartouche
It is recommended that @code{Calendar.Clock} and @code{Real_Time.Clock}
be implemented as transformations of the same time base.
@end cartouche
Followed. 

@sp 1
@cartouche
It is recommended that the @dfn{best} time base which exists in
the underlying system be available to the application through
@code{Clock}. @dfn{Best} may mean highest accuracy or largest range.
@end cartouche
Followed. 

@cindex Partition communication subsystem
@cindex PCS
@item E.5(28-29): Partition Communication Subsystem
@sp 1
@cartouche
Whenever possible, the PCS on the called partition should allow for
multiple tasks to call the RPC-receiver with different messages and
should allow them to block until the corresponding subprogram body
returns.
@end cartouche
Followed by GLADE, a separately supplied PCS that can be used with
GNAT. For information on GLADE, contact Ada Core Technologies.

@sp 1
@cartouche
The @code{Write} operation on a stream of type @code{Params_Stream_Type}
should raise @code{Storage_Error} if it runs out of space trying to
write the @code{Item} into the stream.
@end cartouche
Followed by GLADE, a separately supplied PCS that can be used with
GNAT@. For information on GLADE, contact Ada Core Technologies.

@cindex COBOL support
@item F(7): COBOL Support
@sp 1
@cartouche
If COBOL (respectively, C) is widely supported in the target
environment, implementations supporting the Information Systems Annex
should provide the child package @code{Interfaces.COBOL} (respectively,
@code{Interfaces.C}) specified in Annex B and should support a
@code{convention_identifier} of COBOL (respectively, C) in the interfacing
pragmas (see Annex B), thus allowing Ada programs to interface with
programs written in that language.
@end cartouche
Followed. 

@cindex Decimal radix support
@item F.1(2): Decimal Radix Support
@sp 1
@cartouche
Packed decimal should be used as the internal representation for objects
of subtype @var{S} when @var{S}'Machine_Radix = 10.
@end cartouche
Not followed. GNAT ignores @var{S}'Machine_Radix and always uses binary
representations.

@cindex Numerics
@item G: Numerics
@sp 2
@cartouche
If Fortran (respectively, C) is widely supported in the target
environment, implementations supporting the Numerics Annex
should provide the child package @code{Interfaces.Fortran} (respectively,
@code{Interfaces.C}) specified in Annex B and should support a
@code{convention_identifier} of Fortran (respectively, C) in the interfacing
pragmas (see Annex B), thus allowing Ada programs to interface with
programs written in that language.
@end cartouche
Followed.

@cindex Complex types
@item G.1.1(56-58): Complex Types
@sp 2
@cartouche
Because the usual mathematical meaning of multiplication of a complex
operand and a real operand is that of the scaling of both components of
the former by the latter, an implementation should not perform this
operation by first promoting the real operand to complex type and then
performing a full complex multiplication. In systems that, in the
future, support an Ada binding to IEC 559:1989, the latter technique
will not generate the required result when one of the components of the
complex operand is infinite. (Explicit multiplication of the infinite
component by the zero component obtained during promotion yields a NaN
that propagates into the final result.) Analogous advice applies in the
case of multiplication of a complex operand and a pure-imaginary
operand, and in the case of division of a complex operand by a real or
pure-imaginary operand.
@end cartouche
Not followed. 

@sp 1
@cartouche
Similarly, because the usual mathematical meaning of addition of a
complex operand and a real operand is that the imaginary operand remains
unchanged, an implementation should not perform this operation by first
promoting the real operand to complex type and then performing a full
complex addition. In implementations in which the @code{Signed_Zeros}
attribute of the component type is @code{True} (and which therefore
conform to IEC 559:1989 in regard to the handling of the sign of zero in
predefined arithmetic operations), the latter technique will not
generate the required result when the imaginary component of the complex
operand is a negatively signed zero. (Explicit addition of the negative
zero to the zero obtained during promotion yields a positive zero.)
Analogous advice applies in the case of addition of a complex operand
and a pure-imaginary operand, and in the case of subtraction of a
complex operand and a real or pure-imaginary operand.
@end cartouche
Not followed. 

@sp 1
@cartouche
Implementations in which @code{Real'Signed_Zeros} is @code{True} should
attempt to provide a rational treatment of the signs of zero results and
result components. As one example, the result of the @code{Argument}
function should have the sign of the imaginary component of the
parameter @code{X} when the point represented by that parameter lies on
the positive real axis; as another, the sign of the imaginary component
of the @code{Compose_From_Polar} function should be the same as
(respectively, the opposite of) that of the @code{Argument} parameter when that
parameter has a value of zero and the @code{Modulus} parameter has a
nonnegative (respectively, negative) value.
@end cartouche
Followed. 

@cindex Complex elementary functions
@item G.1.2(49): Complex Elementary Functions
@sp 1
@cartouche
Implementations in which @code{Complex_Types.Real'Signed_Zeros} is
@code{True} should attempt to provide a rational treatment of the signs
of zero results and result components. For example, many of the complex
elementary functions have components that are odd functions of one of
the parameter components; in these cases, the result component should
have the sign of the parameter component at the origin. Other complex
elementary functions have zero components whose sign is opposite that of
a parameter component at the origin, or is always positive or always
negative.
@end cartouche
Followed. 

@cindex Accuracy requirements
@item G.2.4(19): Accuracy Requirements
@sp 1
@cartouche
The versions of the forward trigonometric functions without a
@code{Cycle} parameter should not be implemented by calling the
corresponding version with a @code{Cycle} parameter of
@code{2.0*Numerics.Pi}, since this will not provide the required
accuracy in some portions of the domain. For the same reason, the
version of @code{Log} without a @code{Base} parameter should not be
implemented by calling the corresponding version with a @code{Base}
parameter of @code{Numerics.e}.
@end cartouche
Followed. 

@cindex Complex arithmetic accuracy
@cindex Accuracy, complex arithmetic
@item G.2.6(15): Complex Arithmetic Accuracy

@sp 1
@cartouche
The version of the @code{Compose_From_Polar} function without a
@code{Cycle} parameter should not be implemented by calling the
corresponding version with a @code{Cycle} parameter of
@code{2.0*Numerics.Pi}, since this will not provide the required
accuracy in some portions of the domain.
@end cartouche
Followed.

@end table
@node Implementation Defined Characteristics
@chapter Implementation Defined Characteristics
In addition to the implementation dependent pragmas and attributes, and
the implementation advice, there are a number of other features of Ada
95 that are potentially implementation dependent. These are mentioned
throughout the Ada 95 Reference Manual, and are summarized in annex M@.

A requirement for conforming Ada compilers is that they provide
documentation describing how the implementation deals with each of these
issues. In this chapter, you will find each point in annex M listed
followed by a description in italic font of how GNAT
@c SGI info:
@ignore
in the ProDev Ada
implementation on IRIX 5.3 operating system or greater 
@end ignore
handles the implementation dependence.

You can use this chapter as a guide to minimizing implementation
dependent features in your programs if portability to other compilers
and other operating systems is an important consideration.  The numbers
in each section below correspond to the paragraph number in the Ada 95
Reference Manual.

@sp 1
@cartouche
@noindent
@strong{2}.  Whether or not each recommendation given in Implementation
Advice is followed. See 1.1.2(37).
@end cartouche
@noindent
@xref{Implementation Advice}.

@sp 1
@cartouche
@noindent
@strong{3}.  Capacity limitations of the implementation. See 1.1.3(3).
@end cartouche
@noindent
The complexity of programs that can be processed is limited only by the
total amount of available virtual memory, and disk space for the
generated object files.

@sp 1
@cartouche
@noindent
@strong{4}.  Variations from the standard that are impractical to avoid
given the implementation's execution environment. See 1.1.3(6).
@end cartouche
@noindent
There are no variations from the standard.

@sp 1
@cartouche
@noindent
@strong{5}.  Which @code{code_statement}s cause external
interactions. See 1.1.3(10).
@end cartouche
@noindent
Any @code{code_statement} can potentially cause external interactions.

@sp 1
@cartouche
@noindent
@strong{6}.  The coded representation for the text of an Ada
program. See 2.1(4).
@end cartouche
@noindent
See separate section on source representation.

@sp 1
@cartouche
@noindent
@strong{7}.  The control functions allowed in comments. See 2.1(14).
@end cartouche
@noindent
See separate section on source representation.

@sp 1
@cartouche
@noindent
@strong{8}.  The representation for an end of line. See 2.2(2).
@end cartouche
@noindent
See separate section on source representation.

@sp 1
@cartouche
@noindent
@strong{9}.  Maximum supported line length and lexical element
length. See 2.2(15).
@end cartouche
@noindent
The maximum line length is 255 characters an the maximum length of a
lexical element is also 255 characters.

@sp 1
@cartouche
@noindent
@strong{10}.  Implementation defined pragmas. See 2.8(14).
@end cartouche
@noindent

@xref{Implementation Defined Pragmas}.

@sp 1
@cartouche
@noindent
@strong{11}.  Effect of pragma @code{Optimize}. See 2.8(27).
@end cartouche
@noindent
Pragma @code{Optimize}, if given with a @code{Time} or @code{Space}
parameter, checks that the optimization flag is set, and aborts if it is
not.

@sp 1
@cartouche
@noindent
@strong{12}.  The sequence of characters of the value returned by
@code{@var{S}'Image} when some of the graphic characters of
@code{@var{S}'Wide_Image} are not defined in @code{Character}. See
3.5(37).
@end cartouche
@noindent
The sequence of characters is as defined by the wide character encoding
method used for the source. See section on source representation for
further details.

@sp 1
@cartouche
@noindent
@strong{13}.  The predefined integer types declared in
@code{Standard}. See 3.5.4(25).
@end cartouche
@noindent
@table @code
@item Short_Short_Integer
8 bit signed
@item Short_Integer
(Short) 16 bit signed
@item Integer
32 bit signed
@item Long_Integer
64 bit signed (Alpha OpenVMS only)
32 bit signed (all other targets)
@item Long_Long_Integer
64 bit signed
@end table

@sp 1
@cartouche
@noindent
@strong{14}.  Any nonstandard integer types and the operators defined
for them. See 3.5.4(26).
@end cartouche
@noindent
There are no nonstandard integer types. 

@sp 1
@cartouche
@noindent
@strong{15}.  Any nonstandard real types and the operators defined for
them. See 3.5.6(8).
@end cartouche
@noindent
There are no nonstandard real types. 

@sp 1
@cartouche
@noindent
@strong{16}.  What combinations of requested decimal precision and range
are supported for floating point types. See 3.5.7(7).
@end cartouche
@noindent
The precision and range is as defined by the IEEE standard. 

@sp 1
@cartouche
@noindent
@strong{17}.  The predefined floating point types declared in
@code{Standard}. See 3.5.7(16).
@end cartouche
@noindent
@table @code
@item Short_Float 
32 bit IEEE short
@item Float 
(Short) 32 bit IEEE short 
@item Long_Float 
64 bit IEEE long 
@item Long_Long_Float 
64 bit IEEE long (80 bit IEEE long on x86 processors)
@end table

@sp 1
@cartouche
@noindent
@strong{18}.  The small of an ordinary fixed point type. See 3.5.9(8).
@end cartouche
@noindent
@code{Fine_Delta} is 2**(-63) 

@sp 1
@cartouche
@noindent
@strong{19}.  What combinations of small, range, and digits are
supported for fixed point types. See 3.5.9(10).
@end cartouche
@noindent
Any combinations are permitted that do not result in a small less than
@code{Fine_Delta} and do not result in a mantissa larger than 63 bits.
If the mantissa is larger than 53 bits on machines where Long_Long_Float
is 64 bits (true of all architectures except ia32), then the output from
Text_IO is accurate to only 53 bits, rather than the full mantissa. This
is because floating-point conversions are used to convert fixed point.

@sp 1
@cartouche
@noindent
@strong{20}.  The result of @code{Tags.Expanded_Name} for types declared
within an unnamed @code{block_statement}. See 3.9(10).
@end cartouche
@noindent
Block numbers of the form @code{B@var{nnn}}, where @var{nnn} is a
decimal integer are allocated.

@sp 1
@cartouche
@noindent
@strong{21}.  Implementation-defined attributes. See 4.1.4(12).
@end cartouche
@noindent
@xref{Implementation Defined Attributes}.

@sp 1
@cartouche
@noindent
@strong{22}.  Any implementation-defined time types. See 9.6(6).
@end cartouche
@noindent
There are no implementation-defined time types. 

@sp 1
@cartouche
@noindent
@strong{23}.  The time base associated with relative delays.
@end cartouche
@noindent
See 9.6(20). The time base used is that provided by the C library
function @code{gettimeofday}.

@sp 1
@cartouche
@noindent
@strong{24}.  The time base of the type @code{Calendar.Time}. See
9.6(23).
@end cartouche
@noindent
The time base used is that provided by the C library function
@code{gettimeofday}.

@sp 1
@cartouche
@noindent
@strong{25}.  The time zone used for package @code{Calendar}
operations. See 9.6(24).
@end cartouche
@noindent
The time zone used by package @code{Calendar} is the current system time zone
setting for local time, as accessed by the C library function
@code{localtime}.

@sp 1
@cartouche
@noindent
@strong{26}.  Any limit on @code{delay_until_statements} of
@code{select_statements}. See 9.6(29).
@end cartouche
@noindent
There are no such limits. 

@sp 1
@cartouche
@noindent
@strong{27}.  Whether or not two non overlapping parts of a composite
object are independently addressable, in the case where packing, record
layout, or @code{Component_Size} is specified for the object. See
9.10(1).
@end cartouche
@noindent
Separate components are independently addressable if they do not share
overlapping storage units.

@sp 1
@cartouche
@noindent
@strong{28}.  The representation for a compilation. See 10.1(2).
@end cartouche
@noindent
A compilation is represented by a sequence of files presented to the
compiler in a single invocation of the @file{gcc} command.

@sp 1
@cartouche
@noindent
@strong{29}.  Any restrictions on compilations that contain multiple
compilation_units. See 10.1(4).
@end cartouche
@noindent
No single file can contain more than one compilation unit, but any
sequence of files can be presented to the compiler as a single
compilation.

@sp 1
@cartouche
@noindent
@strong{30}.  The mechanisms for creating an environment and for adding
and replacing compilation units. See 10.1.4(3).
@end cartouche
@noindent
See separate section on compilation model. 

@sp 1
@cartouche
@noindent
@strong{31}.  The manner of explicitly assigning library units to a
partition. See 10.2(2).
@end cartouche
@noindent
If a unit contains an Ada main program, then the Ada units for the partition
are determined by recursive application of the rules in the Ada Reference
Manual section 10.2(2-6). In other words, the Ada units will be those that
are needed by the main program, and then this definition of need is applied
recursively to those units, and the partition contains the transitive
closure determined by this relationship. In short, all the necessary units
are included, with no need to explicitly specify the list. If additional
units are required, e.g.@: by foreign language units, then all units must be
mentioned in the context clause of one of the needed Ada units.

If the partition contains no main program, or if the main program is in
a language other than Ada, then GNAT 
provides the binder options -z and -n respectively, and in this case a
list of units can be explicitly supplied to the binder for inclusion in
the partition (all units needed by these units will also be included
automatically). For full details on the use of these options, refer to
the User Guide sections on Binding and Linking.

@sp 1
@cartouche
@noindent
@strong{32}.  The implementation-defined means, if any, of specifying
which compilation units are needed by a given compilation unit. See
10.2(2).
@end cartouche
@noindent
The units needed by a given compilation unit are as defined in
the Ada Reference Manual section 10.2(2-6). There are no 
implementation-defined pragmas or other implementation-defined
means for specifying needed units.

@sp 1
@cartouche
@noindent
@strong{33}.  The manner of designating the main subprogram of a
partition. See 10.2(7).
@end cartouche
@noindent
The main program is designated by providing the name of the
corresponding ali file as the input parameter to the binder.

@sp 1
@cartouche
@noindent
@strong{34}.  The order of elaboration of @code{library_items}. See
10.2(18).
@end cartouche
@noindent
The first constraint on ordering is that it meets the requirements of
chapter 10 of the Ada 95 Reference Manual. This still leaves some
implementation dependent choices, which are resolved by first
elaborating bodies as early as possible (i.e.@: in preference to specs
where there is a choice), and second by evaluating the immediate with
clauses of a unit to determine the probably best choice, and
third by elaborating in alphabetical order of unit names
where a choice still remains.

@sp 1
@cartouche
@noindent
@strong{35}.  Parameter passing and function return for the main
subprogram. See 10.2(21).
@end cartouche
@noindent
The main program has no parameters. It may be a procedure, or a function
returning an integer type. In the latter case, the returned integer
value is the return code of the program.

@sp 1
@cartouche
@noindent
@strong{36}.  The mechanisms for building and running partitions. See
10.2(24).
@end cartouche
@noindent
GNAT itself supports programs with only a single partition. The GNATDIST
tool provided with the GLADE package (which also includes an implementation
of the PCS) provides a completely flexible method for building and running
programs consisting of multiple partitions. See the separate GLADE manual
for details.

@sp 1
@cartouche
@noindent
@strong{37}.  The details of program execution, including program
termination. See 10.2(25).
@end cartouche
@noindent
See separate section on compilation model.

@sp 1
@cartouche
@noindent
@strong{38}.  The semantics of any non-active partitions supported by the
implementation. See 10.2(28).
@end cartouche
@noindent
Passive partitions are supported on targets where shared memory is
provided by the operating system. See the GLADE reference manual for
further details.

@sp 1
@cartouche
@noindent
@strong{39}.  The information returned by @code{Exception_Message}. See
11.4.1(10).
@end cartouche
@noindent
Exception message returns the null string unless a specific message has
been passed by the program.

@sp 1
@cartouche
@noindent
@strong{40}.  The result of @code{Exceptions.Exception_Name} for types
declared within an unnamed @code{block_statement}. See 11.4.1(12).
@end cartouche
@noindent
Blocks have implementation defined names of the form @code{B@var{nnn}}
where @var{nnn} is an integer.

@sp 1
@cartouche
@noindent
@strong{41}.  The information returned by
@code{Exception_Information}. See 11.4.1(13).
@end cartouche
@noindent
@code{Exception_Information} contains the expanded name of the exception
in upper case, and no other information.

@sp 1
@cartouche
@noindent
@strong{42}.  Implementation-defined check names. See 11.5(27).
@end cartouche
@noindent
No implementation-defined check names are supported. 

@sp 1
@cartouche
@noindent
@strong{43}.  The interpretation of each aspect of representation. See
13.1(20).
@end cartouche
@noindent
See separate section on data representations.

@sp 1
@cartouche
@noindent
@strong{44}.  Any restrictions placed upon representation items. See
13.1(20).
@end cartouche
@noindent
See separate section on data representations.

@sp 1
@cartouche
@noindent
@strong{45}.  The meaning of @code{Size} for indefinite subtypes. See
13.3(48).
@end cartouche
@noindent
Size for an indefinite subtype is the maximum possible size, except that
for the case of a subprogram parameter, the size of the parameter object
is the actual size.

@sp 1
@cartouche
@noindent
@strong{46}.  The default external representation for a type tag. See
13.3(75).
@end cartouche
@noindent
The default external representation for a type tag is the fully expanded
name of the type in upper case letters.

@sp 1
@cartouche
@noindent
@strong{47}.  What determines whether a compilation unit is the same in
two different partitions. See 13.3(76).
@end cartouche
@noindent
A compilation unit is the same in two different partitions if and only
if it derives from the same source file.

@sp 1
@cartouche
@noindent
@strong{48}.  Implementation-defined components. See 13.5.1(15).
@end cartouche
@noindent
The only implementation defined component is the tag for a tagged type,
which contains a pointer to the dispatching table.

@sp 1
@cartouche
@noindent
@strong{49}.  If @code{Word_Size} = @code{Storage_Unit}, the default bit
ordering. See 13.5.3(5).
@end cartouche
@noindent
@code{Word_Size} (32) is not the same as @code{Storage_Unit} (8) for this
implementation, so no non-default bit ordering is supported. The default
bit ordering corresponds to the natural endianness of the target architecture.

@sp 1
@cartouche
@noindent
@strong{50}.  The contents of the visible part of package @code{System}
and its language-defined children. See 13.7(2).
@end cartouche
@noindent
See the definition of these packages in files @file{system.ads} and
@file{s-stoele.ads}.

@sp 1
@cartouche
@noindent
@strong{51}.  The contents of the visible part of package
@code{System.Machine_Code}, and the meaning of
@code{code_statements}. See 13.8(7).
@end cartouche
@noindent
See the definition and documentation in file @file{s-maccod.ads}.

@sp 1
@cartouche
@noindent
@strong{52}.  The effect of unchecked conversion. See 13.9(11).
@end cartouche
@noindent
Unchecked conversion between types of the same size
and results in an uninterpreted transmission of the bits from one type
to the other. If the types are of unequal sizes, then in the case of
discrete types, a shorter source is first zero or sign extended as
necessary, and a shorter target is simply truncated on the left.
For all non-discrete types, the source is first copied if necessary
to ensure that the alignment requirements of the target are met, then
a pointer is constructed to the source value, and the result is obtained
by dereferencing this pointer after converting it to be a pointer to the
target type.

@sp 1
@cartouche
@noindent
@strong{53}.  The manner of choosing a storage pool for an access type
when @code{Storage_Pool} is not specified for the type. See 13.11(17).
@end cartouche
@noindent
There are 3 different standard pools used by the compiler when
@code{Storage_Pool} is not specified depending whether the type is local
to a subprogram or defined at the library level and whether
@code{Storage_Size}is specified or not. See documentation in the runtime
library units @code{System.Pool_Global}, @code{System.Pool_Size} and
@code{System.Pool_Local} in files @file{s-poosiz.ads},
@file{s-pooglo.ads} and @file{s-pooloc.ads} for full details on the
default pools used.

@sp 1
@cartouche
@noindent
@strong{54}.  Whether or not the implementation provides user-accessible
names for the standard pool type(s). See 13.11(17).
@end cartouche
@noindent

See documentation in the sources of the run time mentioned in paragraph
@strong{53} . All these pools are accessible by means of @code{with}'ing
these units.

@sp 1
@cartouche
@noindent
@strong{55}.  The meaning of @code{Storage_Size}. See 13.11(18).
@end cartouche
@noindent
@code{Storage_Size} is measured in storage units, and refers to the
total space available for an access type collection, or to the primary
stack space for a task.

@sp 1
@cartouche
@noindent
@strong{56}.  Implementation-defined aspects of storage pools. See
13.11(22).
@end cartouche
@noindent
See documentation in the sources of the run time mentioned in paragraph
@strong{53} for details on GNAT-defined aspects of storage pools.

@sp 1
@cartouche
@noindent
@strong{57}.  The set of restrictions allowed in a pragma
@code{Restrictions}. See 13.12(7).
@end cartouche
@noindent
All RM defined Restriction identifiers are implemented. The following
additional restriction identifiers are provided. There are two separate
lists of implementation dependent restriction identifiers. The first
set requires consistency throughout a partition (in other words, if the
restriction identifier is used for any compilation unit in the partition,
then all compilation units in the partition must obey the restriction.

@table @code

@item Boolean_Entry_Barriers
@findex Boolean_Entry_Barriers
This restriction ensures at compile time that barriers in entry declarations
for protected types are restricted to references to simple boolean variables
defined in the private part of the protected type. No other form of entry
barriers is permitted. This is one of the restrictions of the Ravenscar
profile for limited tasking (see also pragma Ravenscar).

@item Max_Entry_Queue_Depth => Expr
@findex Max_Entry_Queue_Depth
This restriction is a declaration that any protected entry compiled in
the scope of the restriction has at most the specified number of
tasks waiting on the entry
at any one time, and so no queue is required. This restriction is not
checked at compile time. A program execution is erroneous if an attempt
is made to queue more than the specified number of tasks on such an entry.

@item No_Calendar
@findex No_Calendar
This restriction ensures at compile time that there is no implicit or
explicit dependence on the package @code{Ada.Calendar}.

@item No_Dynamic_Interrupts
@findex No_Dynamic_Interrupts
This restriction ensures at compile time that there is no attempt to
dynamically associate interrupts. Only static association is allowed.

@item No_Enumeration_Maps
@findex No_Enumeration_Maps
This restriction ensures at compile time that no operations requiring
enumeration maps are used (that is Image and Value attributes applied
to enumeration types).

@item No_Entry_Calls_In_Elaboration_Code
@findex No_Entry_Calls_In_Elaboration_Code
This restriction ensures at compile time that no task or protected entry
calls are made during elaboration code. As a result of the use of this
restriction, the compiler can assume that no code past an accept statement
in a task can be executed at elaboration time.

@item No_Exception_Handlers
@findex No_Exception_Handlers
This restriction ensures at compile time that there are no explicit
exception handlers.

@item No_Implicit_Conditionals
@findex No_Implicit_Conditionals
This restriction ensures that the generated code does not contain any
implicit conditionals, either by modifying the generated code where possible,
or by rejecting any construct that would otherwise generate an implicit
conditional. The details and use of this restriction are described in
more detail in the High Integrity product documentation.

@item No_Implicit_Loops
@findex No_Implicit_Loops
This restriction ensures that the generated code does not contain any
implicit @code{for} loops, either by modifying
the generated code where possible,
or by rejecting any construct that would otherwise generate an implicit
@code{for} loop. The details and use of this restriction are described in
more detail in the GNORT Reference Manual.

@item No_Local_Protected_Objects
@findex No_Local_Protected_Objects
This restriction ensures at compile time that protected objects are
only declared at the library level.

@item No_Protected_Type_Allocators
@findex No_Protected_Type_Allocators
This restriction ensures at compile time that there are no allocator
expressions that attempt to allocate protected objects.

@item No_Select_Statements
@findex No_Select_Statements
This restriction ensures at compile time no select statements of any kind
are permitted, that is the keyword @code{select} may not appear. 
This is one of the restrictions of the Ravenscar
profile for limited tasking (see also pragma Ravenscar).

@item No_Standard_Storage_Pools
@findex No_Standard_Storage_Pools
This restriction ensures at compile time that no access types
use the standard default storage pool. Any access type declared must
have an explicit Storage_Pool attribute defined specifying a
user-defined storage pool.

@item No_Streams
@findex No_Streams
This restriction ensures at compile time that there are no implicit or
explicit dependencies on the package @code{Ada.Streams}.

@item No_Task_Attributes
@findex No_Task_Attributes
This restriction ensures at compile time that there are no implicit or
explicit dependencies on the package @code{Ada.Task_Attributes}.

@item No_Task_Termination
@findex No_Task_Termination
This restriction ensures at compile time that no terminate alternatives
appear in any task body.

@item No_Wide_Characters
@findex No_Wide_Characters
This restriction ensures at compile time that no uses of the types
@code{Wide_Character} or @code{Wide_String}
appear, and that no wide character literals
appear in the program (that is literals representing characters not in
type @code{Character}.

@item Static_Priorities
@findex Static_Priorities
This restriction ensures at compile time that all priority expressions
are static, and that there are no dependencies on the package
@code{Ada.Dynamic_Priorities}.

@item Static_Storage_Size
@findex Static_Storage_Size
This restriction ensures at compile time that any expression appearing
in a Storage_Size pragma or attribute definition clause is static.

@end table

@noindent
The second set of implementation dependent restriction identifiers
does not require partition-wide consistency.
The restriction may be enforced for a single
compilation unit without any effect on any of the
other compilation units in the partition.

@table @code

@item No_Elaboration_Code
@findex No_Elaboration_Code
This restriction ensures at compile time that no elaboration code is
generated. Note that this is not the same condition as is enforced
by pragma Preelaborate. There are cases in which pragma Preelaborate
still permits code to be generated (e.g.@: code to initialize a large
array to all zeroes), and there are cases of units which do not meet
the requirements for pragma Preelaborate, but for which no elaboration
code is generated. Generally, it is the case that preelaborable units
will meet the restrictions, with the exception of large aggregates
initialized with an others_clause, and exception declarations (which
generate calls to a run-time registry procedure). Note that this restriction
is enforced on a unit by unit basis, it need not be obeyed consistently
throughout a partition.

@item No_Entry_Queue
@findex No_Entry_Queue
This restriction is a declaration that any protected entry compiled in
the scope of the restriction has at most one task waiting on the entry
at any one time, and so no queue is required. This restriction is not
checked at compile time. A program execution is erroneous if an attempt
is made to queue a second task on such an entry.

@item No_Implementation_Attributes
@findex No_Implementation_Attributes
This restriction checks at compile time that no GNAT-defined attributes
are present. With this restriction, the only attributes that can be used
are those defined in the Ada 95 Reference Manual.

@item No_Implementation_Pragmas
@findex No_Implementation_Pragmas
This restriction checks at compile time that no GNAT-defined pragmas
are present. With this restriction, the only pragmas that can be used
are those defined in the Ada 95 Reference Manual.

@item No_Implementation_Restrictions
@findex No_Implementation_Restrictions
This restriction checks at compile time that no GNAT-defined restriction
identifiers (other than @code{No_Implementation_Restrictions} itself)
are present. With this restriction, the only other restriction identifiers
that can be used are those defined in the Ada 95 Reference Manual.

@end table

@sp 1
@cartouche
@noindent
@strong{58}.  The consequences of violating limitations on
@code{Restrictions} pragmas. See 13.12(9).
@end cartouche
@noindent
Restrictions that can be checked at compile time result in illegalities
if violated. Currently there are no other consequences of violating
restrictions.

@sp 1
@cartouche
@noindent
@strong{59}.  The representation used by the @code{Read} and
@code{Write} attributes of elementary types in terms of stream
elements. See 13.13.2(9).
@end cartouche
@noindent
The representation is the in-memory representation of the base type of
the type, using the number of bits corresponding to the
@code{@var{type}'Size} value, and the natural ordering of the machine.

@sp 1
@cartouche
@noindent
@strong{60}.  The names and characteristics of the numeric subtypes
declared in the visible part of package @code{Standard}. See A.1(3).
@end cartouche
@noindent
See items describing the integer and floating-point types supported.

@sp 1
@cartouche
@noindent
@strong{61}.  The accuracy actually achieved by the elementary
functions. See A.5.1(1).
@end cartouche
@noindent
The elementary functions correspond to the functions available in the C
library. Only fast math mode is implemented.

@sp 1
@cartouche
@noindent
@strong{62}.  The sign of a zero result from some of the operators or
functions in @code{Numerics.Generic_Elementary_Functions}, when
@code{Float_Type'Signed_Zeros} is @code{True}. See A.5.1(46).
@end cartouche
@noindent
The sign of zeroes follows the requirements of the IEEE 754 standard on
floating-point.

@sp 1
@cartouche
@noindent
@strong{63}.  The value of
@code{Numerics.Float_Random.Max_Image_Width}. See A.5.2(27).
@end cartouche
@noindent
Maximum image width is 649, see library file @file{a-numran.ads}.

@sp 1
@cartouche
@noindent
@strong{64}.  The value of
@code{Numerics.Discrete_Random.Max_Image_Width}. See A.5.2(27).
@end cartouche
@noindent
Maximum image width is 80, see library file @file{a-nudira.ads}.

@sp 1
@cartouche
@noindent
@strong{65}.  The algorithms for random number generation. See
A.5.2(32).
@end cartouche
@noindent
The algorithm is documented in the source files @file{a-numran.ads} and
@file{a-numran.adb}.

@sp 1
@cartouche
@noindent
@strong{66}.  The string representation of a random number generator's
state. See A.5.2(38).
@end cartouche
@noindent
See the documentation contained in the file @file{a-numran.adb}. 

@sp 1
@cartouche
@noindent
@strong{67}.  The minimum time interval between calls to the
time-dependent Reset procedure that are guaranteed to initiate different
random number sequences. See A.5.2(45).
@end cartouche
@noindent
The minimum period between reset calls to guarantee distinct series of
random numbers is one microsecond.

@sp 1
@cartouche
@noindent
@strong{68}.  The values of the @code{Model_Mantissa},
@code{Model_Emin}, @code{Model_Epsilon}, @code{Model},
@code{Safe_First}, and @code{Safe_Last} attributes, if the Numerics
Annex is not supported. See A.5.3(72).
@end cartouche
@noindent
See the source file @file{ttypef.ads} for the values of all numeric
attributes.

@sp 1
@cartouche
@noindent
@strong{69}.  Any implementation-defined characteristics of the
input-output packages. See A.7(14).
@end cartouche
@noindent
There are no special implementation defined characteristics for these
packages.

@sp 1
@cartouche
@noindent
@strong{70}.  The value of @code{Buffer_Size} in @code{Storage_IO}. See
A.9(10).
@end cartouche
@noindent
All type representations are contiguous, and the @code{Buffer_Size} is
the value of @code{@var{type}'Size} rounded up to the next storage unit
boundary.

@sp 1
@cartouche
@noindent
@strong{71}.  External files for standard input, standard output, and
standard error See A.10(5).
@end cartouche
@noindent
These files are mapped onto the files provided by the C streams
libraries. See source file @file{i-cstrea.ads} for further details.

@sp 1
@cartouche
@noindent
@strong{72}.  The accuracy of the value produced by @code{Put}. See
A.10.9(36).
@end cartouche
@noindent
If more digits are requested in the output than are represented by the
precision of the value, zeroes are output in the corresponding least
significant digit positions.

@sp 1
@cartouche
@noindent
@strong{73}.  The meaning of @code{Argument_Count}, @code{Argument}, and
@code{Command_Name}. See A.15(1).
@end cartouche
@noindent
These are mapped onto the @code{argv} and @code{argc} parameters of the
main program in the natural manner.

@sp 1
@cartouche
@noindent
@strong{74}.  Implementation-defined convention names. See B.1(11).
@end cartouche
@noindent
The following convention names are supported

@table @code
@item  Ada
Ada
@item Asm 
Assembly language 
@item Assembler 
Assembly language 
@item C
C
@item C_Pass_By_Copy
Treated like C, except for record types
@item COBOL 
COBOL 
@item CPP 
C++ 
@item Default
Treated the same as C
@item DLL
DLL (used for Windows implementations only) is handled like the Stdcall
convention. This convention is used to access variables and functions
(with Stdcall convention) in a DLL@.
@item Win32
Win32 (used for Windows implementations only) is handled like the Stdcall
convention. This convention is used to access variables and functions
(with Stdcall convention) in a DLL@.
@item External
Treated the same as C
@item Fortran 
Fortran 
@item Intrinsic 
For support of pragma @code{Import} with convention Intrinsic, see
separate section on Intrinsic Subprograms.
@item Stdcall 
Stdcall (used for Windows implementations only). This convention correspond
to the WINAPI (previously called Pascal convention) C/C++ convention under
Windows. A function with this convention clean the stack before exit.
@item Stubbed
Stubbed is a special convention used to indicate that the body of the
subprogram will be entirely ignored. Any call to the subprogram
is converted into a raise of the @code{Program_Error} exception. If a
pragma @code{Import} specifies convention @code{stubbed} then no body need
be present at all. This convention is useful during development for the
inclusion of subprograms whose body has not yet been written.

@end table
@noindent
In addition, all otherwise unrecognized convention names are also
treated as being synonymous with convention C@. In all implementations
except for VMS, use of such other names results in a warning. In VMS
implementations, these names are accepted silently.

@sp 1
@cartouche
@noindent
@strong{75}.  The meaning of link names. See B.1(36).
@end cartouche
@noindent
Link names are the actual names used by the linker.

@sp 1
@cartouche
@noindent
@strong{76}.  The manner of choosing link names when neither the link
name nor the address of an imported or exported entity is specified. See
B.1(36).
@end cartouche
@noindent
The default linker name is that which would be assigned by the relevant
external language, interpreting the Ada name as being in all lower case
letters.

@sp 1
@cartouche
@noindent
@strong{77}.  The effect of pragma @code{Linker_Options}. See B.1(37).
@end cartouche
@noindent
The string passed to @code{Linker_Options} is presented uninterpreted as
an argument to the link command, unless it contains Ascii.NUL characters.
NUL characters if they appear act as argument separators, so for example

@smallexample
pragma Linker_Options ("-labc" & ASCII.Nul & "-ldef");
@end smallexample

@noindent
causes two separate arguments "-labc" and "-ldef" to be passed to the
linker with a guarantee that the order is preserved (no such guarantee
exists for the use of separate Linker_Options pragmas).

In addition, GNAT allow multiple arguments to @code{Linker_Options}
with exactly the same meaning, so the above pragma could also be
written as:

@smallexample
pragma Linker_Options ("-labc", "-ldef");
@end smallexample

@noindent
The above multiple argument form is a GNAT extension.

@sp 1
@cartouche
@noindent
@strong{78}.  The contents of the visible part of package
@code{Interfaces} and its language-defined descendants. See B.2(1).
@end cartouche
@noindent
See files with prefix @file{i-} in the distributed library.

@sp 1
@cartouche
@noindent
@strong{79}.  Implementation-defined children of package
@code{Interfaces}. The contents of the visible part of package
@code{Interfaces}. See B.2(11).
@end cartouche
@noindent
See files with prefix @file{i-} in the distributed library.

@sp 1
@cartouche
@noindent
@strong{80}.  The types @code{Floating}, @code{Long_Floating},
@code{Binary}, @code{Long_Binary}, @code{Decimal_ Element}, and
@code{COBOL_Character}; and the initialization of the variables
@code{Ada_To_COBOL} and @code{COBOL_To_Ada}, in
@code{Interfaces.COBOL}. See B.4(50).
@end cartouche
@noindent
@table @code
@item Floating
Float
@item Long_Floating 
(Floating) Long_Float 
@item Binary 
Integer 
@item Long_Binary 
Long_Long_Integer 
@item Decimal_Element 
Character 
@item COBOL_Character 
Character 
@end table

For initialization, see the file @file{i-cobol.ads} in the distributed library.

@sp 1
@cartouche
@noindent
@strong{81}.  Support for access to machine instructions. See C.1(1).
@end cartouche
@noindent
See documentation in file @file{s-maccod.ads} in the distributed library.

@sp 1
@cartouche
@noindent
@strong{82}.  Implementation-defined aspects of access to machine
operations. See C.1(9).
@end cartouche
@noindent
See documentation in file @file{s-maccod.ads} in the distributed library.

@sp 1
@cartouche
@noindent
@strong{83}.  Implementation-defined aspects of interrupts. See C.3(2).
@end cartouche
@noindent
Interrupts are mapped to signals or conditions as appropriate. See
definition of unit
@code{Ada.Interrupt_Names} in source file @file{a-intnam.ads} for details
on the interrupts supported on a particular target.

@sp 1
@cartouche
@noindent
@strong{84}.  Implementation-defined aspects of pre-elaboration. See
C.4(13).
@end cartouche
@noindent
GNAT does not permit a partition to be restarted without reloading,
except under control of the debugger.

@sp 1
@cartouche
@noindent
@strong{85}.  The semantics of pragma @code{Discard_Names}. See C.5(7).
@end cartouche
@noindent
Pragma @code{Discard_Names} causes names of enumeration literals to
be suppressed. In the presence of this pragma, the Image attribute 
provides the image of the Pos of the literal, and Value accepts
Pos values.

@sp 1
@cartouche
@noindent
@strong{86}.  The result of the @code{Task_Identification.Image}
attribute. See C.7.1(7).
@end cartouche
@noindent
The result of this attribute is an 8-digit hexadecimal string
representing the virtual address of the task control block.

@sp 1
@cartouche
@noindent
@strong{87}.  The value of @code{Current_Task} when in a protected entry
or interrupt handler. See C.7.1(17).
@end cartouche
@noindent
Protected entries or interrupt handlers can be executed by any
convenient thread, so the value of @code{Current_Task} is undefined.

@sp 1
@cartouche
@noindent
@strong{88}.  The effect of calling @code{Current_Task} from an entry
body or interrupt handler. See C.7.1(19).
@end cartouche
@noindent
The effect of calling @code{Current_Task} from an entry body or
interrupt handler is to return the identification of the task currently
executing the code.

@sp 1
@cartouche
@noindent
@strong{89}.  Implementation-defined aspects of
@code{Task_Attributes}. See C.7.2(19).
@end cartouche
@noindent
There are no implementation-defined aspects of @code{Task_Attributes}.

@sp 1
@cartouche
@noindent
@strong{90}.  Values of all @code{Metrics}. See D(2).
@end cartouche
@noindent
The metrics information for GNAT depends on the performance of the
underlying operating system. The sources of the run-time for tasking
implementation, together with the output from @code{-gnatG} can be
used to determine the exact sequence of operating systems calls made
to implement various tasking constructs. Together with appropriate
information on the performance of the underlying operating system,
on the exact target in use, this information can be used to determine
the required metrics.

@sp 1
@cartouche
@noindent
@strong{91}.  The declarations of @code{Any_Priority} and
@code{Priority}. See D.1(11).
@end cartouche
@noindent
See declarations in file @file{system.ads}.

@sp 1
@cartouche
@noindent
@strong{92}.  Implementation-defined execution resources. See D.1(15).
@end cartouche
@noindent
There are no implementation-defined execution resources.

@sp 1
@cartouche
@noindent
@strong{93}.  Whether, on a multiprocessor, a task that is waiting for
access to a protected object keeps its processor busy. See D.2.1(3).
@end cartouche
@noindent
On a multi-processor, a task that is waiting for access to a protected
object does not keep its processor busy.

@sp 1
@cartouche
@noindent
@strong{94}.  The affect of implementation defined execution resources
on task dispatching. See D.2.1(9).
@end cartouche
@noindent
@c SGI info
@ignore
Tasks map to IRIX threads, and the dispatching policy is as defined by
the IRIX implementation of threads.
@end ignore
Tasks map to threads in the threads package used by GNAT@. Where possible
and appropriate, these threads correspond to native threads of the
underlying operating system.

@sp 1
@cartouche
@noindent
@strong{95}.  Implementation-defined @code{policy_identifiers} allowed
in a pragma @code{Task_Dispatching_Policy}. See D.2.2(3).
@end cartouche
@noindent
There are no implementation-defined policy-identifiers allowed in this
pragma.

@sp 1
@cartouche
@noindent
@strong{96}.  Implementation-defined aspects of priority inversion. See
D.2.2(16).
@end cartouche
@noindent
Execution of a task cannot be preempted by the implementation processing
of delay expirations for lower priority tasks.

@sp 1
@cartouche
@noindent
@strong{97}.  Implementation defined task dispatching. See D.2.2(18).
@end cartouche
@noindent
@c SGI info:
@ignore
Tasks map to IRIX threads, and the dispatching policy is as defied by
the IRIX implementation of threads.
@end ignore
The policy is the same as that of the underlying threads implementation.

@sp 1
@cartouche
@noindent
@strong{98}.  Implementation-defined @code{policy_identifiers} allowed
in a pragma @code{Locking_Policy}. See D.3(4).
@end cartouche
@noindent
The only implementation defined policy permitted in GNAT is
@code{Inheritance_Locking}. On targets that support this policy, locking
is implemented by inheritance, i.e.@: the task owning the lock operates
at a priority equal to the highest priority of any task currently
requesting the lock.

@sp 1
@cartouche
@noindent
@strong{99}.  Default ceiling priorities. See D.3(10).
@end cartouche
@noindent
The ceiling priority of protected objects of the type
@code{System.Interrupt_Priority'Last} as described in the Ada 95
Reference Manual D.3(10),

@sp 1
@cartouche
@noindent
@strong{100}.  The ceiling of any protected object used internally by
the implementation. See D.3(16).
@end cartouche
@noindent
The ceiling priority of internal protected objects is
@code{System.Priority'Last}.

@sp 1
@cartouche
@noindent
@strong{101}.  Implementation-defined queuing policies. See D.4(1).
@end cartouche
@noindent
There are no implementation-defined queueing policies. 

@sp 1
@cartouche
@noindent
@strong{102}.  On a multiprocessor, any conditions that cause the
completion of an aborted construct to be delayed later than what is
specified for a single processor. See D.6(3).
@end cartouche
@noindent
The semantics for abort on a multi-processor is the same as on a single
processor, there are no further delays.

@sp 1
@cartouche
@noindent
@strong{103}.  Any operations that implicitly require heap storage
allocation. See D.7(8).
@end cartouche
@noindent
The only operation that implicitly requires heap storage allocation is
task creation.

@sp 1
@cartouche
@noindent
@strong{104}.  Implementation-defined aspects of pragma
@code{Restrictions}. See D.7(20).
@end cartouche
@noindent
There are no such implementation-defined aspects. 

@sp 1
@cartouche
@noindent
@strong{105}.  Implementation-defined aspects of package
@code{Real_Time}. See D.8(17).
@end cartouche
@noindent
There are no implementation defined aspects of package @code{Real_Time}.

@sp 1
@cartouche
@noindent
@strong{106}.  Implementation-defined aspects of
@code{delay_statements}. See D.9(8).
@end cartouche
@noindent
Any difference greater than one microsecond will cause the task to be
delayed (see D.9(7)).

@sp 1
@cartouche
@noindent
@strong{107}.  The upper bound on the duration of interrupt blocking
caused by the implementation. See D.12(5).
@end cartouche
@noindent
The upper bound is determined by the underlying operating system. In
no cases is it more than 10 milliseconds.

@sp 1
@cartouche
@noindent
@strong{108}.  The means for creating and executing distributed
programs. See E(5).
@end cartouche
@noindent
The GLADE package provides a utility GNATDIST for creating and executing
distributed programs. See the GLADE reference manual for further details.

@sp 1
@cartouche
@noindent
@strong{109}.  Any events that can result in a partition becoming
inaccessible. See E.1(7).
@end cartouche
@noindent
See the GLADE reference manual for full details on such events.

@sp 1
@cartouche
@noindent
@strong{110}.  The scheduling policies, treatment of priorities, and
management of shared resources between partitions in certain cases. See
E.1(11).
@end cartouche
@noindent
See the GLADE reference manual for full details on these aspects of
multi-partition execution.

@sp 1
@cartouche
@noindent
@strong{111}.  Events that cause the version of a compilation unit to
change. See E.3(5).
@end cartouche
@noindent
Editing the source file of a compilation unit, or the source files of
any units on which it is dependent in a significant way cause the version
to change. No other actions cause the version number to change. All changes
are significant except those which affect only layout, capitalization or
comments.

@sp 1
@cartouche
@noindent
@strong{112}.  Whether the execution of the remote subprogram is
immediately aborted as a result of cancellation. See E.4(13).
@end cartouche
@noindent
See the GLADE reference manual for details on the effect of abort in
a distributed application.

@sp 1
@cartouche
@noindent
@strong{113}.  Implementation-defined aspects of the PCS@. See E.5(25).
@end cartouche
@noindent
See the GLADE reference manual for a full description of all implementation
defined aspects of the PCS@.

@sp 1
@cartouche
@noindent
@strong{114}.  Implementation-defined interfaces in the PCS@. See
E.5(26).
@end cartouche
@noindent
See the GLADE reference manual for a full description of all
implementation defined interfaces.

@sp 1
@cartouche
@noindent
@strong{115}.  The values of named numbers in the package
@code{Decimal}. See F.2(7).
@end cartouche
@noindent
@table @code
@item Max_Scale
+18
@item Min_Scale
-18
@item Min_Delta
1.0E-18
@item Max_Delta
1.0E+18
@item Max_Decimal_Digits
18
@end table

@sp 1
@cartouche
@noindent
@strong{116}.  The value of @code{Max_Picture_Length} in the package
@code{Text_IO.Editing}. See F.3.3(16).
@end cartouche
@noindent
64

@sp 1
@cartouche
@noindent
@strong{117}.  The value of @code{Max_Picture_Length} in the package
@code{Wide_Text_IO.Editing}. See F.3.4(5).
@end cartouche
@noindent
64

@sp 1
@cartouche
@noindent
@strong{118}.  The accuracy actually achieved by the complex elementary
functions and by other complex arithmetic operations. See G.1(1).
@end cartouche
@noindent
Standard library functions are used for the complex arithmetic
operations. Only fast math mode is currently supported.

@sp 1
@cartouche
@noindent
@strong{119}.  The sign of a zero result (or a component thereof) from
any operator or function in @code{Numerics.Generic_Complex_Types}, when
@code{Real'Signed_Zeros} is True. See G.1.1(53).
@end cartouche
@noindent
The signs of zero values are as recommended by the relevant
implementation advice.

@sp 1
@cartouche
@noindent
@strong{120}.  The sign of a zero result (or a component thereof) from
any operator or function in
@code{Numerics.Generic_Complex_Elementary_Functions}, when
@code{Real'Signed_Zeros} is @code{True}. See G.1.2(45).
@end cartouche
@noindent
The signs of zero values are as recommended by the relevant
implementation advice.

@sp 1
@cartouche
@noindent
@strong{121}.  Whether the strict mode or the relaxed mode is the
default. See G.2(2).
@end cartouche
@noindent
The strict mode is the default. There is no separate relaxed mode. GNAT
provides a highly efficient implementation of strict mode.

@sp 1
@cartouche
@noindent
@strong{122}.  The result interval in certain cases of fixed-to-float
conversion. See G.2.1(10).
@end cartouche
@noindent
For cases where the result interval is implementation dependent, the
accuracy is that provided by performing all operations in 64-bit IEEE
floating-point format.

@sp 1
@cartouche
@noindent
@strong{123}.  The result of a floating point arithmetic operation in
overflow situations, when the @code{Machine_Overflows} attribute of the
result type is @code{False}. See G.2.1(13).
@end cartouche
@noindent
Infinite and Nan values are produced as dictated by the IEEE
floating-point standard.

@sp 1
@cartouche
@noindent
@strong{124}.  The result interval for division (or exponentiation by a
negative exponent), when the floating point hardware implements division
as multiplication by a reciprocal. See G.2.1(16).
@end cartouche
@noindent
Not relevant, division is IEEE exact. 

@sp 1
@cartouche
@noindent
@strong{125}.  The definition of close result set, which determines the
accuracy of certain fixed point multiplications and divisions. See
G.2.3(5).
@end cartouche
@noindent
Operations in the close result set are performed using IEEE long format
floating-point arithmetic. The input operands are converted to
floating-point, the operation is done in floating-point, and the result
is converted to the target type.

@sp 1
@cartouche
@noindent
@strong{126}.  Conditions on a @code{universal_real} operand of a fixed
point multiplication or division for which the result shall be in the
perfect result set. See G.2.3(22).
@end cartouche
@noindent
The result is only defined to be in the perfect result set if the result
can be computed by a single scaling operation involving a scale factor
representable in 64-bits.

@sp 1
@cartouche
@noindent
@strong{127}.  The result of a fixed point arithmetic operation in
overflow situations, when the @code{Machine_Overflows} attribute of the
result type is @code{False}. See G.2.3(27).
@end cartouche
@noindent
Not relevant, @code{Machine_Overflows} is @code{True} for fixed-point
types.

@sp 1
@cartouche
@noindent
@strong{128}.  The result of an elementary function reference in
overflow situations, when the @code{Machine_Overflows} attribute of the
result type is @code{False}. See G.2.4(4).
@end cartouche
@noindent
IEEE infinite and Nan values are produced as appropriate.

@sp 1
@cartouche
@noindent
@strong{129}.  The value of the angle threshold, within which certain
elementary functions, complex arithmetic operations, and complex
elementary functions yield results conforming to a maximum relative
error bound. See G.2.4(10).
@end cartouche
@noindent
Information on this subject is not yet available.

@sp 1
@cartouche
@noindent
@strong{130}.  The accuracy of certain elementary functions for
parameters beyond the angle threshold. See G.2.4(10).
@end cartouche
@noindent
Information on this subject is not yet available.

@sp 1
@cartouche
@noindent
@strong{131}.  The result of a complex arithmetic operation or complex
elementary function reference in overflow situations, when the
@code{Machine_Overflows} attribute of the corresponding real type is
@code{False}. See G.2.6(5).
@end cartouche
@noindent
IEEE infinite and Nan values are produced as appropriate. 

@sp 1
@cartouche
@noindent
@strong{132}.  The accuracy of certain complex arithmetic operations and
certain complex elementary functions for parameters (or components
thereof) beyond the angle threshold. See G.2.6(8).
@end cartouche
@noindent
Information on those subjects is not yet available.

@sp 1
@cartouche
@noindent
@strong{133}.  Information regarding bounded errors and erroneous
execution. See H.2(1).
@end cartouche
@noindent
Information on this subject is not yet available.

@sp 1
@cartouche
@noindent
@strong{134}.  Implementation-defined aspects of pragma
@code{Inspection_Point}. See H.3.2(8).
@end cartouche
@noindent
Pragma @code{Inspection_Point} ensures that the variable is live and can
be examined by the debugger at the inspection point.

@sp 1
@cartouche
@noindent
@strong{135}.  Implementation-defined aspects of pragma
@code{Restrictions}. See H.4(25).
@end cartouche
@noindent
There are no implementation-defined aspects of pragma @code{Restrictions}. The
use of pragma @code{Restrictions [No_Exceptions]} has no effect on the
generated code. Checks must suppressed by use of pragma @code{Suppress}.

@sp 1
@cartouche
@noindent
@strong{136}.  Any restrictions on pragma @code{Restrictions}. See
H.4(27).
@end cartouche
@noindent
There are no restrictions on pragma @code{Restrictions}.

@node Intrinsic Subprograms
@chapter Intrinsic Subprograms
@cindex Intrinsic Subprograms

@menu
* Intrinsic Operators::
* Enclosing_Entity::
* Exception_Information::
* Exception_Message::
* Exception_Name::
* File::
* Line::
* Rotate_Left::
* Rotate_Right::
* Shift_Left::
* Shift_Right::
* Shift_Right_Arithmetic::
* Source_Location::
@end menu

GNAT allows a user application program to write the declaration:

@smallexample
   pragma Import (Intrinsic, name);
@end smallexample

@noindent
providing that the name corresponds to one of the implemented intrinsic
subprograms in GNAT, and that the parameter profile of the referenced
subprogram meets the requirements. This chapter describes the set of
implemented intrinsic subprograms, and the requirements on parameter profiles.
Note that no body is supplied; as with other uses of pragma Import, the
body is supplied elsewhere (in this case by the compiler itself). Note
that any use of this feature is potentially non-portable, since the
Ada standard does not require Ada compilers to implement this feature.

@node Intrinsic Operators
@section Intrinsic Operators
@cindex Intrinsic operator

@noindent
All predefined operators can be used in @code{pragma Import (Intrinsic,..)}
declarations. In the binary operator case, the operands must have the same
size. The operand or operands must also be appropriate for
the operator. For example, for addition, the operands must 
both be floating-point or both be fixed-point. You can use an intrinsic
operator declaration as in the following example:

@smallexample
   type Int1 is new Integer;
   type Int2 is new Integer;

   function "+" (X1 : Int1; X2 : Int2) return Int1;
   function "+" (X1 : Int1; X2 : Int2) return Int2;
   pragma Import (Intrinsic, "+");
@end smallexample

@noindent
This declaration would permit "mixed mode" arithmetic on items
of the differing types Int1 and Int2.

@node Enclosing_Entity
@section Enclosing_Entity
@cindex Enclosing_Entity
@noindent
This intrinsic subprogram is used in the implementation of the
library routine @code{GNAT.Source_Info}. The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@code{GNAT.Source_Info.Enclosing_Entity} to obtain the name of
the current subprogram, package, task, entry, or protected subprogram.

@node Exception_Information
@section Exception_Information
@cindex Exception_Information'
@noindent
This intrinsic subprogram is used in the implementation of the
library routine @code{GNAT.Current_Exception}. The only useful
use of the intrinsic import in this case is the one in this unit,
so an application program should simply call the function
@code{GNAT.Current_Exception.Exception_Information} to obtain
the exception information associated with the current exception.

@node Exception_Message
@section Exception_Message
@cindex Exception_Message
@noindent
This intrinsic subprogram is used in the implementation of the
library routine @code{GNAT.Current_Exception}. The only useful
use of the intrinsic import in this case is the one in this unit,
so an application program should simply call the function
@code{GNAT.Current_Exception.Exception_Message} to obtain
the message associated with the current exception.

@node Exception_Name
@section Exception_Name
@cindex Exception_Name
@noindent
This intrinsic subprogram is used in the implementation of the
library routine @code{GNAT.Current_Exception}. The only useful
use of the intrinsic import in this case is the one in this unit,
so an application program should simply call the function
@code{GNAT.Current_Exception.Exception_Name} to obtain
the name of the current exception.

@node File
@section File
@cindex File
@noindent
This intrinsic subprogram is used in the implementation of the
library routine @code{GNAT.Source_Info}. The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@code{GNAT.Source_Info.File} to obtain the name of the current
file.

@node Line
@section Line
@cindex Line
@noindent
This intrinsic subprogram is used in the implementation of the
library routine @code{GNAT.Source_Info}. The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@code{GNAT.Source_Info.Line} to obtain the number of the current
source line.

@node Rotate_Left
@section Rotate_Left
@cindex Rotate_Left
@noindent
In standard Ada 95, the @code{Rotate_Left} function is available only
for the predefined modular types in package @code{Interfaces}. However, in
GNAT it is possible to define a Rotate_Left function for a user
defined modular type or any signed integer type as in this example:

@smallexample
   function Shift_Left
     (Value  : My_Modular_Type;
      Amount : Natural)
      return   My_Modular_Type;
@end smallexample

@noindent
The requirements are that the profile be exactly as in the example
above. The only modifications allowed are in the formal parameter
names, and in the type of @code{Value} and the return type, which
must be the same, and must be either a signed integer type, or
a modular integer type with a binary modulus, and the size must
be 8. 16, 32 or 64 bits.

@node Rotate_Right
@section Rotate_Right
@cindex Rotate_Right
@noindent
A @code{Rotate_Right} function can be defined for any user defined
binary modular integer type, or signed integer type, as described
above for @code{Rotate_Left}.

@node Shift_Left
@section Shift_Left
@cindex Shift_Left
@noindent
A @code{Shift_Left} function can be defined for any user defined
binary modular integer type, or signed integer type, as described
above for @code{Rotate_Left}.

@node Shift_Right
@section Shift_Right
@cindex Shift_Right
@noindent
A @code{Shift_Right} function can be defined for any user defined
binary modular integer type, or signed integer type, as described
above for @code{Rotate_Left}.

@node Shift_Right_Arithmetic
@section Shift_Right_Arithmetic
@cindex Shift_Right_Arithmetic
@noindent
A @code{Shift_Right_Arithmetic} function can be defined for any user
defined binary modular integer type, or signed integer type, as described
above for @code{Rotate_Left}.

@node Source_Location
@section Source_Location
@cindex Source_Location
@noindent
This intrinsic subprogram is used in the implementation of the
library routine @code{GNAT.Source_Info}. The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@code{GNAT.Source_Info.Source_Location} to obtain the current
source file location.

@node Representation Clauses and Pragmas
@chapter Representation Clauses and Pragmas
@cindex Representation Clauses

@menu
* Alignment Clauses::
* Size Clauses::
* Storage_Size Clauses::
* Size of Variant Record Objects::
* Biased Representation ::
* Value_Size and Object_Size Clauses::
* Component_Size Clauses::
* Bit_Order Clauses::
* Effect of Bit_Order on Byte Ordering::
* Pragma Pack for Arrays::
* Pragma Pack for Records::
* Record Representation Clauses::
* Enumeration Clauses::
* Address Clauses::
* Effect of Convention on Representation::
* Determining the Representations chosen by GNAT::
@end menu

@noindent
@cindex Representation Clause
@cindex Representation Pragma
@cindex Pragma, representation
This section describes the representation clauses accepted by GNAT, and
their effect on the representation of corresponding data objects.

GNAT fully implements Annex C (Systems Programming). This means that all
the implementation advice sections in chapter 13 are fully implemented.
However, these sections only require a minimal level of support for
representation clauses. GNAT provides much more extensive capabilities,
and this section describes the additional capabilities provided.

@node Alignment Clauses
@section Alignment Clauses
@cindex Alignment Clause

@noindent
GNAT requires that all alignment clauses specify a power of 2, and all
default alignments are always a power of 2. The default alignment
values are as follows:

@itemize @bullet
@item Primitive Types
For primitive types, the alignment is the maximum of the actual size of
objects of the type, and the maximum alignment supported by the target.
For example, for type Long_Float, the object size is 8 bytes, and the
default alignment will be 8 on any target that supports alignments
this large, but on some targets, the maximum alignment may be smaller
than 8, in which case objects of type Long_Float will be maximally
aligned.

@item Arrays
For arrays, the alignment is equal to the alignment of the component type
for the normal case where no packing or component size is given. If the
array is packed, and the packing is effective (see separate section on
packed arrays), then the alignment will be one for long packed arrays,
or arrays whose length is not known at compile time. For short packed
arrays, which are handled internally as modular types, the alignment
will be as described for primitive types, e.g.@: a packed array of length
31 bits will have an object size of four bytes, and an alignment of 4.

@item Records
For the normal non-packed case, the alignment of a record is equal to
the maximum alignment of any of its components. For tagged records, this
includes the implicit access type used for the tag. If a pragma Pack is
used and all fields are packable (see separate section on pragma Pack),
then the resulting alignment is 1.

@end itemize

@noindent
An alignment clause may
always specify a larger alignment than the default value, up to some
maximum value dependent on the target (obtainable by using the
attribute reference System'Maximum_Alignment). The only case in which
it is permissible to specify a smaller alignment than the default value
is in the case of a record for which a record representation clause is
given. In this case, packable fields for which a component clause is
given still result in a default alignment corresponding to the original
type, but this may be overridden, since these components in fact only
require an alignment of one byte. For example, given

@smallexample
  type v is record
     a : integer;
  end record;

  for v use record
     a at 0  range 0 .. 31;
  end record;

  for v'alignment use 1;
@end smallexample

@noindent
@cindex Alignment, default
The default alignment for the type @code{v} is 4, as a result of the
integer field in the record, but since this field is placed with a
component clause, it is permissible, as shown, to override the default
alignment of the record to a smaller value.

@node Size Clauses
@section Size Clauses
@cindex Size Clause

@noindent
The default size of types is as specified in the reference manual. For
objects, GNAT will generally increase the type size so that the object
size is a multiple of storage units, and also a multiple of the
alignment. For example

@smallexample
   type Smallint is range 1 .. 6;

   type Rec is record
      y1 : integer;
      y2 : boolean;
   end record;
@end smallexample

@noindent
In this example, @code{Smallint}
has a size of 3, as specified by the RM rules,
but objects of this type will have a size of 8, 
since objects by default occupy an integral number
of storage units. On some targets, notably older
versions of the Digital Alpha, the size of stand
alone objects of this type may be 32, reflecting
the inability of the hardware to do byte load/stores.

Similarly, the size of type @code{Rec} is 40 bits, but
the alignment is 4, so objects of this type will have
their size increased to 64 bits so that it is a multiple
of the alignment. The reason for this decision, which is
in accordance with the specific note in RM 13.3(43):

@smallexample
A Size clause should be supported for an object if the specified
Size is at least as large as its subtype's Size, and corresponds
to a size in storage elements that is a multiple of the object's
Alignment (if the Alignment is nonzero).
@end smallexample

@noindent
An explicit size clause may be used to override the default size by
increasing it. For example, if we have:

@smallexample
   type My_Boolean is new Boolean;
   for My_Boolean'Size use 32;
@end smallexample

@noindent
then objects of this type will always be 32 bits long. In the case of
discrete types, the size can be increased up to 64 bits, with the effect
that the entire specified field is used to hold the value, sign- or
zero-extended as appropriate. If more than 64 bits is specified, then
padding space is allocated after the value, and a warning is issued that
there are unused bits.

Similarly the size of records and arrays may be increased, and the effect
is to add padding bits after the value. This also causes a warning message
to be generated.

The largest Size value permitted in GNAT is 2**32-1. Since this is a
Size in bits, this corresponds to an object of size 256 megabytes (minus
one). This limitation is true on all targets. The reason for this
limitation is that it improves the quality of the code in many cases
if it is known that a Size value can be accommodated in an object of
type Integer.

@node Storage_Size Clauses
@section Storage_Size Clauses
@cindex Storage_Size Clause

@noindent
For tasks, the @code{Storage_Size} clause specifies the amount of space
to be allocated for the task stack. This cannot be extended, and if the
stack is exhausted, then @code{Storage_Error} will be raised if stack
checking is enabled. If the default size of 20K bytes is insufficient,  
then you need to use a @code{Storage_Size} attribute definition clause,
or a @code{Storage_Size} pragma in the task definition to set the
appropriate required size. A useful technique is to include in every
task definition a pragma of the form:

@smallexample
   pragma Storage_Size (Default_Stack_Size);
@end smallexample

@noindent
Then Default_Stack_Size can be defined in a global package, and modified
as required. Any tasks requiring different task stack sizes from the
default can have an appropriate alternative reference in the pragma.

For access types, the @code{Storage_Size} clause specifies the maximum
space available for allocation of objects of the type. If this space is
exceeded then @code{Storage_Error} will be raised by an allocation attempt.
In the case where the access type is declared local to a subprogram, the
use of a @code{Storage_Size} clause triggers automatic use of a special
predefined storage pool (@code{System.Pool_Size}) that ensures that all
space for the pool is automatically reclaimed on exit from the scope in
which the type is declared.

A special case recognized by the compiler is the specification of a
@code{Storage_Size} of zero for an access type. This means that no
items can be allocated from the pool, and this is recognized at compile
time, and all the overhead normally associated with maintaining a fixed
size storage pool is eliminated. Consider the following example:

@smallexample
   procedure p is
      type R is array (Natural) of Character;
      type P is access all R;
      for P'Storage_Size use 0;
      --  Above access type intended only for interfacing purposes
   
      y : P;
   
      procedure g (m : P);
      pragma Import (C, g);
   
      --  ...
   
   begin
      --  ...
      y := new R;
   end;
@end smallexample

@noindent
As indicated in this example, these dummy storage pools are often useful in
connection with interfacing where no object will ever be allocated. If you
compile the above example, you get the warning:

@smallexample
   p.adb:16:09: warning: allocation from empty storage pool
   p.adb:16:09: warning: Storage_Error will be raised at run time
@end smallexample

@noindent
Of course in practice, there will not be any explicit allocators in the
case of such an access declaration.

@node Size of Variant Record Objects
@section Size of Variant Record Objects
@cindex Size, variant record objects
@cindex Variant record objects, size

@noindent
An issue arises in the case of variant record objects of whether Size gives
information about a particular variant, or the maximum size required
for any variant. Consider the following program

@smallexample
with Text_IO; use Text_IO;
procedure q is
   type R1 (A : Boolean := False) is record
     case A is
       when True  => X : Character;
       when False => null;
     end case;
   end record;
   
   V1 : R1 (False);
   V2 : R1;

begin
   Put_Line (Integer'Image (V1'Size));
   Put_Line (Integer'Image (V2'Size));
end q;
@end smallexample

@noindent
Here we are dealing with a variant record, where the True variant
requires 16 bits, and the False variant requires 8 bits.
In the above example, both V1 and V2 contain the False variant,
which is only 8 bits long. However, the result of running the
program is:

@smallexample
8
16
@end smallexample

@noindent
The reason for the difference here is that the discriminant value of
V1 is fixed, and will always be False. It is not possible to assign
a True variant value to V1, therefore 8 bits is sufficient. On the
other hand, in the case of V2, the initial discriminant value is
False (from the default), but it is possible to assign a True
variant value to V2, therefore 16 bits must be allocated for V2
in the general case, even fewer bits may be needed at any particular
point during the program execution.

As can be seen from the output of this program, the @code{'Size}
attribute applied to such an object in GNAT gives the actual allocated
size of the variable, which is the largest size of any of the variants.
The Ada Reference Manual is not completely clear on what choice should
be made here, but the GNAT behavior seems most consistent with the
language in the RM@.

In some cases, it may be desirable to obtain the size of the current
variant, rather than the size of the largest variant. This can be
achieved in GNAT by making use of the fact that in the case of a
subprogram parameter, GNAT does indeed return the size of the current
variant (because a subprogram has no way of knowing how much space
is actually allocated for the actual).

Consider the following modified version of the above program:

@smallexample
with Text_IO; use Text_IO;
procedure q is
   type R1 (A : Boolean := False) is record
     case A is
       when True  => X : Character;
       when False => null;
     end case;
   end record;
   
   V2 : R1;

   function Size (V : R1) return Integer is
   begin
      return V'Size;
   end Size;

begin
   Put_Line (Integer'Image (V2'Size));
   Put_Line (Integer'IMage (Size (V2)));
   V2 := (True, 'x');
   Put_Line (Integer'Image (V2'Size));
   Put_Line (Integer'IMage (Size (V2)));
end q;
@end smallexample

@noindent
The output from this program is

@smallexample
16
8
16
16
@end smallexample

@noindent
Here we see that while the @code{'Size} attribute always returns
the maximum size, regardless of the current variant value, the
@code{Size} function does indeed return the size of the current
variant value.

@node Biased Representation
@section Biased Representation
@cindex Size for biased representation
@cindex Biased representation

@noindent
In the case of scalars with a range starting at other than zero, it is
possible in some cases to specify a size smaller than the default minimum
value, and in such cases, @code{GNAT} uses an unsigned biased representation,
in which zero is used to represent the lower bound, and successive values
represent successive values of the type.

For example, suppose we have the declaration:

@smallexample
   type Small is range -7 .. -4;
   for Small'Size use 2;
@end smallexample

@noindent
Although the default size of type @code{Small} is 4, the @code{Size}
clause is accepted by GNAT and results in the following representation
scheme:

@smallexample
  -7 is represented as 2#00#
  -6 is represented as 2#01#
  -5 is represented as 2#10#
  -4 is represented as 2#11#
@end smallexample

@noindent
Biased representation is only used if the specified @code{Size} clause
cannot be accepted in any other manner. These reduced sizes that force
biased representation can be used for all discrete types except for 
enumeration types for which a representation clause is given.

@node Value_Size and Object_Size Clauses
@section Value_Size and Object_Size Clauses
@findex Value_Size
@findex Object_Size
@cindex Size, of objects

@noindent
In Ada 95, the @code{Size} of a discrete type is the minimum number of bits
required to hold values of the type. Although this interpretation was
allowed in Ada 83, it was not required, and this requirement in practice
can cause some significant difficulties. For example, in most Ada 83
compilers, @code{Natural'Size} was 32. However, in Ada-95,
@code{Natural'Size} is
typically 31. This means that code may change in behavior when moving
from Ada 83 to Ada 95. For example, consider:

@smallexample
   type Rec is record;
      A : Natural;
      B : Natural;
   end record;

   for Rec use record
      for A use at 0  range 0 .. Natural'Size - 1;
      for B use at 0  range Natural'Size .. 2 * Natural'Size - 1;
   end record;
@end smallexample

@noindent
In the above code, since the typical size of @code{Natural} objects
is 32 bits and @code{Natural'Size} is 31, the above code can cause
unexpected inefficient packing in Ada 95, and in general there are
surprising cases where the fact that the object size can exceed the
size of the type causes surprises.

To help get around this problem GNAT provides two implementation
dependent attributes @code{Value_Size} and @code{Object_Size}. When 
applied to a type, these attributes yield the size of the type
(corresponding to the RM defined size attribute), and the size of
objects of the type respectively.

The @code{Object_Size} is used for determining the default size of
objects and components. This size value can be referred to using the
@code{Object_Size} attribute. The phrase "is used" here means that it is
the basis of the determination of the size. The backend is free to
pad this up if necessary for efficiency, e.g.@: an 8-bit stand-alone
character might be stored in 32 bits on a machine with no efficient
byte access instructions such as the Alpha.

The default rules for the value of @code{Object_Size} for fixed-point and
discrete types are as follows:

@itemize @bullet
@item
The @code{Object_Size} for base subtypes reflect the natural hardware
size in bits (run the utility gnatpsta to find those values for numeric types). 
Enumeration types and fixed-point base subtypes have 8. 16. 32 or 64
bits for this size, depending on the range of values to be stored.

@item
The @code{Object_Size} of a subtype is the same as the
@code{Object_Size} of
the type from which it is obtained.

@item
The @code{Object_Size} of a derived base type is copied from the parent
base type, and the @code{Object_Size} of a derived first subtype is copied
from the parent first subtype.
@end itemize

@noindent
The @code{Value_Size} attribute
is the number of bits required to store a value
of the type. This size can be referred to using the @code{Value_Size}
attribute. This value is used to determine how tightly to pack
records or arrays with components of this type, and also affects
the semantics of unchecked conversion (unchecked conversions where
the @code{Value_Size} values differ generate a warning, and are potentially
target dependent).

The default rules for the value of @code{Value_Size} are as follows:

@itemize @bullet
@item
The @code{Value_Size} for a base subtype is the minimum number of bits
required to store all values of the type (including the sign bit
only if negative values are possible).

@item
If a subtype statically matches the first subtype of a given type, then it has
by default the same @code{Value_Size} as the first subtype. This is a
consequence of RM 13.1(14) ("if two subtypes statically match,
then their subtype-specific aspects are the same".)

@item
All other subtypes have a @code{Value_Size} corresponding to the minimum
number of bits required to store all values of the subtype. For
dynamic bounds, it is assumed that the value can range down or up
to the corresponding bound of the ancestor
@end itemize

@noindent
The RM defined attribute @code{Size} corresponds to the
@code{Value_Size} attribute.

The @code{Size} attribute may be defined for a first-named subtype. This sets
the @code{Value_Size} of
the first-named subtype to the given value, and the
@code{Object_Size} of this first-named subtype to the given value padded up
to an appropriate boundary. It is a consequence of the default rules
above that this @code{Object_Size} will apply to all further subtypes. On the
other hand, @code{Value_Size} is affected only for the first subtype, any
dynamic subtypes obtained from it directly, and any statically matching
subtypes. The @code{Value_Size} of any other static subtypes is not affected.

@code{Value_Size} and
@code{Object_Size} may be explicitly set for any subtype using
an attribute definition clause. Note that the use of these attributes
can cause the RM 13.1(14) rule to be violated. If two access types
reference aliased objects whose subtypes have differing @code{Object_Size}
values as a result of explicit attribute definition clauses, then it
is erroneous to convert from one access subtype to the other.

At the implementation level, Esize stores the Object_SIze and the
RM_Size field stores the @code{Value_Size} (and hence the value of the
@code{Size} attribute,
which, as noted above, is equivalent to @code{Value_Size}).

To get a feel for the difference, consider the following examples (note
that in each case the base is short_short_integer with a size of 8):

@smallexample
                                       Object_Size     Value_Size

type x1 is range 0..5;                      8               3

type x2 is range 0..5;
for x2'size use 12;                        12              12

subtype x3 is x2 range 0 .. 3;             12               2

subtype x4 is x2'base range 0 .. 10;        8               4

subtype x5 is x2 range 0 .. dynamic;       12              (7)

subtype x6 is x2'base range 0 .. dynamic;   8              (7)

@end smallexample

@noindent
Note: the entries marked (7) are not actually specified by the Ada 95 RM,
but it seems in the spirit of the RM rules to allocate the minimum number
of bits known to be large enough to hold the given range of values.

So far, so good, but GNAT has to obey the RM rules, so the question is
under what conditions must the RM @code{Size} be used.
The following is a list
of the occasions on which the RM @code{Size} must be used:

@itemize @bullet
@item
Component size for packed arrays or records

@item
Value of the attribute @code{Size} for a type

@item
Warning about sizes not matching for unchecked conversion
@end itemize

@noindent
For types other than discrete and fixed-point types, the @code{Object_Size}
and Value_Size are the same (and equivalent to the RM attribute @code{Size}).
Only @code{Size} may be specified for such types.

@node Component_Size Clauses
@section Component_Size Clauses
@cindex Component_Size Clause

@noindent
Normally, the value specified in a component clause must be consistent
with the subtype of the array component with regard to size and alignment.
In other words, the value specified must be at least equal to the size
of this subtype, and must be a multiple of the alignment value.

In addition, component size clauses are allowed which cause the array
to be packed, by specifying a smaller value. The cases in which this
is allowed are for component size values in the range 1-63. The value
specified must not be smaller than the Size of the subtype. GNAT will
accurately honor all packing requests in this range. For example, if
we have:

@smallexample
type r is array (1 .. 8) of Natural;
for r'Size use 31;
@end smallexample

@noindent
then the resulting array has a length of 31 bytes (248 bits = 8 * 31).
Of course access to the components of such an array is considerably
less efficient than if the natural component size of 32 is used.

@node Bit_Order Clauses
@section Bit_Order Clauses
@cindex Bit_Order Clause
@cindex bit ordering
@cindex ordering, of bits

@noindent
For record subtypes, GNAT permits the specification of the @code{Bit_Order}
attribute. The specification may either correspond to the default bit
order for the target, in which case the specification has no effect and
places no additional restrictions, or it may be for the non-standard
setting (that is the opposite of the default).

In the case where the non-standard value is specified, the effect is
to renumber bits within each byte, but the ordering of bytes is not
affected. There are certain
restrictions placed on component clauses as follows:

@itemize @bullet

@item Components fitting within a single storage unit.
@noindent
These are unrestricted, and the effect is merely to renumber bits. For
example if we are on a little-endian machine with @code{Low_Order_First}
being the default, then the following two declarations have exactly
the same effect:

@smallexample
   type R1 is record
      A : Boolean;
      B : Integer range 1 .. 120;
   end record;

   for R1 use record
      A at 0 range 0 .. 0;
      B at 0 range 1 .. 7;
   end record;

   type R2 is record
      A : Boolean;
      B : Integer range 1 .. 120;
   end record;

   for R2'Bit_Order use High_Order_First;

   for R2 use record
      A at 0 range 7 .. 7;
      B at 0 range 0 .. 6;
   end record;
@end smallexample

@noindent
The useful application here is to write the second declaration with the
@code{Bit_Order} attribute definition clause, and know that it will be treated
the same, regardless of whether the target is little-endian or big-endian.

@item Components occupying an integral number of bytes.
@noindent
These are components that exactly fit in two or more bytes. Such component
declarations are allowed, but have no effect, since it is important to realize
that the @code{Bit_Order} specification does not affect the ordering of bytes.
In particular, the following attempt at getting an endian-independent integer
does not work:

@smallexample
   type R2 is record
      A : Integer;
   end record;

   for R2'Bit_Order use High_Order_First;

   for R2 use record
      A at 0 range 0 .. 31;
   end record;
@end smallexample

@noindent
This declaration will result in a little-endian integer on a
little-endian machine, and a big-endian integer on a big-endian machine.
If byte flipping is required for interoperability between big- and
little-endian machines, this must be explicitly programmed. This capability
is not provided by @code{Bit_Order}.

@item Components that are positioned across byte boundaries
@noindent
but do not occupy an integral number of bytes. Given that bytes are not
reordered, such fields would occupy a non-contiguous sequence of bits
in memory, requiring non-trivial code to reassemble. They are for this
reason not permitted, and any component clause specifying such a layout
will be flagged as illegal by GNAT@.

@end itemize

@noindent
Since the misconception that Bit_Order automatically deals with all
endian-related incompatibilities is a common one, the specification of
a component field that is an integral number of bytes will always
generate a warning. This warning may be suppressed using
@code{pragma Suppress} if desired. The following section contains additional
details regarding the issue of byte ordering.

@node Effect of Bit_Order on Byte Ordering
@section Effect of Bit_Order on Byte Ordering
@cindex byte ordering
@cindex ordering, of bytes

@noindent
In this section we will review the effect of the @code{Bit_Order} attribute
definition clause on byte ordering. Briefly, it has no effect at all, but
a detailed example will be helpful. Before giving this
example, let us review the precise
definition of the effect of defining @code{Bit_Order}. The effect of a
non-standard bit order is described in section 15.5.3 of the Ada
Reference Manual:

@smallexample
2   A bit ordering is a method of interpreting the meaning of
the storage place attributes.
@end smallexample

@noindent
To understand the precise definition of storage place attributes in
this context, we visit section 13.5.1 of the manual:

@smallexample
13   A record_representation_clause (without the mod_clause)
specifies the layout. The storage place attributes (see 13.5.2)
are taken from the values of the position, first_bit, and last_bit
expressions after normalizing those values so that first_bit is
less than Storage_Unit.
@end smallexample

@noindent
The critical point here is that storage places are taken from
the values after normalization, not before. So the @code{Bit_Order}
interpretation applies to normalized values. The interpretation
is described in the later part of the 15.5.3 paragraph:

@smallexample
2   A bit ordering is a method of interpreting the meaning of
the storage place attributes.  High_Order_First (known in the
vernacular as "big endian") means that the first bit of a
storage element (bit 0) is the most significant bit (interpreting
the sequence of bits that represent a component as an unsigned
integer value). Low_Order_First (known in the vernacular as
"little endian") means the opposite: the first bit is the
least significant.
@end smallexample

@noindent
Note that the numbering is with respect to the bits of a storage
unit. In other words, the specification affects only the numbering
of bits within a single storage unit.

We can make the effect clearer by giving an example.

Suppose that we have an external device which presents two bytes, the first
byte presented, which is the first (low addressed byte) of the two byte
record is called Master, and the second byte is called Slave.

The left most (most significant bit is called Control for each byte, and
the remaing 7 bits are called V1, V2 .. V7, where V7 is the right most
(least significant bit).

On a big-endian machine, we can write the following representation clause

@smallexample
   type Data is record
      Master_Control : Bit;
      Master_V1      : Bit;
      Master_V2      : Bit;
      Master_V3      : Bit;
      Master_V4      : Bit;
      Master_V5      : Bit;
      Master_V6      : Bit;
      Master_V7      : Bit;
      Slave_Control  : Bit;
      Slave_V1       : Bit;
      Slave_V2       : Bit;
      Slave_V3       : Bit;
      Slave_V4       : Bit;
      Slave_V5       : Bit;
      Slave_V6       : Bit;
      Slave_V7       : Bit;
   end record;

   for Data use record
      Master_Control at 0 range 0 .. 0;
      Master_V1      at 0 range 1 .. 1;
      Master_V2      at 0 range 2 .. 2;
      Master_V3      at 0 range 3 .. 3;
      Master_V4      at 0 range 4 .. 4;
      Master_V5      at 0 range 5 .. 5;
      Master_V6      at 0 range 6 .. 6;
      Master_V7      at 0 range 7 .. 7;
      Slave_Control  at 1 range 0 .. 0;
      Slave_V1       at 1 range 1 .. 1;
      Slave_V2       at 1 range 2 .. 2;
      Slave_V3       at 1 range 3 .. 3;
      Slave_V4       at 1 range 4 .. 4;
      Slave_V5       at 1 range 5 .. 5;
      Slave_V6       at 1 range 6 .. 6;
      Slave_V7       at 1 range 7 .. 7;
   end record;
@end smallexample

@noindent
Now if we move this to a little endian machine, then the bit ordering within
the byte is backwards, so we have to rewrite the record rep clause as:

@smallexample
   for Data use record
      Master_Control at 0 range 7 .. 7;
      Master_V1      at 0 range 6 .. 6;
      Master_V2      at 0 range 5 .. 5;
      Master_V3      at 0 range 4 .. 4;
      Master_V4      at 0 range 3 .. 3;
      Master_V5      at 0 range 2 .. 2;
      Master_V6      at 0 range 1 .. 1;
      Master_V7      at 0 range 0 .. 0;
      Slave_Control  at 1 range 7 .. 7;
      Slave_V1       at 1 range 6 .. 6;
      Slave_V2       at 1 range 5 .. 5;
      Slave_V3       at 1 range 4 .. 4;
      Slave_V4       at 1 range 3 .. 3;
      Slave_V5       at 1 range 2 .. 2;
      Slave_V6       at 1 range 1 .. 1;
      Slave_V7       at 1 range 0 .. 0;
   end record;
@end smallexample

It is a nuisance to have to rewrite the clause, especially if
the code has to be maintained on both machines. However,
this is a case that we can handle with the
@code{Bit_Order} attribute if it is implemented.
Note that the implementation is not required on byte addressed
machines, but it is indeed implemented in @code{GNAT}.
This means that we can simply use the
first record clause, together with the declaration

@smallexample
   for Data'Bit_Order use High_Order_First;
@end smallexample

@noindent
and the effect is what is desired, namely the layout is exactly the same,
independent of whether the code is compiled on a big-endial or little-endian
machine.

The important point to understand is that byte ordering is not affected.
A @code{Bit_Order} attribute definition never affects which byte a field
ends up in, only where it ends up in that byte.
To make this clear, let us rewrite the record rep clause of the previous
example as:

@smallexample
   for Data'Bit_Order use High_Order_First;
   for Data use record
      Master_Control at 0 range  0 ..  0;
      Master_V1      at 0 range  1 ..  1;
      Master_V2      at 0 range  2 ..  2;
      Master_V3      at 0 range  3 ..  3;
      Master_V4      at 0 range  4 ..  4;
      Master_V5      at 0 range  5 ..  5;
      Master_V6      at 0 range  6 ..  6;
      Master_V7      at 0 range  7 ..  7;
      Slave_Control  at 0 range  8 ..  8;
      Slave_V1       at 0 range  9 ..  9;
      Slave_V2       at 0 range 10 .. 10;
      Slave_V3       at 0 range 11 .. 11;
      Slave_V4       at 0 range 12 .. 12;
      Slave_V5       at 0 range 13 .. 13;
      Slave_V6       at 0 range 14 .. 14;
      Slave_V7       at 0 range 15 .. 15;
   end record;
@end smallexample

@noindent
This is exactly equivalent to saying (a repeat of the first example):

@smallexample
   for Data'Bit_Order use High_Order_First;
   for Data use record
      Master_Control at 0 range 0 .. 0;
      Master_V1      at 0 range 1 .. 1;
      Master_V2      at 0 range 2 .. 2;
      Master_V3      at 0 range 3 .. 3;
      Master_V4      at 0 range 4 .. 4;
      Master_V5      at 0 range 5 .. 5;
      Master_V6      at 0 range 6 .. 6;
      Master_V7      at 0 range 7 .. 7;
      Slave_Control  at 1 range 0 .. 0;
      Slave_V1       at 1 range 1 .. 1;
      Slave_V2       at 1 range 2 .. 2;
      Slave_V3       at 1 range 3 .. 3;
      Slave_V4       at 1 range 4 .. 4;
      Slave_V5       at 1 range 5 .. 5;
      Slave_V6       at 1 range 6 .. 6;
      Slave_V7       at 1 range 7 .. 7;
   end record;
@end smallexample

@noindent
Why are they equivalent? Well take a specific field, the @code{Slave_V2}
field. The storage place attributes are obtained by normalizing the
values given so that the @code{First_Bit} value is less than 8. After
nromalizing the values (0,10,10) we get (1,2,2) which is exactly what
we specified in the other case.

Now one might expect that the @code{Bit_Order} attribute might affect
bit numbering within the entire record component (two bytes in this
case, thus affecting which byte fields end up in), but that is not
the way this feature is defined, it only affects numbering of bits,
not which byte they end up in.

Consequently it never makes sense to specify a starting bit number
greater than 7 (for a byte addressable field) if an attribute
definition for @code{Bit_Order} has been given, and indeed it
may be actively confusing to specify such a value, so the compiler
generates a warning for such usage.

If you do need to control byte ordering then appropriate conditional
values must be used. If in our example, the slave byte came first on
some machines we might write:

@smallexample
   Master_Byte_First constant Boolean := ...;

   Master_Byte : constant Natural :=
                   1 - Boolean'Pos (Master_Byte_First);
   Slave_Byte  : constant Natural :=
                   Boolean'Pos (Master_Byte_First);

   for Data'Bit_Order use High_Order_First;
   for Data use record
      Master_Control at Master_Byte range 0 .. 0;
      Master_V1      at Master_Byte range 1 .. 1;
      Master_V2      at Master_Byte range 2 .. 2;
      Master_V3      at Master_Byte range 3 .. 3;
      Master_V4      at Master_Byte range 4 .. 4;
      Master_V5      at Master_Byte range 5 .. 5;
      Master_V6      at Master_Byte range 6 .. 6;
      Master_V7      at Master_Byte range 7 .. 7;
      Slave_Control  at Slave_Byte  range 0 .. 0;
      Slave_V1       at Slave_Byte  range 1 .. 1;
      Slave_V2       at Slave_Byte  range 2 .. 2;
      Slave_V3       at Slave_Byte  range 3 .. 3;
      Slave_V4       at Slave_Byte  range 4 .. 4;
      Slave_V5       at Slave_Byte  range 5 .. 5;
      Slave_V6       at Slave_Byte  range 6 .. 6;
      Slave_V7       at Slave_Byte  range 7 .. 7;
   end record;
@end smallexample

@noindent
Now to switch between machines, all that is necessary is
to set the boolean constant @code{Master_Byte_First} in
an appropriate manner.

@node Pragma Pack for Arrays
@section Pragma Pack for Arrays
@cindex Pragma Pack (for arrays)

@noindent
Pragma Pack applied to an array has no effect unless the component type
is packable. For a component type to be packable, it must be one of the
following cases:

@itemize @bullet
@item
Any scalar type
@item
Any fixed-point type
@item
Any type whose size is specified with a size clause
@item
Any packed array type with a static size
@end itemize

@noindent
For all these cases, if the component subtype size is in the range
1- 63, then the effect of the pragma Pack is exactly as though a
component size were specified giving the component subtype size.
For example if we have:

@smallexample
   type r is range 0 .. 17;
 
   type ar is array (1 .. 8) of r;
   pragma Pack (ar);
@end smallexample

@noindent
Then the component size of @code{ar} will be set to 5 (i.e.@: to @code{r'size},
and the size of the array @code{ar} will be exactly 40 bits.

Note that in some cases this rather fierce approach to packing can produce
unexpected effects. For example, in Ada 95, type Natural typically has a
size of 31, meaning that if you pack an array of Natural, you get 31-bit
close packing, which saves a few bits, but results in far less efficient
access. Since many other Ada compilers will ignore such a packing request,
GNAT will generate a warning on some uses of pragma Pack that it guesses
might not be what is intended. You can easily remove this warning by
using an explicit Component_Size setting instead, which never generates
a warning, since the intention of the programmer is clear in this case.

GNAT treats packed arrays in one of two ways. If the size of the array is
known at compile time and is less than 64 bits, then internally the array
is represented as a single modular type, of exactly the appropriate number
of bits. If the length is greater than 63 bits, or is not known at compile
time, then the packed array is represented as an array of bytes, and the
length is always a multiple of 8 bits.

@node Pragma Pack for Records
@section Pragma Pack for Records
@cindex Pragma Pack (for records)

@noindent
Pragma Pack applied to a record will pack the components to reduce wasted
space from alignment gaps and by reducing the amount of space taken by
components. We distinguish between package components and non-packable
components. Components of the following types are considered packable:

@itemize @bullet
@item
All scalar types are packable.

@item
All fixed-point types are represented internally as integers, and
are packable.

@item
Small packed arrays, whose size does not exceed 64 bits, and where the
size is statically known at compile time, are represented internally
as modular integers, and so they are also packable.

@end itemize

@noindent
All packable components occupy the exact number of bits corresponding to
their @code{Size} value, and are packed with no padding bits, i.e.@: they
can start on an arbitrary bit boundary.

All other types are non-packable, they occupy an integral number of
storage units, and 
are placed at a boundary corresponding to their alignment requirements.

For example, consider the record

@smallexample
   type Rb1 is array (1 .. 13) of Boolean;
   pragma Pack (rb1);

   type Rb2 is array (1 .. 65) of Boolean;
   pragma Pack (rb2);

   type x2 is record
      l1 : Boolean;
      l2 : Duration;
      l3 : Float;
      l4 : Boolean;
      l5 : Rb1;
      l6 : Rb2;
   end record;
   pragma Pack (x2);
@end smallexample

@noindent
The representation for the record x2 is as follows:

@smallexample
for x2'Size use 224;
for x2 use record
   l1 at  0 range  0 ..  0;
   l2 at  0 range  1 .. 64;
   l3 at 12 range  0 .. 31;
   l4 at 16 range  0 ..  0;
   l5 at 16 range  1 .. 13;
   l6 at 18 range  0 .. 71;
end record;
@end smallexample

@noindent
Studying this example, we see that the packable fields @code{l1}
and @code{l2} are
of length equal to their sizes, and placed at specific bit boundaries (and
not byte boundaries) to
eliminate padding. But @code{l3} is of a non-packable float type, so
it is on the next appropriate alignment boundary. 

The next two fields are fully packable, so @code{l4} and @code{l5} are
minimally packed with no gaps. However, type @code{Rb2} is a packed
array that is longer than 64 bits, so it is itself non-packable. Thus
the @code{l6} field is aligned to the next byte boundary, and takes an
integral number of bytes, i.e.@: 72 bits.

@node Record Representation Clauses
@section Record Representation Clauses
@cindex Record Representation Clause

@noindent
Record representation clauses may be given for all record types, including
types obtained by record extension. Component clauses are allowed for any
static component. The restrictions on component clauses depend on the type
of the component.

@cindex Component Clause
For all components of an elementary type, the only restriction on component
clauses is that the size must be at least the 'Size value of the type
(actually the Value_Size). There are no restrictions due to alignment,
and such components may freely cross storage boundaries.

Packed arrays with a size up to and including 64-bits are represented
internally using a modular type with the appropriate number of bits, and
thus the same lack of restriction applies. For example, if you declare:

@smallexample
   type R is array (1 .. 49) of Boolean;
   pragma Pack (R);
   for R'Size use 49;
@end smallexample

@noindent
then a component clause for a component of type R may start on any
specified bit boundary, and may specify a value of 49 bits or greater.

For non-primitive types, including packed arrays with a size greater than
64-bits, component clauses must respect the alignment requirement of the
type, in particular, always starting on a byte boundary, and the length
must be a multiple of the storage unit.

The tag field of a tagged type always occupies an address sized field at
the start of the record. No component clause may attempt to overlay this
tag.

In the case of a record extension T1, of a type T, no component clause applied
to the type T1 can specify a storage location that would overlap the first
T'Size bytes of the record.

@node Enumeration Clauses
@section Enumeration Clauses

The only restriction on enumeration clauses is that the range of values
must be representable. For the signed case, if one or more of the
representation values are negative, all values must be in the range:

@smallexample
   System.Min_Int .. System.Max_Int
@end smallexample

@noindent
For the unsigned case, where all values are non negative, the values must
be in the range:

@smallexample
   0 .. System.Max_Binary_Modulus;
@end smallexample

@noindent
A "confirming" representation clause is one in which the values range
from 0 in sequence, i.e.@: a clause that confirms the default representation
for an enumeration type.
Such a confirming representation
is permitted by these rules, and is specially recognized by the compiler so
that no extra overhead results from the use of such a clause.

If an array has an index type which is an enumeration type to which an
enumeration clause has been applied, then the array is stored in a compact
manner. Consider the declarations:

@smallexample
   type r is (A, B, C);
   for r use (A => 1, B => 5, C => 10);
   type t is array (r) of Character;
@end smallexample

@noindent
The array type t corresponds to a vector with exactly three elements and
has a default size equal to @code{3*Character'Size}. This ensures efficient
use of space, but means that accesses to elements of the array will incur
the overhead of converting representation values to the corresponding
positional values, (i.e.@: the value delivered by the  @code{Pos} attribute).

@node Address Clauses
@section Address Clauses
@cindex Address Clause

The reference manual allows a general restriction on representation clauses,
as found in RM 13.1(22):

@smallexample
   An implementation need not support representation
   items containing nonstatic expressions, except that
   an implementation should support a representation item
   for a given entity if each nonstatic expression in the
   representation item is a name that statically denotes
   a constant declared before the entity.
@end smallexample

@noindent
In practice this is applicable only to address clauses, since this is the
only case in which a non-static expression is permitted by the syntax. As
the AARM notes in sections 13.1 (22.a-22.h):

@smallexample
  22.a   Reason: This is to avoid the following sort
         of thing:

  22.b        X : Integer := F(...);
              Y : Address := G(...);
              for X'Address use Y;

  22.c   In the above, we have to evaluate the
         initialization expression for X before we
         know where to put the result. This seems
         like an unreasonable implementation burden.

  22.d   The above code should instead be written
         like this:

  22.e        Y : constant Address := G(...);
              X : Integer := F(...);
              for X'Address use Y;

  22.f   This allows the expression ``Y'' to be safely
         evaluated before X is created.

  22.g   The constant could be a formal parameter of mode in.

  22.h   An implementation can support other nonstatic
         expressions if it wants to. Expressions of type
         Address are hardly ever static, but their value
         might be known at compile time anyway in many
         cases.
@end smallexample

@noindent
GNAT does indeed permit many additional cases of non-static expressions. In
particular, if the type involved is elementary there are no restrictions
(since in this case, holding a temporary copy of the initialization value,
if one is present, is inexpensive). In addition, if there is no implicit or
explicit initialization, then there are no restrictions. GNAT will reject
only the case where all three of these conditions hold:

@itemize @bullet

@item
The type of the item is non-elementary (e.g.@: a record or array).

@item
There is explicit or implicit initialization required for the object.

@item
The address value is non-static. Here GNAT is more permissive than the
RM, and allows the address value to be the address of a previously declared
stand-alone variable, as long as it does not itself have an address clause.

@smallexample
           Anchor : Some_Initialized_Type;
           Overlay : Some_Initialized_Type;
           for Overlay'Address use Anchor'Address;
@end smallexample

However, the prefix of the address clause cannot be an array component, or
a component of a discriminated record.

@end itemize

@noindent
As noted above in section 22.h, address values are typically non-static. In
particular the To_Address function, even if applied to a literal value, is
a non-static function call. To avoid this minor annoyance, GNAT provides
the implementation defined attribute 'To_Address. The following two 
expressions have identical values:

@findex Attribute
@findex To_Address
@smallexample
   To_Address (16#1234_0000#)
   System'To_Address (16#1234_0000#);
@end smallexample

@noindent
except that the second form is considered to be a static expression, and
thus when used as an address clause value is always permitted.

@noindent
Additionally, GNAT treats as static an address clause that is an
unchecked_conversion of a static integer value. This simplifies the porting
of legacy code, and provides a portable equivalent to the GNAT attribute
To_Address.

@findex Export
An address clause cannot be given for an exported object. More
understandably the real restriction is that objects with an address
clause cannot be exported. This is because such variables are not
defined by the Ada program, so there is no external object so export.

@findex Import
It is permissible to give an address clause and a pragma Import for the
same object. In this case, the variable is not really defined by the
Ada program, so there is no external symbol to be linked. The link name
and the external name are ignored in this case. The reason that we allow this
combination is that it provides a useful idiom to avoid unwanted
initializations on objects with address clauses.

When an address clause is given for an object that has implicit or
explicit initialization, then by default initialization takes place. This
means that the effect of the object declaration is to overwrite the
memory at the specified address. This is almost always not what the
programmer wants, so GNAT will output a warning:

@smallexample
  with System;
  package G is
     type R is record
        M : Integer := 0;
     end record;
  
     Ext : R;
     for Ext'Address use System'To_Address (16#1234_1234#);
         |
  >>> warning: implicit initialization of "Ext" may
      modify overlaid storage
  >>> warning: use pragma Import for "Ext" to suppress
      initialization (RM B(24))
  
  end G;
@end smallexample

@noindent
As indicated by the warning message, the solution is to use a (dummy) pragma
Import to suppress this initialization. The pragma tell the compiler that the
object is declared and initialized elsewhere. The following package compiles
without warnings (and the initialization is suppressed):

@smallexample
   with System;
   package G is
      type R is record
         M : Integer := 0;
      end record;
   
      Ext : R;
      for Ext'Address use System'To_Address (16#1234_1234#);
      pragma Import (Ada, Ext);
   end G;
@end smallexample

@node Effect of Convention on Representation
@section Effect of Convention on Representation
@cindex Convention, effect on representation

@noindent
Normally the specification of a foreign language convention for a type or
an object has no effect on the chosen representation. In particular, the
representation chosen for data in GNAT generally meets the standard system
conventions, and for example records are laid out in a manner that is
consistent with C@. This means that specifying convention C (for example)
has no effect.

There are three exceptions to this general rule:

@itemize @bullet

@item Convention Fortran and array subtypes
If pragma Convention Fortran is specified for an array subtype, then in
accordance with the implementation advice in section 3.6.2(11) of the
Ada Reference Manual, the array will be stored in a Fortran-compatible
column-major manner, instead of the normal default row-major order.

@item Convention C and enumeration types
GNAT normally stores enumeration types in 8, 16, or 32 bits as required
to accommodate all values of the type. For example, for the enumeration
type declared by:

@smallexample
   type Color is (Red, Green, Blue);
@end smallexample

@noindent
8 bits is sufficient to store all values of the type, so by default, objects
of type @code{Color} will be represented using 8 bits. However, normal C
convention is to use 32-bits for all enum values in C, since enum values
are essentially of type int. If pragma Convention C is specified for an
Ada enumeration type, then the size is modified as necessary (usually to
32 bits) to be consistent with the C convention for enum values.

@item Convention C/Fortran and Boolean types
In C, the usual convention for boolean values, that is values used for
conditions, is that zero represents false, and nonzero values represent
true. In Ada, the normal convention is that two specific values, typically
0/1, are used to represent false/true respectively.

Fortran has a similar convention for @code{LOGICAL} values (any nonzero
value represents true).

To accommodate the Fortran and C conventions, if a pragma Convention specifies
C or Fortran convention for a derived Boolean, as in the following example:

@smallexample
   type C_Switch is new Boolean;
   pragma Convention (C, C_Switch);
@end smallexample

@noindent
then the GNAT generated code will treat any nonzero value as true. For truth
values generated by GNAT, the conventional value 1 will be used for True, but
when one of these values is read, any nonzero value is treated as True.

@end itemize

@node Determining the Representations chosen by GNAT
@section Determining the Representations chosen by GNAT
@cindex Representation, determination of
@cindex -gnatR switch

@noindent
Although the descriptions in this section are intended to be complete, it is
often easier to simply experiment to see what GNAT accepts and what the
effect is on the layout of types and objects.

As required by the Ada RM, if a representation clause is not accepted, then
it must be rejected as illegal by the compiler. However, when a representation
clause or pragma is accepted, there can still be questions of what the
compiler actually does. For example, if a partial record representation
clause specifies the location of some components and not others, then where
are the non-specified components placed? Or if pragma pack is used on a
record, then exactly where are the resulting fields placed? The section
on pragma Pack in this chapter can be used to answer the second question,
but it is often easier to just see what the compiler does.

For this purpose, GNAT provides the option @code{-gnatR}. If you compile
with this option, then the compiler will output information on the actual
representations chosen, in a format similar to source representation
clauses. For example, if we compile the package:

@smallexample
package q is
   type r (x : boolean) is tagged record
      case x is
         when True => S : String (1 .. 100);
         when False => null;
      end case;
   end record;

   type r2 is new r (false) with record
      y2 : integer;
   end record;

   for r2 use record
      y2 at 16 range 0 .. 31;
   end record;

   type x is record
      y : character;
   end record;

   type x1 is array (1 .. 10) of x;
   for x1'component_size use 11;

   type ia is access integer;

   type Rb1 is array (1 .. 13) of Boolean;
   pragma Pack (rb1);

   type Rb2 is array (1 .. 65) of Boolean;
   pragma Pack (rb2);

   type x2 is record
      l1 : Boolean;
      l2 : Duration;
      l3 : Float;
      l4 : Boolean;
      l5 : Rb1;
      l6 : Rb2;
   end record;
   pragma Pack (x2);
end q;
@end smallexample

@noindent
using the switch @code{-gnatR} we obtain the following output:

@smallexample
Representation information for unit q
-------------------------------------

for r'Size use ??;
for r'Alignment use 4;
for r use record
   x    at 4 range  0 ..  7;
   _tag at 0 range  0 .. 31;
   s    at 5 range  0 .. 799;
end record;

for r2'Size use 160;
for r2'Alignment use 4;
for r2 use record
   x       at  4 range  0 ..  7;
   _tag    at  0 range  0 .. 31;
   _parent at  0 range  0 .. 63;
   y2      at 16 range  0 .. 31;
end record;

for x'Size use 8;
for x'Alignment use 1;
for x use record
   y at 0 range  0 ..  7;
end record;

for x1'Size use 112;
for x1'Alignment use 1;
for x1'Component_Size use 11;

for rb1'Size use 13;
for rb1'Alignment use 2;
for rb1'Component_Size use 1;

for rb2'Size use 72;
for rb2'Alignment use 1;
for rb2'Component_Size use 1;

for x2'Size use 224;
for x2'Alignment use 4;
for x2 use record
   l1 at  0 range  0 ..  0;
   l2 at  0 range  1 .. 64;
   l3 at 12 range  0 .. 31;
   l4 at 16 range  0 ..  0;
   l5 at 16 range  1 .. 13;
   l6 at 18 range  0 .. 71;
end record;
@end smallexample

@noindent
The Size values are actually the Object_Size, i.e.@: the default size that
will be allocated for objects of the type.
The ?? size for type r indicates that we have a variant record, and the
actual size of objects will depend on the discriminant value.

The Alignment values show the actual alignment chosen by the compiler
for each record or array type.

The record representation clause for type r shows where all fields
are placed, including the compiler generated tag field (whose location
cannot be controlled by the programmer).

The record representation clause for the type extension r2 shows all the
fields present, including the parent field, which is a copy of the fields
of the parent type of r2, i.e.@: r1.

The component size and size clauses for types rb1 and rb2 show
the exact effect of pragma Pack on these arrays, and the record
representation clause for type x2 shows how pragma Pack affects
this record type.

In some cases, it may be useful to cut and paste the representation clauses
generated by the compiler into the original source to fix and guarantee
the actual representation to be used.

@node Standard Library Routines
@chapter Standard Library Routines

@noindent
The Ada 95 Reference Manual contains in Annex A a full description of an
extensive set of standard library routines that can be used in any Ada
program, and which must be provided by all Ada compilers. They are
analogous to the standard C library used by C programs.

GNAT implements all of the facilities described in annex A, and for most
purposes the description in the Ada 95
reference manual, or appropriate Ada
text book, will be sufficient for making use of these facilities.

In the case of the input-output facilities, @xref{The Implementation of
Standard I/O}, gives details on exactly how GNAT interfaces to the
file system. For the remaining packages, the Ada 95 reference manual
should be sufficient. The following is a list of the packages included,
together with a brief description of the functionality that is provided.

For completeness, references are included to other predefined library
routines defined in other sections of the Ada 95 reference manual (these are
cross-indexed from annex A).

@table @code
@item Ada (A.2)
This is a parent package for all the standard library packages. It is
usually included implicitly in your program, and itself contains no
useful data or routines.

@item Ada.Calendar (9.6)
@code{Calendar} provides time of day access, and routines for
manipulating times and durations.

@item Ada.Characters (A.3.1)
This is a dummy parent package that contains no useful entities

@item Ada.Characters.Handling (A.3.2)
This package provides some basic character handling capabilities,
including classification functions for classes of characters (e.g.@: test
for letters, or digits).

@item Ada.Characters.Latin_1 (A.3.3)
This package includes a complete set of definitions of the characters
that appear in type CHARACTER@. It is useful for writing programs that
will run in international environments. For example, if you want an
upper case E with an acute accent in a string, it is often better to use
the definition of @code{UC_E_Acute} in this package. Then your program
will print in an understandable manner even if your environment does not
support these extended characters.

@item Ada.Command_Line (A.15)
This package provides access to the command line parameters and the name
of the current program (analogous to the use of argc and argv in C), and
also allows the exit status for the program to be set in a
system-independent manner.

@item Ada.Decimal (F.2)
This package provides constants describing the range of decimal numbers
implemented, and also a decimal divide routine (analogous to the COBOL
verb DIVIDE .. GIVING .. REMAINDER ..)

@item Ada.Direct_IO (A.8.4)
This package provides input-output using a model of a set of records of
fixed-length, containing an arbitrary definite Ada type, indexed by an
integer record number.

@item Ada.Dynamic_Priorities (D.5)
This package allows the priorities of a task to be adjusted dynamically
as the task is running.

@item Ada.Exceptions (11.4.1)
This package provides additional information on exceptions, and also
contains facilities for treating exceptions as data objects, and raising
exceptions with associated messages.

@item Ada.Finalization (7.6)
This package contains the declarations and subprograms to support the
use of controlled types, providing for automatic initialization and
finalization (analogous to the constructors and destructors of C++)

@item Ada.Interrupts (C.3.2)
This package provides facilities for interfacing to interrupts, which
includes the set of signals or conditions that can be raised and
recognized as interrupts.

@item Ada.Interrupts.Names (C.3.2)
This package provides the set of interrupt names (actually signal
or condition names) that can be handled by GNAT@.

@item Ada.IO_Exceptions (A.13)
This package defines the set of exceptions that can be raised by use of
the standard IO packages.

@item Ada.Numerics
This package contains some standard constants and exceptions used
throughout the numerics packages. Note that the constants pi and e are
defined here, and it is better to use these definitions than rolling
your own.

@item Ada.Numerics.Complex_Elementary_Functions
Provides the implementation of standard elementary functions (such as
log and trigonometric functions) operating on complex numbers using the
standard @code{Float} and the @code{Complex} and @code{Imaginary} types
created by the package @code{Numerics.Complex_Types}.

@item Ada.Numerics.Complex_Types
This is a predefined instantiation of
@code{Numerics.Generic_Complex_Types} using @code{Standard.Float} to
build the type @code{Complex} and @code{Imaginary}.

@item Ada.Numerics.Discrete_Random
This package provides a random number generator suitable for generating
random integer values from a specified range.

@item Ada.Numerics.Float_Random
This package provides a random number generator suitable for generating
uniformly distributed floating point values.

@item Ada.Numerics.Generic_Complex_Elementary_Functions
This is a generic version of the package that provides the
implementation of standard elementary functions (such as log an
trigonometric functions) for an arbitrary complex type.

The following predefined instantiations of this package exist

@table @code
@item Short_Float
@code{Ada.Numerics.Short_Complex_Elementary_Functions}
@item Float
@code{Ada.Numerics.Complex_Elementary_Functions}
@item Long_Float
@code{Ada.Numerics.
 Long_Complex_Elementary_Functions}
@end table

@item Ada.Numerics.Generic_Complex_Types
This is a generic package that allows the creation of complex types,
with associated complex arithmetic operations.

The following predefined instantiations of this package exist
@table @code
@item Short_Float
@code{Ada.Numerics.Short_Complex_Complex_Types}
@item Float
@code{Ada.Numerics.Complex_Complex_Types}
@item Long_Float
@code{Ada.Numerics.Long_Complex_Complex_Types}
@end table

@item Ada.Numerics.Generic_Elementary_Functions
This is a generic package that provides the implementation of standard
elementary functions (such as log an trigonometric functions) for an
arbitrary float type.

The following predefined instantiations of this package exist

@table @code
@item Short_Float
@code{Ada.Numerics.Short_Elementary_Functions}
@item Float
@code{Ada.Numerics.Elementary_Functions}
@item Long_Float
@code{Ada.Numerics.Long_Elementary_Functions}
@end table

@item Ada.Real_Time (D.8)
This package provides facilities similar to those of @code{Calendar}, but
operating with a finer clock suitable for real time control.

@item Ada.Sequential_IO (A.8.1)
This package provides input-output facilities for sequential files,
which can contain a sequence of values of a single type, which can be
any Ada type, including indefinite (unconstrained) types.

@item Ada.Storage_IO (A.9)
This package provides a facility for mapping arbitrary Ada types to and
from a storage buffer. It is primarily intended for the creation of new
IO packages.

@item Ada.Streams (13.13.1)
This is a generic package that provides the basic support for the
concept of streams as used by the stream attributes (@code{Input},
@code{Output}, @code{Read} and @code{Write}).

@item Ada.Streams.Stream_IO (A.12.1)
This package is a specialization of the type @code{Streams} defined in
package @code{Streams} together with a set of operations providing
Stream_IO capability. The Stream_IO model permits both random and
sequential access to a file which can contain an arbitrary set of values
of one or more Ada types.

@item Ada.Strings (A.4.1)
This package provides some basic constants used by the string handling
packages.

@item Ada.Strings.Bounded (A.4.4)
This package provides facilities for handling variable length
strings. The bounded model requires a maximum length. It is thus
somewhat more limited than the unbounded model, but avoids the use of
dynamic allocation or finalization.

@item Ada.Strings.Fixed (A.4.3)
This package provides facilities for handling fixed length strings.

@item Ada.Strings.Maps (A.4.2)
This package provides facilities for handling character mappings and
arbitrarily defined subsets of characters. For instance it is useful in
defining specialized translation tables.

@item Ada.Strings.Maps.Constants (A.4.6)
This package provides a standard set of predefined mappings and
predefined character sets. For example, the standard upper to lower case
conversion table is found in this package. Note that upper to lower case
conversion is non-trivial if you want to take the entire set of
characters, including extended characters like E with an acute accent,
into account. You should use the mappings in this package (rather than
adding 32 yourself) to do case mappings.

@item Ada.Strings.Unbounded (A.4.5)
This package provides facilities for handling variable length
strings. The unbounded model allows arbitrary length strings, but
requires the use of dynamic allocation and finalization.

@item Ada.Strings.Wide_Bounded (A.4.7)
@itemx Ada.Strings.Wide_Fixed (A.4.7)
@itemx Ada.Strings.Wide_Maps (A.4.7)
@itemx Ada.Strings.Wide_Maps.Constants (A.4.7)
@itemx Ada.Strings.Wide_Unbounded (A.4.7)
These package provide analogous capabilities to the corresponding
packages without @samp{Wide_} in the name, but operate with the types
@code{Wide_String} and @code{Wide_Character} instead of @code{String}
and @code{Character}.

@item Ada.Synchronous_Task_Control (D.10)
This package provides some standard facilities for controlling task
communication in a synchronous manner.

@item Ada.Tags
This package contains definitions for manipulation of the tags of tagged
values.

@item Ada.Task_Attributes
This package provides the capability of associating arbitrary
task-specific data with separate tasks.

@item Ada.Text_IO
This package provides basic text input-output capabilities for
character, string and numeric data. The subpackages of this
package are listed next.

@item Ada.Text_IO.Decimal_IO
Provides input-output facilities for decimal fixed-point types

@item Ada.Text_IO.Enumeration_IO 
Provides input-output facilities for enumeration types.

@item Ada.Text_IO.Fixed_IO
Provides input-output facilities for ordinary fixed-point types.

@item Ada.Text_IO.Float_IO
Provides input-output facilities for float types. The following
predefined instantiations of this generic package are available:

@table @code
@item Short_Float
@code{Short_Float_Text_IO}
@item Float
@code{Float_Text_IO}
@item Long_Float
@code{Long_Float_Text_IO}
@end table

@item Ada.Text_IO.Integer_IO
Provides input-output facilities for integer types. The following
predefined instantiations of this generic package are available:

@table @code
@item Short_Short_Integer
@code{Ada.Short_Short_Integer_Text_IO}
@item Short_Integer
@code{Ada.Short_Integer_Text_IO}
@item Integer
@code{Ada.Integer_Text_IO}
@item Long_Integer
@code{Ada.Long_Integer_Text_IO}
@item Long_Long_Integer
@code{Ada.Long_Long_Integer_Text_IO}
@end table

@item Ada.Text_IO.Modular_IO
Provides input-output facilities for modular (unsigned) types

@item Ada.Text_IO.Complex_IO (G.1.3)
This package provides basic text input-output capabilities for complex
data.

@item Ada.Text_IO.Editing (F.3.3)
This package contains routines for edited output, analogous to the use
of pictures in COBOL@. The picture formats used by this package are a
close copy of the facility in COBOL@.

@item Ada.Text_IO.Text_Streams (A.12.2)
This package provides a facility that allows Text_IO files to be treated
as streams, so that the stream attributes can be used for writing
arbitrary data, including binary data, to Text_IO files.

@item Ada.Unchecked_Conversion (13.9)
This generic package allows arbitrary conversion from one type to
another of the same size, providing for breaking the type safety in
special circumstances.

If the types have the same Size (more accurately the same Value_Size),
then the effect is simply to transfer the bits from the source to the
target type without any modification. This usage is well defined, and
for simple types whose representation is typically the same across
all implementations, gives a portable method of performing such
conversions.

If the types do not have the same size, then the result is implementation
defined, and thus may be non-portable. The following describes how GNAT
handles such unchecked conversion cases.

If the types are of different sizes, and are both discrete types, then
the effect is of a normal type conversion without any constraint checking.
In particular if the result type has a larger size, the result will be
zero or sign extended. If the result type has a smaller size, the result
will be truncated by ignoring high order bits.

If the types are of different sizes, and are not both discrete types,
then the conversion works as though pointers were created to the source
and target, and the pointer value is converted. The effect is that bits
are copied from successive low order storage units and bits of the source
up to the length of the target type.

A warning is issued if the lengths differ, since the effect in this
case is implementation dependent, and the above behavior may not match
that of some other compiler.

A pointer to one type may be converted to a pointer to another type using
unchecked conversion. The only case in which the effect is undefined is
when one or both pointers are pointers to unconstrained array types. In
this case, the bounds information may get incorrectly transferred, and in
particular, GNAT uses double size pointers for such types, and it is
meaningless to convert between such pointer types. GNAT will issue a
warning if the alignment of the target designated type is more strict
than the alignment of the source designated type (since the result may
be unaligned in this case).

A pointer other than a pointer to an unconstrained array type may be
converted to and from System.Address. Such usage is common in Ada 83
programs, but note that Ada.Address_To_Access_Conversions is the
preferred method of performing such conversions in Ada 95. Neither
unchecked conversion nor Ada.Address_To_Access_Conversions should be
used in conjunction with pointers to unconstrained objects, since
the bounds information cannot be handled correctly in this case.

@item Ada.Unchecked_Deallocation (13.11.2)
This generic package allows explicit freeing of storage previously
allocated by use of an allocator.

@item Ada.Wide_Text_IO (A.11)
This package is similar to @code{Ada.Text_IO}, except that the external
file supports wide character representations, and the internal types are
@code{Wide_Character} and @code{Wide_String} instead of @code{Character}
and @code{String}. It contains generic subpackages listed next.

@item Ada.Wide_Text_IO.Decimal_IO
Provides input-output facilities for decimal fixed-point types

@item Ada.Wide_Text_IO.Enumeration_IO
Provides input-output facilities for enumeration types.

@item Ada.Wide_Text_IO.Fixed_IO
Provides input-output facilities for ordinary fixed-point types.

@item Ada.Wide_Text_IO.Float_IO
Provides input-output facilities for float types. The following
predefined instantiations of this generic package are available:

@table @code
@item Short_Float
@code{Short_Float_Wide_Text_IO}
@item Float
@code{Float_Wide_Text_IO}
@item Long_Float
@code{Long_Float_Wide_Text_IO}
@end table

@item Ada.Wide_Text_IO.Integer_IO
Provides input-output facilities for integer types. The following
predefined instantiations of this generic package are available:

@table @code
@item Short_Short_Integer
@code{Ada.Short_Short_Integer_Wide_Text_IO}
@item Short_Integer
@code{Ada.Short_Integer_Wide_Text_IO}
@item Integer
@code{Ada.Integer_Wide_Text_IO}
@item Long_Integer
@code{Ada.Long_Integer_Wide_Text_IO}
@item Long_Long_Integer
@code{Ada.Long_Long_Integer_Wide_Text_IO}
@end table

@item Ada.Wide_Text_IO.Modular_IO
Provides input-output facilities for modular (unsigned) types

@item Ada.Wide_Text_IO.Complex_IO (G.1.3)
This package is similar to @code{Ada.Text_IO.Complex_IO}, except that the
external file supports wide character representations.

@item Ada.Wide_Text_IO.Editing (F.3.4)
This package is similar to @code{Ada.Text_IO.Editing}, except that the
types are @code{Wide_Character} and @code{Wide_String} instead of
@code{Character} and @code{String}.

@item Ada.Wide_Text_IO.Streams (A.12.3)
This package is similar to @code{Ada.Text_IO.Streams}, except that the
types are @code{Wide_Character} and @code{Wide_String} instead of
@code{Character} and @code{String}.
@end table
@node The Implementation of Standard I/O
@chapter The Implementation of Standard I/O

@noindent
GNAT implements all the required input-output facilities described in
A.6 through A.14. These sections of the Ada 95 reference manual describe the
required behavior of these packages from the Ada point of view, and if
you are writing a portable Ada program that does not need to know the
exact manner in which Ada maps to the outside world when it comes to
reading or writing external files, then you do not need to read this
chapter. As long as your files are all regular files (not pipes or
devices), and as long as you write and read the files only from Ada, the
description in the Ada 95 reference manual is sufficient.

However, if you want to do input-output to pipes or other devices, such
as the keyboard or screen, or if the files you are dealing with are
either generated by some other language, or to be read by some other
language, then you need to know more about the details of how the GNAT
implementation of these input-output facilities behaves.

In this chapter we give a detailed description of exactly how GNAT
interfaces to the file system. As always, the sources of the system are
available to you for answering questions at an even more detailed level,
but for most purposes the information in this chapter will suffice.

Another reason that you may need to know more about how input-output is
implemented arises when you have a program written in mixed languages
where, for example, files are shared between the C and Ada sections of
the same program. GNAT provides some additional facilities, in the form
of additional child library packages, that facilitate this sharing, and
these additional facilities are also described in this chapter.

@menu
* Standard I/O Packages::       
* FORM Strings::                
* Direct_IO::                   
* Sequential_IO::               
* Text_IO::                     
* Wide_Text_IO::                
* Stream_IO::                   
* Shared Files::                
* Open Modes::                  
* Operations on C Streams::     
* Interfacing to C Streams::    
@end menu

@node Standard I/O Packages
@section Standard I/O Packages

@noindent
The Standard I/O packages described in Annex A for

@itemize @bullet
@item
Ada.Text_IO
@item
Ada.Text_IO.Complex_IO
@item
Ada.Text_IO.Text_Streams,
@item
Ada.Wide_Text_IO
@item
Ada.Wide_Text_IO.Complex_IO,
@item
Ada.Wide_Text_IO.Text_Streams
@item
Ada.Stream_IO
@item
Ada.Sequential_IO
@item
Ada.Direct_IO
@end itemize

@noindent
are implemented using the C
library streams facility; where

@itemize @bullet
@item
All files are opened using @code{fopen}.
@item
All input/output operations use @code{fread}/@code{fwrite}.
@end itemize

There is no internal buffering of any kind at the Ada library level. The
only buffering is that provided at the system level in the
implementation of the C library routines that support streams. This
facilitates shared use of these streams by mixed language programs.

@node FORM Strings
@section FORM Strings

@noindent
The format of a FORM string in GNAT is:

@smallexample
"keyword=value,keyword=value,...,keyword=value"
@end smallexample

@noindent
where letters may be in upper or lower case, and there are no spaces
between values. The order of the entries is not important. Currently
there are two keywords defined.

@smallexample
SHARED=[YES|NO]
WCEM=[n|h|u|s\e]
@end smallexample

The use of these parameters is described later in this section.

@node Direct_IO
@section Direct_IO

@noindent
Direct_IO can only be instantiated for definite types. This is a
restriction of the Ada language, which means that the records are fixed
length (the length being determined by @code{@var{type}'Size}, rounded
up to the next storage unit boundary if necessary).

The records of a Direct_IO file are simply written to the file in index
sequence, with the first record starting at offset zero, and subsequent
records following. There is no control information of any kind. For
example, if 32-bit integers are being written, each record takes
4-bytes, so the record at index @var{K} starts at offset (@var{K} -
1)*4.

There is no limit on the size of Direct_IO files, they are expanded as
necessary to accommodate whatever records are written to the file.

@node Sequential_IO
@section Sequential_IO

@noindent
Sequential_IO may be instantiated with either a definite (constrained)
or indefinite (unconstrained) type.

For the definite type case, the elements written to the file are simply
the memory images of the data values with no control information of any
kind. The resulting file should be read using the same type, no validity
checking is performed on input.

For the indefinite type case, the elements written consist of two
parts. First is the size of the data item, written as the memory image
of a @code{Interfaces.C.size_t} value, followed by the memory image of
the data value. The resulting file can only be read using the same
(unconstrained) type. Normal assignment checks are performed on these
read operations, and if these checks fail, @code{Data_Error} is
raised. In particular, in the array case, the lengths must match, and in
the variant record case, if the variable for a particular read operation
is constrained, the discriminants must match.

Note that it is not possible to use Sequential_IO to write variable
length array items, and then read the data back into different length
arrays. For example, the following will raise @code{Data_Error}:

@smallexample
 package IO is new Sequential_IO (String);
 F : IO.File_Type;
 S : String (1..4);
 ...
 IO.Create (F)
 IO.Write (F, "hello!")
 IO.Reset (F, Mode=>In_File);
 IO.Read (F, S);
 Put_Line (S);

@end smallexample

On some Ada implementations, this will print @samp{hell}, but the program is
clearly incorrect, since there is only one element in the file, and that
element is the string @samp{hello!}.

In Ada 95, this kind of behavior can be legitimately achieved using
Stream_IO, and this is the preferred mechanism. In particular, the above
program fragment rewritten to use Stream_IO will work correctly.

@node Text_IO
@section Text_IO

@noindent
Text_IO files consist of a stream of characters containing the following
special control characters:

@smallexample
LF (line feed, 16#0A#) Line Mark
FF (form feed, 16#0C#) Page Mark
@end smallexample

A canonical Text_IO file is defined as one in which the following
conditions are met:

@itemize @bullet
@item
The character @code{LF} is used only as a line mark, i.e.@: to mark the end
of the line.

@item
The character @code{FF} is used only as a page mark, i.e.@: to mark the
end of a page and consequently can appear only immediately following a
@code{LF} (line mark) character.

@item
The file ends with either @code{LF} (line mark) or @code{LF}-@code{FF}
(line mark, page mark). In the former case, the page mark is implicitly
assumed to be present.
@end itemize

A file written using Text_IO will be in canonical form provided that no
explicit @code{LF} or @code{FF} characters are written using @code{Put}
or @code{Put_Line}. There will be no @code{FF} character at the end of
the file unless an explicit @code{New_Page} operation was performed
before closing the file.

A canonical Text_IO file that is a regular file, i.e.@: not a device or a
pipe, can be read using any of the routines in Text_IO@. The
semantics in this case will be exactly as defined in the Ada 95 reference
manual and all the routines in Text_IO are fully implemented.

A text file that does not meet the requirements for a canonical Text_IO
file has one of the following:

@itemize @bullet
@item
The file contains @code{FF} characters not immediately following a
@code{LF} character.

@item
The file contains @code{LF} or @code{FF} characters written by
@code{Put} or @code{Put_Line}, which are not logically considered to be
line marks or page marks.

@item
The file ends in a character other than @code{LF} or @code{FF},
i.e.@: there is no explicit line mark or page mark at the end of the file.
@end itemize

Text_IO can be used to read such non-standard text files but subprograms
to do with line or page numbers do not have defined meanings. In
particular, a @code{FF} character that does not follow a @code{LF}
character may or may not be treated as a page mark from the point of
view of page and line numbering. Every @code{LF} character is considered
to end a line, and there is an implied @code{LF} character at the end of
the file.

@menu
* Text_IO Stream Pointer Positioning::  
* Text_IO Reading and Writing Non-Regular Files::  
* Get_Immediate::               
* Treating Text_IO Files as Streams::
* Text_IO Extensions::
* Text_IO Facilities for Unbounded Strings::
@end menu
@node Text_IO Stream Pointer Positioning

@subsection Stream Pointer Positioning

@noindent
@code{Ada.Text_IO} has a definition of current position for a file that
is being read. No internal buffering occurs in Text_IO, and usually the
physical position in the stream used to implement the file corresponds
to this logical position defined by Text_IO@. There are two exceptions:

@itemize @bullet
@item
After a call to @code{End_Of_Page} that returns @code{True}, the stream
is positioned past the @code{LF} (line mark) that precedes the page
mark. Text_IO maintains an internal flag so that subsequent read
operations properly handle the logical position which is unchanged by
the @code{End_Of_Page} call.

@item
After a call to @code{End_Of_File} that returns @code{True}, if the
Text_IO file was positioned before the line mark at the end of file
before the call, then the logical position is unchanged, but the stream
is physically positioned right at the end of file (past the line mark,
and past a possible page mark following the line mark. Again Text_IO
maintains internal flags so that subsequent read operations properly
handle the logical position.
@end itemize

These discrepancies have no effect on the observable behavior of
Text_IO, but if a single Ada stream is shared between a C program and
Ada program, or shared (using @samp{shared=yes} in the form string)
between two Ada files, then the difference may be observable in some
situations.

@node Text_IO Reading and Writing Non-Regular Files
@subsection Reading and Writing Non-Regular Files

@noindent
A non-regular file is a device (such as a keyboard), or a pipe. Text_IO
can be used for reading and writing. Writing is not affected and the
sequence of characters output is identical to the normal file case, but
for reading, the behavior of Text_IO is modified to avoid undesirable
look-ahead as follows:

An input file that is not a regular file is considered to have no page
marks. Any @code{Ascii.FF} characters (the character normally used for a
page mark) appearing in the file are considered to be data
characters. In particular:

@itemize @bullet
@item
@code{Get_Line} and @code{Skip_Line} do not test for a page mark
following a line mark. If a page mark appears, it will be treated as a
data character.

@item
This avoids the need to wait for an extra character to be typed or
entered from the pipe to complete one of these operations.

@item
@code{End_Of_Page} always returns @code{False}

@item
@code{End_Of_File} will return @code{False} if there is a page mark at
the end of the file.
@end itemize

Output to non-regular files is the same as for regular files. Page marks
may be written to non-regular files using @code{New_Page}, but as noted
above they will not be treated as page marks on input if the output is
piped to another Ada program.

Another important discrepancy when reading non-regular files is that the end
of file indication is not "sticky". If an end of file is entered, e.g.@: by
pressing the @code{EOT} key,
then end of file
is signalled once (i.e.@: the test @code{End_Of_File}
will yield @code{True}, or a read will
raise @code{End_Error}), but then reading can resume
to read data past that end of
file indication, until another end of file indication is entered.

@node Get_Immediate
@subsection Get_Immediate
@cindex Get_Immediate

@noindent
Get_Immediate returns the next character (including control characters)
from the input file. In particular, Get_Immediate will return LF or FF
characters used as line marks or page marks. Such operations leave the
file positioned past the control character, and it is thus not treated
as having its normal function. This means that page, line and column
counts after this kind of Get_Immediate call are set as though the mark
did not occur. In the case where a Get_Immediate leaves the file
positioned between the line mark and page mark (which is not normally
possible), it is undefined whether the FF character will be treated as a
page mark.

@node Treating Text_IO Files as Streams
@subsection Treating Text_IO Files as Streams
@cindex Stream files

@noindent
The package @code{Text_IO.Streams} allows a Text_IO file to be treated
as a stream. Data written to a Text_IO file in this stream mode is
binary data. If this binary data contains bytes 16#0A# (@code{LF}) or
16#0C# (@code{FF}), the resulting file may have non-standard
format. Similarly if read operations are used to read from a Text_IO
file treated as a stream, then @code{LF} and @code{FF} characters may be
skipped and the effect is similar to that described above for
@code{Get_Immediate}.

@node Text_IO Extensions
@subsection Text_IO Extensions
@cindex Text_IO extensions

@noindent
A package GNAT.IO_Aux in the GNAT library provides some useful extensions
to the standard @code{Text_IO} package:

@itemize @bullet
@item function File_Exists (Name : String) return Boolean;
Determines if a file of the given name exists and can be successfully
opened (without actually performing the open operation).

@item function Get_Line return String;
Reads a string from the standard input file. The value returned is exactly
the length of the line that was read.

@item function Get_Line (File : Ada.Text_IO.File_Type) return String;
Similar, except that the parameter File specifies the file from which
the string is to be read.

@end itemize

@node Text_IO Facilities for Unbounded Strings
@subsection Text_IO Facilities for Unbounded Strings
@cindex Text_IO for unbounded strings
@cindex Unbounded_String, Text_IO operations

@noindent
The package @code{Ada.Strings.Unbounded.Text_IO}
in library files @code{a-suteio.ads/adb} contains some GNAT-specific
subprograms useful for Text_IO operations on unbounded strings:

@itemize @bullet

@item function Get_Line (File : File_Type) return Unbounded_String;
Reads a line from the specified file
and returns the result as an unbounded string.

@item procedure Put (File : File_Type; U : Unbounded_String);
Writes the value of the given unbounded string to the specified file
Similar to the effect of
@code{Put (To_String (U))} except that an extra copy is avoided.

@item procedure Put_Line (File : File_Type; U : Unbounded_String);
Writes the value of the given unbounded string to the specified file,
followed by a @code{New_Line}.
Similar to the effect of @code{Put_Line (To_String (U))} except
that an extra copy is avoided.
@end itemize

@noindent
In the above procedures, @code{File} is of type @code{Ada.Text_IO.File_Type}
and is optional. If the parameter is omitted, then the standard input or
output file is referenced as appropriate.

The package @code{Ada.Strings.Wide_Unbounded.Wide_Text_IO} in library
files @code{a-swuwti.ads/adb} provides similar extended @code{Wide_Text_IO}
functionality for unbounded wide strings.

@node Wide_Text_IO
@section Wide_Text_IO

@noindent
@code{Wide_Text_IO} is similar in most respects to Text_IO, except that
both input and output files may contain special sequences that represent
wide character values. The encoding scheme for a given file may be
specified using a FORM parameter:

@smallexample
WCEM=@var{x}
@end smallexample

@noindent
as part of the FORM string (WCEM = wide character encoding method),
where @var{x} is one of the following characters

@table @samp
@item h
Hex ESC encoding
@item u
Upper half encoding
@item s
Shift-JIS encoding
@item e
EUC Encoding
@item 8
UTF-8 encoding
@item b
Brackets encoding
@end table

The encoding methods match those that
can be used in a source
program, but there is no requirement that the encoding method used for
the source program be the same as the encoding method used for files,
and different files may use different encoding methods.

The default encoding method for the standard files, and for opened files
for which no WCEM parameter is given in the FORM string matches the
wide character encoding specified for the main program (the default
being brackets encoding if no coding method was specified with -gnatW).

@table @asis
@item Hex Coding
In this encoding, a wide character is represented by a five character
sequence:

@smallexample
ESC a b c d
@end smallexample

where @var{a}, @var{b}, @var{c}, @var{d} are the four hexadecimal
characters (using upper case letters) of the wide character code. For
example, ESC A345 is used to represent the wide character with code
16#A345#. This scheme is compatible with use of the full
@code{Wide_Character} set.

@item Upper Half Coding
The wide character with encoding 16#abcd#, where the upper bit is on
(i.e.@: a is in the range 8-F) is represented as two bytes 16#ab# and
16#cd#. The second byte may never be a format control character, but is
not required to be in the upper half. This method can be also used for
shift-JIS or EUC where the internal coding matches the external coding. 

@item Shift JIS Coding
A wide character is represented by a two character sequence 16#ab# and
16#cd#, with the restrictions described for upper half encoding as
described above. The internal character code is the corresponding JIS
character according to the standard algorithm for Shift-JIS
conversion. Only characters defined in the JIS code set table can be
used with this encoding method.

@item EUC Coding
A wide character is represented by a two character sequence 16#ab# and
16#cd#, with both characters being in the upper half. The internal
character code is the corresponding JIS character according to the EUC
encoding algorithm. Only characters defined in the JIS code set table
can be used with this encoding method.

@item UTF-8 Coding
A wide character is represented using
UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO
10646-1/Am.2.  Depending on the character value, the representation
is a one, two, or three byte sequence:

@smallexample
16#0000#-16#007f#: 2#0xxxxxxx#
16#0080#-16#07ff#: 2#110xxxxx# 2#10xxxxxx#
16#0800#-16#ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#
@end smallexample

where the xxx bits correspond to the left-padded bits of the
16-bit character value. Note that all lower half ASCII characters
are represented as ASCII bytes and all upper half characters and
other wide characters are represented as sequences of upper-half
(The full UTF-8 scheme allows for encoding 31-bit characters as
6-byte sequences, but in this implementation, all UTF-8 sequences
of four or more bytes length will raise a Constraint_Error, as
will all illegal UTF-8 sequences.)

@item Brackets Coding
In this encoding, a wide character is represented by the following eight
character sequence:

@smallexample
[ " a b c d " ]
@end smallexample

Where @code{a}, @code{b}, @code{c}, @code{d} are the four hexadecimal
characters (using uppercase letters) of the wide character code. For
example, @code{["A345"]} is used to represent the wide character with code
@code{16#A345#}.
This scheme is compatible with use of the full Wide_Character set.
On input, brackets coding can also be used for upper half characters,
e.g.@: @code{["C1"]} for lower case a. However, on output, brackets notation
is only used for wide characters with a code greater than @code{16#FF#}.

@end table

For the coding schemes other than Hex and Brackets encoding,
not all wide character
values can be represented. An attempt to output a character that cannot
be represented using the encoding scheme for the file causes
Constraint_Error to be raised. An invalid wide character sequence on
input also causes Constraint_Error to be raised.

@menu
* Wide_Text_IO Stream Pointer Positioning::  
* Wide_Text_IO Reading and Writing Non-Regular Files::  
@end menu

@node Wide_Text_IO Stream Pointer Positioning
@subsection Stream Pointer Positioning

@noindent
@code{Ada.Wide_Text_IO} is similar to @code{Ada.Text_IO} in its handling
of stream pointer positioning (@pxref{Text_IO}). There is one additional
case:

If @code{Ada.Wide_Text_IO.Look_Ahead} reads a character outside the
normal lower ASCII set (i.e.@: a character in the range:

@smallexample
Wide_Character'Val (16#0080#) .. Wide_Character'Val (16#FFFF#)
@end smallexample

@noindent
then although the logical position of the file pointer is unchanged by
the @code{Look_Ahead} call, the stream is physically positioned past the
wide character sequence. Again this is to avoid the need for buffering
or backup, and all @code{Wide_Text_IO} routines check the internal
indication that this situation has occurred so that this is not visible
to a normal program using @code{Wide_Text_IO}. However, this discrepancy
can be observed if the wide text file shares a stream with another file.

@node Wide_Text_IO Reading and Writing Non-Regular Files
@subsection Reading and Writing Non-Regular Files

@noindent
As in the case of Text_IO, when a non-regular file is read, it is
assumed that the file contains no page marks (any form characters are
treated as data characters), and @code{End_Of_Page} always returns
@code{False}. Similarly, the end of file indication is not sticky, so
it is possible to read beyond an end of file.

@node Stream_IO
@section Stream_IO

@noindent
A stream file is a sequence of bytes, where individual elements are
written to the file as described in the Ada 95 reference manual. The type
@code{Stream_Element} is simply a byte. There are two ways to read or
write a stream file.

@itemize @bullet
@item
The operations @code{Read} and @code{Write} directly read or write a
sequence of stream elements with no control information.

@item
The stream attributes applied to a stream file transfer data in the
manner described for stream attributes.
@end itemize

@node Shared Files
@section Shared Files

@noindent
Section A.14 of the Ada 95 Reference Manual allows implementations to
provide a wide variety of behavior if an attempt is made to access the
same external file with two or more internal files.

To provide a full range of functionality, while at the same time
minimizing the problems of portability caused by this implementation
dependence, GNAT handles file sharing as follows:

@itemize @bullet
@item
In the absence of a @samp{shared=@var{xxx}} form parameter, an attempt
to open two or more files with the same full name is considered an error
and is not supported. The exception @code{Use_Error} will be
raised. Note that a file that is not explicitly closed by the program
remains open until the program terminates.

@item
If the form parameter @samp{shared=no} appears in the form string, the
file can be opened or created with its own separate stream identifier,
regardless of whether other files sharing the same external file are
opened. The exact effect depends on how the C stream routines handle
multiple accesses to the same external files using separate streams.

@item
If the form parameter @samp{shared=yes} appears in the form string for
each of two or more files opened using the same full name, the same
stream is shared between these files, and the semantics are as described
in Ada 95 Reference Manual, Section A.14.
@end itemize

When a program that opens multiple files with the same name is ported
from another Ada compiler to GNAT, the effect will be that
@code{Use_Error} is raised.

The documentation of the original compiler and the documentation of the
program should then be examined to determine if file sharing was
expected, and @samp{shared=@var{xxx}} parameters added to @code{Open}
and @code{Create} calls as required.

When a program is ported from GNAT to some other Ada compiler, no
special attention is required unless the @samp{shared=@var{xxx}} form
parameter is used in the program. In this case, you must examine the
documentation of the new compiler to see if it supports the required
file sharing semantics, and form strings modified appropriately. Of
course it may be the case that the program cannot be ported if the
target compiler does not support the required functionality. The best
approach in writing portable code is to avoid file sharing (and hence
the use of the @samp{shared=@var{xxx}} parameter in the form string)
completely.

One common use of file sharing in Ada 83 is the use of instantiations of
Sequential_IO on the same file with different types, to achieve
heterogeneous input-output. Although this approach will work in GNAT if
@samp{shared=yes} is specified, it is preferable in Ada 95 to use Stream_IO
for this purpose (using the stream attributes)

@node Open Modes
@section Open Modes

@noindent
@code{Open} and @code{Create} calls result in a call to @code{fopen}
using the mode shown in Table 6.1

@sp 2
@center Table 6-1 @code{Open} and @code{Create} Call Modes
@smallexample
                               @b{OPEN }           @b{CREATE}
Append_File                    "r+"             "w+"
In_File                        "r"              "w+"
Out_File (Direct_IO)           "r+"             "w"
Out_File (all other cases)     "w"              "w"
Inout_File                     "r+"             "w+"
@end smallexample

If text file translation is required, then either @samp{b} or @samp{t}
is added to the mode, depending on the setting of Text. Text file
translation refers to the mapping of CR/LF sequences in an external file
to LF characters internally. This mapping only occurs in DOS and
DOS-like systems, and is not relevant to other systems.

A special case occurs with Stream_IO@. As shown in the above table, the
file is initially opened in @samp{r} or @samp{w} mode for the
@code{In_File} and @code{Out_File} cases. If a @code{Set_Mode} operation
subsequently requires switching from reading to writing or vice-versa,
then the file is reopened in @samp{r+} mode to permit the required operation.

@node Operations on C Streams
@section Operations on C Streams
The package @code{Interfaces.C_Streams} provides an Ada program with direct
access to the C library functions for operations on C streams:

@smallexample
package Interfaces.C_Streams is
  -- Note: the reason we do not use the types that are in
  -- Interfaces.C is that we want to avoid dragging in the
  -- code in this unit if possible.
  subtype chars is System.Address;
  -- Pointer to null-terminated array of characters
  subtype FILEs is System.Address;
  -- Corresponds to the C type FILE*
  subtype voids is System.Address;
  -- Corresponds to the C type void*
  subtype int is Integer;
  subtype long is Long_Integer;
  -- Note: the above types are subtypes deliberately, and it
  -- is part of this spec that the above correspondences are
  -- guaranteed. This means that it is legitimate to, for
  -- example, use Integer instead of int. We provide these
  -- synonyms for clarity, but in some cases it may be
  -- convenient to use the underlying types (for example to
  -- avoid an unnecessary dependency of a spec on the spec
  -- of this unit).
  type size_t is mod 2 ** Standard'Address_Size;
  NULL_Stream : constant FILEs;
  -- Value returned (NULL in C) to indicate an
  -- fdopen/fopen/tmpfile error
  ----------------------------------
  -- Constants Defined in stdio.h --
  ----------------------------------
  EOF : constant int;
  -- Used by a number of routines to indicate error or 
  -- end of file
  IOFBF : constant int;
  IOLBF : constant int;
  IONBF : constant int;
  -- Used to indicate buffering mode for setvbuf call
  SEEK_CUR : constant int;
  SEEK_END : constant int;
  SEEK_SET : constant int;
  -- Used to indicate origin for fseek call
  function stdin return FILEs;
  function stdout return FILEs;
  function stderr return FILEs;
  -- Streams associated with standard files
  --------------------------
  -- Standard C functions --
  --------------------------
  -- The functions selected below are ones that are
  -- available in DOS, OS/2, UNIX and Xenix (but not
  -- necessarily in ANSI C). These are very thin interfaces
  -- which copy exactly the C headers. For more
  -- documentation on these functions, see the Microsoft C 
  -- "Run-Time Library Reference" (Microsoft Press, 1990,
  -- ISBN 1-55615-225-6), which includes useful information
  -- on system compatibility.
  procedure clearerr (stream : FILEs);
  function fclose (stream : FILEs) return int;
  function fdopen (handle : int; mode : chars) return FILEs; 
  function feof (stream : FILEs) return int; 
  function ferror (stream : FILEs) return int; 
  function fflush (stream : FILEs) return int; 
  function fgetc (stream : FILEs) return int; 
  function fgets (strng : chars; n : int; stream : FILEs) 
      return chars; 
  function fileno (stream : FILEs) return int; 
  function fopen (filename : chars; Mode : chars) 
      return FILEs;
  -- Note: to maintain target independence, use
  -- text_translation_required, a boolean variable defined in
  -- a-sysdep.c to deal with the target dependent text
  -- translation requirement. If this variable is set, 
  -- then  b/t should be appended to the standard mode
  -- argument to set the text translation mode off or on 
  -- as required.
  function fputc (C : int; stream : FILEs) return int;
  function fputs (Strng : chars; Stream : FILEs) return int;
  function fread
     (buffer : voids;
      size : size_t;
      count : size_t;
      stream : FILEs)
      return size_t;
  function freopen
     (filename : chars;
      mode : chars;
      stream : FILEs)
      return FILEs;
  function fseek
     (stream : FILEs;
      offset : long;
      origin : int)
      return int;
  function ftell (stream : FILEs) return long;
  function fwrite
     (buffer : voids;
      size : size_t;
      count : size_t;
      stream : FILEs)
      return size_t; 
  function isatty (handle : int) return int;
  procedure mktemp (template : chars);
  -- The return value (which is just a pointer to template)
  -- is discarded
  procedure rewind (stream : FILEs);
  function rmtmp return int;
  function setvbuf
     (stream : FILEs;
      buffer : chars;
      mode : int;
      size : size_t)
      return int;

  function tmpfile return FILEs;
  function ungetc (c : int; stream : FILEs) return int;
  function unlink (filename : chars) return int;
  ---------------------
  -- Extra functions --
  ---------------------
  -- These functions supply slightly thicker bindings than
  -- those above. They are derived from functions in the 
  -- C Run-Time Library, but may do a bit more work than
  -- just directly calling one of the Library functions.
  function is_regular_file (handle : int) return int;
  -- Tests if given handle is for a regular file (result 1)
  -- or for a non-regular file (pipe or device, result 0).
  ---------------------------------
  -- Control of Text/Binary Mode --
  ---------------------------------
  -- If text_translation_required is true, then the following
  -- functions may be used to dynamically switch a file from
  -- binary to text mode or vice versa. These functions have
  -- no effect if text_translation_required is false (i.e. in
  -- normal UNIX mode). Use fileno to get a stream handle.
  procedure set_binary_mode (handle : int);
  procedure set_text_mode (handle : int);
  ----------------------------
  -- Full Path Name support --
  ----------------------------
  procedure full_name (nam : chars; buffer : chars);
  -- Given a NUL terminated string representing a file
  -- name, returns in buffer a NUL terminated string
  -- representing the full path name for the file name. 
  -- On systems where it is relevant the   drive is also
  -- part of the full path name. It is the responsibility 
  -- of the caller to pass an actual parameter for buffer
  -- that is big enough for any full path name. Use
  -- max_path_len given below as the size of buffer.
  max_path_len : integer;
  -- Maximum length of an allowable full path name on the
  -- system, including a terminating NUL character.
end Interfaces.C_Streams;
@end smallexample

@node Interfacing to C Streams
@section Interfacing to C Streams

@noindent
The packages in this section permit interfacing Ada files to C Stream
operations.

@smallexample
 with Interfaces.C_Streams;
 package Ada.Sequential_IO.C_Streams is
    function C_Stream (F : File_Type)  
       return Interfaces.C_Streams.FILEs;
    procedure Open
      (File : in out File_Type;
       Mode : in File_Mode;
       C_Stream : in Interfaces.C_Streams.FILEs;
       Form : in String := "");
 end Ada.Sequential_IO.C_Streams;

  with Interfaces.C_Streams;
  package Ada.Direct_IO.C_Streams is
     function C_Stream (F : File_Type) 
        return Interfaces.C_Streams.FILEs;
     procedure Open
       (File : in out File_Type;
        Mode : in File_Mode;
        C_Stream : in Interfaces.C_Streams.FILEs;
        Form : in String := "");
  end Ada.Direct_IO.C_Streams;

  with Interfaces.C_Streams;
  package Ada.Text_IO.C_Streams is
     function C_Stream (F : File_Type)
        return Interfaces.C_Streams.FILEs;
     procedure Open
       (File : in out File_Type;
        Mode : in File_Mode;
        C_Stream : in Interfaces.C_Streams.FILEs;
        Form : in String := "");
  end Ada.Text_IO.C_Streams;

  with Interfaces.C_Streams;
  package Ada.Wide_Text_IO.C_Streams is
     function C_Stream (F : File_Type)  
        return Interfaces.C_Streams.FILEs;
     procedure Open
       (File : in out File_Type;
        Mode : in File_Mode;
        C_Stream : in Interfaces.C_Streams.FILEs;
        Form : in String := "");
 end Ada.Wide_Text_IO.C_Streams;

 with Interfaces.C_Streams;
 package Ada.Stream_IO.C_Streams is
    function C_Stream (F : File_Type)
       return Interfaces.C_Streams.FILEs;
    procedure Open
      (File : in out File_Type;
       Mode : in File_Mode;
       C_Stream : in Interfaces.C_Streams.FILEs;
       Form : in String := "");
 end Ada.Stream_IO.C_Streams;
@end smallexample

In each of these five packages, the @code{C_Stream} function obtains the
@code{FILE} pointer from a currently opened Ada file. It is then
possible to use the @code{Interfaces.C_Streams} package to operate on
this stream, or the stream can be passed to a C program which can
operate on it directly. Of course the program is responsible for
ensuring that only appropriate sequences of operations are executed.

One particular use of relevance to an Ada program is that the
@code{setvbuf} function can be used to control the buffering of the
stream used by an Ada file. In the absence of such a call the standard
default buffering is used.

The @code{Open} procedures in these packages open a file giving an
existing C Stream instead of a file name. Typically this stream is
imported from a C program, allowing an Ada file to operate on an
existing C file.

@node The GNAT Library
@chapter The GNAT Library

@noindent
The GNAT library contains a number of general and special purpose packages.
It represents functionality that the GNAT developers have found useful, and
which is made available to GNAT users. The packages described here are fully
supported, and upwards compatibility will be maintained in future releases,
so you can use these facilities with the confidence that the same functionality
will be available in future releases.

The chapter here simply gives a brief summary of the facilities available.
The full documentation is found in the spec file for the package. The full
sources of these library packages, including both spec and body, are provided
with all GNAT releases. For example, to find out the full specifications of
the SPITBOL pattern matching capability, including a full tutorial and 
extensive examples, look in the g-spipat.ads file in the library.

For each entry here, the package name (as it would appear in a @code{with}
clause) is given, followed by the name of the corresponding spec file in
parentheses. The packages are children in four hierarchies, @code{Ada},
@code{Interfaces}, @code{System}, and @code{GNAT}, the latter being a
GNAT-specific hierarchy.

Note that an application program should only use packages in one of these
four hierarchies if the package is defined in the Ada Reference Manual,
or is listed in this section of the GNAT Programmers Reference Manual.
All other units should be considered internal implementation units and
should not be directly @code{with}'ed by application code. The use of
a @code{with} statement that references one of these internal implementation
units makes an application potentially dependent on changes in versions
of GNAT, and will generate a warning message.

@menu
* Ada.Characters.Wide_Latin_1 (a-cwila1.ads)::
* Ada.Command_Line.Remove (a-colire.ads)::
* Ada.Direct_IO.C_Streams (a-diocst.ads)::
* Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads)::
* Ada.Sequential_IO.C_Streams (a-siocst.ads)::
* Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads)::
* Ada.Strings.Unbounded.Text_IO (a-suteio.ads)::
* Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads)::
* Ada.Text_IO.C_Streams (a-tiocst.ads)::
* Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads)::
* GNAT.AWK (g-awk.ads)::
* GNAT.Bubble_Sort_A (g-busora.ads)::
* GNAT.Bubble_Sort_G (g-busorg.ads)::
* GNAT.Calendar (g-calend.ads)::
* GNAT.Calendar.Time_IO (g-catiio.ads)::
* GNAT.CRC32 (g-crc32.ads)::
* GNAT.Case_Util (g-casuti.ads)::
* GNAT.CGI (g-cgi.ads)::
* GNAT.CGI.Cookie (g-cgicoo.ads)::
* GNAT.CGI.Debug (g-cgideb.ads)::
* GNAT.Command_Line (g-comlin.ads)::
* GNAT.Current_Exception (g-curexc.ads)::
* GNAT.Debug_Pools (g-debpoo.ads)::
* GNAT.Debug_Utilities (g-debuti.ads)::
* GNAT.Directory_Operations (g-dirope.ads)::
* GNAT.Dynamic_Tables (g-dyntab.ads)::
* GNAT.Exception_Traces (g-exctra.ads)::
* GNAT.Expect (g-expect.ads)::
* GNAT.Float_Control (g-flocon.ads)::
* GNAT.Heap_Sort_A (g-hesora.ads)::
* GNAT.Heap_Sort_G (g-hesorg.ads)::
* GNAT.HTable (g-htable.ads)::
* GNAT.IO (g-io.ads)::
* GNAT.IO_Aux (g-io_aux.ads)::
* GNAT.Lock_Files (g-locfil.ads)::
* GNAT.Most_Recent_Exception (g-moreex.ads)::
* GNAT.OS_Lib (g-os_lib.ads)::
* GNAT.Regexp (g-regexp.ads)::
* GNAT.Registry (g-regist.ads)::
* GNAT.Regpat (g-regpat.ads)::
* GNAT.Sockets (g-socket.ads)::
* GNAT.Source_Info (g-souinf.ads)::
* GNAT.Spell_Checker (g-speche.ads)::
* GNAT.Spitbol.Patterns (g-spipat.ads)::
* GNAT.Spitbol (g-spitbo.ads)::
* GNAT.Spitbol.Table_Boolean (g-sptabo.ads)::
* GNAT.Spitbol.Table_Integer (g-sptain.ads)::
* GNAT.Spitbol.Table_VString (g-sptavs.ads)::
* GNAT.Table (g-table.ads)::
* GNAT.Task_Lock (g-tasloc.ads)::
* GNAT.Threads (g-thread.ads)::
* GNAT.Traceback (g-traceb.ads)::
* GNAT.Traceback.Symbolic (g-trasym.ads)::
* Interfaces.C.Extensions (i-cexten.ads)::
* Interfaces.C.Streams (i-cstrea.ads)::
* Interfaces.CPP (i-cpp.ads)::
* Interfaces.Os2lib (i-os2lib.ads)::
* Interfaces.Os2lib.Errors (i-os2err.ads)::
* Interfaces.Os2lib.Synchronization (i-os2syn.ads)::
* Interfaces.Os2lib.Threads (i-os2thr.ads)::
* Interfaces.Packed_Decimal (i-pacdec.ads)::
* Interfaces.VxWorks (i-vxwork.ads)::
* System.Address_Image (s-addima.ads)::
* System.Assertions (s-assert.ads)::
* System.Partition_Interface (s-parint.ads)::
* System.Task_Info (s-tasinf.ads)::
* System.Wch_Cnv (s-wchcnv.ads)::
* System.Wch_Con (s-wchcon.ads)::
@end menu

@node Ada.Characters.Wide_Latin_1 (a-cwila1.ads)
@section Ada.Characters.Wide_Latin_1 (a-cwila1.ads)
@cindex Ada.Characters.Wide_Latin_1 (a-cwila1.ads)
@cindex  Latin_1 constants for Wide_Character

@noindent
This child of @code{Ada.Characters}
provides a set of definitions corresponding to those in the
RM-defined package @code{Ada.Characters.Latin_1} but with the
types of the constants being @code{Wide_Character}
instead of @code{Character}. The provision of such a package
is specifically authorized by the Ada Reference Manual
(RM A.3(27)).

@node Ada.Command_Line.Remove (a-colire.ads)
@section Ada.Command_Line.Remove (a-colire.ads)
@cindex Ada.Command_Line.Remove (a-colire.ads)
@cindex  Removing command line arguments
@cindex  Command line, argument removal

@noindent
This child of @code{Ada.Command_Line}
provides a mechanism for logically removing
arguments from the argument list. Once removed, an argument is not visible
to further calls on the subprograms in @code{Ada.Command_Line} will not
see the removed argument.

@node Ada.Direct_IO.C_Streams (a-diocst.ads)
@section Ada.Direct_IO.C_Streams (a-diocst.ads)
@cindex Ada.Direct_IO.C_Streams (a-diocst.ads)
@cindex  C Streams, Interfacing with Direct_IO

@noindent
This package provides subprograms that allow interfacing between 
C streams and @code{Direct_IO}. The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads)
@section Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads)
@cindex Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads)
@cindex  Null_Occurrence, testing for

@noindent
This child subprogram provides a way of testing for the null 
exception occurrence (@code{Null_Occurrence}) without raising
an exception.

@node Ada.Sequential_IO.C_Streams (a-siocst.ads)
@section Ada.Sequential_IO.C_Streams (a-siocst.ads)
@cindex Ada.Sequential_IO.C_Streams (a-siocst.ads)
@cindex  C Streams, Interfacing with Sequential_IO

@noindent
This package provides subprograms that allow interfacing between 
C streams and @code{Sequential_IO}. The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads)
@section Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads)
@cindex Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads)
@cindex  C Streams, Interfacing with Stream_IO

@noindent
This package provides subprograms that allow interfacing between 
C streams and @code{Stream_IO}. The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada.Strings.Unbounded.Text_IO (a-suteio.ads)
@section Ada.Strings.Unbounded.Text_IO (a-suteio.ads)
@cindex Ada.Strings.Unbounded.Text_IO (a-suteio.ads)
@cindex Unbounded_String, IO support
@cindex Text_IO, extensions for unbounded strings

@noindent
This package provides subprograms for Text_IO for unbounded
strings, avoiding the necessity for an intermediate operation
with ordinary strings.

@node Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads)
@section Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads)
@cindex Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads)
@cindex Unbounded_Wide_String, IO support
@cindex Text_IO, extensions for unbounded wide strings

@noindent
This package provides subprograms for Text_IO for unbounded
wide strings, avoiding the necessity for an intermediate operation
with ordinary wide strings.

@node Ada.Text_IO.C_Streams (a-tiocst.ads)
@section Ada.Text_IO.C_Streams (a-tiocst.ads)
@cindex Ada.Text_IO.C_Streams (a-tiocst.ads)
@cindex  C Streams, Interfacing with Text_IO

@noindent
This package provides subprograms that allow interfacing between 
C streams and @code{Text_IO}. The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads)
@section Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads)
@cindex Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads)
@cindex  C Streams, Interfacing with Wide_Text_IO

@noindent
This package provides subprograms that allow interfacing between 
C streams and @code{Wide_Text_IO}. The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node GNAT.AWK (g-awk.ads)
@section GNAT.AWK (g-awk.ads)
@cindex GNAT.AWK (g-awk.ads)
@cindex Parsing

@noindent
Provides AWK-like parsing functions, with an easy interface for parsing one
or more files containing formatted data. The file is viewed as a database
where each record is a line and a field is a data element in this line.

@node GNAT.Bubble_Sort_A (g-busora.ads)
@section GNAT.Bubble_Sort_A (g-busora.ads)
@cindex GNAT.Bubble_Sort_A (g-busora.ads)
@cindex Sorting

@noindent
Provides a general implementation of bubble sort usable for sorting arbitrary
data items. Move and comparison procedures are provided by passing
access-to-procedure values.

@node GNAT.Bubble_Sort_G (g-busorg.ads)
@section GNAT.Bubble_Sort_G (g-busorg.ads)
@cindex GNAT.Bubble_Sort_G (g-busorg.ads)
@cindex Sorting

@noindent
Similar to @code{Bubble_Sort_A} except that the move and sorting procedures
are provided as generic parameters, this improves efficiency, especially
if the procedures can be inlined, at the expense of duplicating code for
multiple instantiations.

@node GNAT.Calendar (g-calend.ads)
@section GNAT.Calendar (g-calend.ads)
@cindex GNAT.Calendar (g-calend.ads)
@cindex Calendar

@noindent
Extends the facilities provided by @code{Ada.Calendar} to include handling
of days of the week, an extended @code{Split} and @code{Time_Of} capability.
Also provides conversion of @code{Ada.Calendar.Time} values to and from the
C @code{timeval} format.

@node GNAT.Calendar.Time_IO (g-catiio.ads)
@section GNAT.Calendar.Time_IO (g-catiio.ads)
@cindex Calendar
@cindex Time
@cindex GNAT.Calendar.Time_IO (g-catiio.ads)

@node GNAT.CRC32 (g-crc32.ads)
@section GNAT.CRC32 (g-crc32.ads)
@cindex GNAT.CRC32 (g-crc32.ads)
@cindex CRC32

@noindent
This package implements the CRC-32 algorithm. For a full description
of this algorithm you should have a look at:
"Computation of Cyclic Redundancy Checks via Table Look-Up", Communications
of the ACM, Vol.@: 31 No.@: 8, pp.1008-1013 Aug.@: 1988. Sarwate, D.V@.

@noindent
Provides an extended capability for formatted output of time values with
full user control over the format. Modeled on the GNU Date specification.

@node GNAT.Case_Util (g-casuti.ads)
@section GNAT.Case_Util (g-casuti.ads)
@cindex GNAT.Case_Util (g-casuti.ads)
@cindex Casing utilities

@noindent
A set of simple routines for handling upper and lower casing of strings
without the overhead of the full casing tables
in @code{Ada.Characters.Handling}.

@node GNAT.CGI (g-cgi.ads)
@section GNAT.CGI (g-cgi.ads)
@cindex GNAT.CGI (g-cgi.ads)
@cindex CGI (Common Gateway Interface)

@noindent
This is a package for interfacing a GNAT program with a Web server via the
Common Gateway Interface (CGI). Basically this package parse the CGI
parameters which are a set of key/value pairs sent by the Web server. It
builds a table whose index is the key and provides some services to deal
with this table.

@node GNAT.CGI.Cookie (g-cgicoo.ads)
@section GNAT.CGI.Cookie (g-cgicoo.ads)
@cindex GNAT.CGI.Cookie (g-cgicoo.ads)
@cindex CGI (Common Gateway Interface) Cookie support

@noindent
This is a package to interface a GNAT program with a Web server via the
Common Gateway Interface (CGI). It exports services to deal with Web
cookies (piece of information kept in the Web client software).

@node GNAT.CGI.Debug (g-cgideb.ads)
@section GNAT.CGI.Debug (g-cgideb.ads)
@cindex GNAT.CGI.Debug (g-cgideb.ads)
@cindex CGI (Common Gateway Interface) debugging

@noindent
This is a package to help debugging CGI (Common Gateway Interface)
programs written in Ada.

@node GNAT.Command_Line (g-comlin.ads)
@section GNAT.Command_Line (g-comlin.ads)
@cindex GNAT.Command_Line (g-comlin.ads)
@cindex Command line

@noindent
Provides a high level interface to @code{Ada.Command_Line} facilities,
including the ability to scan for named switches with optional parameters
and expand file names using wild card notations.

@node GNAT.Current_Exception (g-curexc.ads)
@section GNAT.Current_Exception (g-curexc.ads)
@cindex GNAT.Current_Exception (g-curexc.ads)
@cindex Current exception
@cindex Exception retrieval

@noindent
Provides access to information on the current exception that has been raised
without the need for using the Ada-95 exception choice parameter specification
syntax. This is particularly useful in mimicking typical facilities for
obtaining information about exceptions provided by Ada 83 compilers.

@node GNAT.Debug_Pools (g-debpoo.ads)
@section GNAT.Debug_Pools (g-debpoo.ads)
@cindex GNAT.Debug_Pools (g-debpoo.ads)
@cindex Debugging

@noindent
Provide a debugging storage pools that helps tracking memory corruption
problems. See section "Finding memory problems with GNAT Debug Pool" in
the GNAT User's guide.

@node GNAT.Debug_Utilities (g-debuti.ads)
@section GNAT.Debug_Utilities (g-debuti.ads)
@cindex GNAT.Debug_Utilities (g-debuti.ads)
@cindex Debugging

@noindent
Provides a few useful utilities for debugging purposes, including conversion
to and from string images of address values.

@node GNAT.Directory_Operations (g-dirope.ads)
@section GNAT.Directory_Operations (g-dirope.ads)
@cindex GNAT.Directory_Operations (g-dirope.ads)
@cindex Directory operations

@noindent
Provides a set of routines for manipulating directories, including changing
the current directory, making new directories, and scanning the files in a
directory.

@node GNAT.Dynamic_Tables (g-dyntab.ads)
@section GNAT.Dynamic_Tables (g-dyntab.ads)
@cindex GNAT.Dynamic_Tables (g-dyntab.ads)
@cindex Table implementation
@cindex Arrays, extendable

@noindent
A generic package providing a single dimension array abstraction where the
length of the array can be dynamically modified.

@noindent
This package provides a facility similar to that of GNAT.Table, except 
that this package declares a type that can be used to define dynamic
instances of the table, while an instantiation of GNAT.Table creates a
single instance of the table type.

@node GNAT.Exception_Traces (g-exctra.ads)
@section GNAT.Exception_Traces (g-exctra.ads)
@cindex GNAT.Exception_Traces (g-exctra.ads)
@cindex Exception traces
@cindex Debugging

@noindent
Provides an interface allowing to control automatic output upon exception
occurrences.

@node GNAT.Expect (g-expect.ads)
@section GNAT.Expect (g-expect.ads)
@cindex GNAT.Expect (g-expect.ads)

@noindent
Provides a set of subprograms similar to what is available
with the standard Tcl Expect tool.
It allows you to easily spawn and communicate with an external process.
You can send commands or inputs to the process, and compare the output
with some expected regular expression.
Currently GNAT.Expect is implemented on all native GNAT ports except for
OpenVMS@. It is not implemented for cross ports, and in particular is not
implemented for VxWorks or LynxOS@.

@node GNAT.Float_Control (g-flocon.ads)
@section GNAT.Float_Control (g-flocon.ads)
@cindex GNAT.Float_Control (g-flocon.ads)
@cindex Floating-Point Processor

@noindent
Provides an interface for resetting the floating-point processor into the
mode required for correct semantic operation in Ada. Some third party
library calls may cause this mode to be modified, and the Reset procedure
in this package can be used to reestablish the required mode.

@node GNAT.Heap_Sort_A (g-hesora.ads)
@section GNAT.Heap_Sort_A (g-hesora.ads)
@cindex GNAT.Heap_Sort_A (g-hesora.ads)
@cindex Sorting

@noindent
Provides a general implementation of heap sort usable for sorting arbitrary
data items. Move and comparison procedures are provided by passing
access-to-procedure values. The algorithm used is a modified heap sort
that performs approximately N*log(N) comparisons in the worst case.

@node GNAT.Heap_Sort_G (g-hesorg.ads)
@section GNAT.Heap_Sort_G (g-hesorg.ads)
@cindex GNAT.Heap_Sort_G (g-hesorg.ads)
@cindex Sorting

@noindent
Similar to @code{Heap_Sort_A} except that the move and sorting procedures
are provided as generic parameters, this improves efficiency, especially
if the procedures can be inlined, at the expense of duplicating code for
multiple instantiations.

@node GNAT.HTable (g-htable.ads)
@section GNAT.HTable (g-htable.ads)
@cindex GNAT.HTable (g-htable.ads)
@cindex Hash tables

@noindent
A generic implementation of hash tables that can be used to hash arbitrary
data. Provides two approaches, one a simple static approach, and the other
allowing arbitrary dynamic hash tables.

@node GNAT.IO (g-io.ads)
@section GNAT.IO (g-io.ads)
@cindex GNAT.IO (g-io.ads)
@cindex Simple I/O
@cindex Input/Output facilities

@noindent
A simple preealborable input-output package that provides a subset of
simple Text_IO functions for reading characters and strings from
Standard_Input, and writing characters, strings and integers to either
Standard_Output or Standard_Error.

@node GNAT.IO_Aux (g-io_aux.ads)
@section GNAT.IO_Aux (g-io_aux.ads)
@cindex GNAT.IO_Aux (g-io_aux.ads)
@cindex Text_IO
@cindex Input/Output facilities

Provides some auxiliary functions for use with Text_IO, including a test
for whether a file exists, and functions for reading a line of text.

@node GNAT.Lock_Files (g-locfil.ads)
@section GNAT.Lock_Files (g-locfil.ads)
@cindex GNAT.Lock_Files (g-locfil.ads)
@cindex File locking
@cindex Locking using files

@noindent
Provides a general interface for using files as locks. Can be used for
providing program level synchronization. 

@node GNAT.Most_Recent_Exception (g-moreex.ads)
@section GNAT.Most_Recent_Exception (g-moreex.ads)
@cindex GNAT.Most_Recent_Exception (g-moreex.ads)
@cindex Exception, obtaining most recent

@noindent
Provides access to the most recently raised exception. Can be used for
various logging purposes, including duplicating functionality of some
Ada 83 implementation dependent extensions.

@node GNAT.OS_Lib (g-os_lib.ads)
@section GNAT.OS_Lib (g-os_lib.ads)
@cindex GNAT.OS_Lib (g-os_lib.ads)
@cindex Operating System interface
@cindex Spawn capability

@noindent
Provides a range of target independent operating system interface functions,
including time/date management, file operations, subprocess management,
including a portable spawn procedure, and access to environment variables
and error return codes.

@node GNAT.Regexp (g-regexp.ads)
@section GNAT.Regexp (g-regexp.ads)
@cindex GNAT.Regexp (g-regexp.ads)
@cindex Regular expressions
@cindex Pattern matching

@noindent
A simple implementation of regular expressions, using a subset of regular
expression syntax copied from familiar Unix style utilities. This is the
simples of the three pattern matching packages provided, and is particularly
suitable for "file globbing" applications.

@node GNAT.Registry (g-regist.ads)
@section GNAT.Registry (g-regist.ads)
@cindex GNAT.Registry (g-regist.ads)
@cindex Windows Registry

@noindent
This is a high level binding to the Windows registry. It is possible to
do simple things like reading a key value, creating a new key. For full
registry API, but at a lower level of abstraction, refer to the Win32.Winreg
package provided with the Win32Ada binding

@node GNAT.Regpat (g-regpat.ads)
@section GNAT.Regpat (g-regpat.ads)
@cindex GNAT.Regpat (g-regpat.ads)
@cindex Regular expressions
@cindex Pattern matching

@noindent
A complete implementation of Unix-style regular expression matching, copied
from the original V7 style regular expression library written in C by
Henry Spencer (and binary compatible with this C library).

@node GNAT.Sockets (g-socket.ads)
@section GNAT.Sockets (g-socket.ads)
@cindex GNAT.Sockets (g-socket.ads)
@cindex Sockets

@noindent
A high level and portable interface to develop sockets based applications.
This package is based on the sockets thin binding found in GNAT.Sockets.Thin.
Currently GNAT.Sockets is implemented on all native GNAT ports except for
OpenVMS@. It is not implemented for cross ports, and in particular is not
implemented for VxWorks or LynxOS@.

@node GNAT.Source_Info (g-souinf.ads)
@section GNAT.Source_Info (g-souinf.ads)
@cindex GNAT.Source_Info (g-souinf.ads)
@cindex Source Information

@noindent
Provides subprograms that give access to source code information known at
compile time, such as the current file name and line number.

@node GNAT.Spell_Checker (g-speche.ads)
@section GNAT.Spell_Checker (g-speche.ads)
@cindex GNAT.Spell_Checker (g-speche.ads)
@cindex Spell checking

@noindent
Provides a function for determining whether one string is a plausible
near misspelling of another string.

@node GNAT.Spitbol.Patterns (g-spipat.ads)
@section GNAT.Spitbol.Patterns (g-spipat.ads)
@cindex GNAT.Spitbol.Patterns (g-spipat.ads)
@cindex SPITBOL pattern matching
@cindex Pattern matching

@noindent
A complete implementation of SNOBOL4 style pattern matching. This is the
most elaborate of the pattern matching packages provided. It fully duplicates
the SNOBOL4 dynamic pattern construction and matching capabilities, using the
efficient algorithm developed by Robert Dewar for the SPITBOL system.

@node GNAT.Spitbol (g-spitbo.ads)
@section GNAT.Spitbol (g-spitbo.ads)
@cindex GNAT.Spitbol (g-spitbo.ads)
@cindex SPITBOL interface

@noindent
The top level package of the collection of SPITBOL-style functionality, this
package provides basic SNOBOL4 string manipulation functions, such as
Pad, Reverse, Trim, Substr capability, as well as a generic table function
useful for constructing arbitrary mappings from strings in the style of
the SNOBOL4 TABLE function.

@node GNAT.Spitbol.Table_Boolean (g-sptabo.ads)
@section GNAT.Spitbol.Table_Boolean (g-sptabo.ads)
@cindex GNAT.Spitbol.Table_Boolean (g-sptabo.ads)
@cindex Sets of strings
@cindex SPITBOL Tables

@noindent
A library level of instantiation of @code{GNAT.Spitbol.Patterns.Table}
for type @code{Standard.Boolean}, giving an implementation of sets of
string values.

@node GNAT.Spitbol.Table_Integer (g-sptain.ads)
@section GNAT.Spitbol.Table_Integer (g-sptain.ads)
@cindex GNAT.Spitbol.Table_Integer (g-sptain.ads)
@cindex Integer maps
@cindex Maps
@cindex SPITBOL Tables

@noindent
A library level of instantiation of @code{GNAT.Spitbol.Patterns.Table}
for type @code{Standard.Integer}, giving an implementation of maps
from string to integer values.

@node GNAT.Spitbol.Table_VString (g-sptavs.ads)
@section GNAT.Spitbol.Table_VString (g-sptavs.ads)
@cindex GNAT.Spitbol.Table_VString (g-sptavs.ads)
@cindex String maps
@cindex Maps
@cindex SPITBOL Tables

@noindent
A library level of instantiation of GNAT.Spitbol.Patterns.Table for 
a variable length string type, giving an implementation of general
maps from strings to strings.

@node GNAT.Table (g-table.ads)
@section GNAT.Table (g-table.ads)
@cindex GNAT.Table (g-table.ads)
@cindex Table implementation
@cindex Arrays, extendable

@noindent
A generic package providing a single dimension array abstraction where the
length of the array can be dynamically modified.

@noindent
This package provides a facility similar to that of GNAT.Dynamic_Tables,
except that this package declares a single instance of the table type,
while an instantiation of GNAT.Dynamic_Tables creates a type that can be
used to define dynamic instances of the table.

@node GNAT.Task_Lock (g-tasloc.ads)
@section GNAT.Task_Lock (g-tasloc.ads)
@cindex GNAT.Task_Lock (g-tasloc.ads)
@cindex Task synchronization
@cindex Task locking
@cindex Locking

@noindent
A very simple facility for locking and unlocking sections of code using a
single global task lock. Appropriate for use in situations where contention
between tasks is very rarely expected.

@node GNAT.Threads (g-thread.ads)
@section GNAT.Threads (g-thread.ads)
@cindex GNAT.Threads (g-thread.ads)
@cindex Foreign threads
@cindex Threads, foreign

@noindent
Provides facilities for creating and destroying threads with explicit calls.
These threads are known to the GNAT run-time system. These subprograms are
exported C-convention procedures intended to be called from foreign code.
By using these primitives rather than directly calling operating systems
routines, compatibility with the Ada tasking runt-time is provided.

@node GNAT.Traceback (g-traceb.ads)
@section GNAT.Traceback (g-traceb.ads)
@cindex GNAT.Traceback (g-traceb.ads)
@cindex Trace back facilities

@noindent
Provides a facility for obtaining non-symbolic traceback information, useful
in various debugging situations.

@node GNAT.Traceback.Symbolic (g-trasym.ads)
@section GNAT.Traceback.Symbolic (g-trasym.ads)
@cindex GNAT.Traceback.Symbolic (g-trasym.ads)
@cindex Trace back facilities

@noindent
Provides symbolic traceback information that includes the subprogram
name and line number information.

@node Interfaces.C.Extensions (i-cexten.ads)
@section Interfaces.C.Extensions (i-cexten.ads)
@cindex Interfaces.C.Extensions (i-cexten.ads)

@noindent
This package contains additional C-related definitions, intended
for use with either manually or automatically generated bindings
to C libraries.

@node Interfaces.C.Streams (i-cstrea.ads)
@section Interfaces.C.Streams (i-cstrea.ads)
@cindex Interfaces.C.Streams (i-cstrea.ads)
@cindex  C streams, interfacing

@noindent
This package is a binding for the most commonly used operations
on C streams.

@node Interfaces.CPP (i-cpp.ads)
@section Interfaces.CPP (i-cpp.ads)
@cindex Interfaces.CPP (i-cpp.ads)
@cindex  C++ interfacing
@cindex  Interfacing, to C++

@noindent
This package provides facilities for use in interfacing to C++. It
is primarily intended to be used in connection with automated tools
for the generation of C++ interfaces.

@node Interfaces.Os2lib (i-os2lib.ads)
@section Interfaces.Os2lib (i-os2lib.ads)
@cindex Interfaces.Os2lib (i-os2lib.ads)
@cindex Interfacing, to OS/2
@cindex OS/2 interfacing

@noindent
This package provides interface definitions to the OS/2 library.
It is a thin binding which is a direct translation of the
various @file{<bse@.h>} files.

@node Interfaces.Os2lib.Errors (i-os2err.ads)
@section Interfaces.Os2lib.Errors (i-os2err.ads)
@cindex Interfaces.Os2lib.Errors (i-os2err.ads)
@cindex OS/2 Error codes
@cindex Interfacing, to OS/2
@cindex OS/2 interfacing

@noindent
This package provides definitions of the OS/2 error codes.

@node Interfaces.Os2lib.Synchronization (i-os2syn.ads)
@section Interfaces.Os2lib.Synchronization (i-os2syn.ads)
@cindex Interfaces.Os2lib.Synchronization (i-os2syn.ads)
@cindex Interfacing, to OS/2
@cindex Synchronization, OS/2
@cindex OS/2 synchronization primitives

@noindent
This is a child package that provides definitions for interfacing
to the @code{OS/2} synchronization primitives.

@node Interfaces.Os2lib.Threads (i-os2thr.ads)
@section Interfaces.Os2lib.Threads (i-os2thr.ads)
@cindex Interfaces.Os2lib.Threads (i-os2thr.ads)
@cindex Interfacing, to OS/2
@cindex Thread control, OS/2
@cindex OS/2 thread interfacing

@noindent
This is a child package that provides definitions for interfacing
to the @code{OS/2} thread primitives.

@node Interfaces.Packed_Decimal (i-pacdec.ads)
@section Interfaces.Packed_Decimal (i-pacdec.ads)
@cindex Interfaces.Packed_Decimal (i-pacdec.ads)
@cindex  IBM Packed Format
@cindex  Packed Decimal

@noindent
This package provides a set of routines for conversions to and
from a packed decimal format compatible with that used on IBM
mainframes.

@node Interfaces.VxWorks (i-vxwork.ads)
@section Interfaces.VxWorks (i-vxwork.ads)
@cindex Interfaces.VxWorks (i-vxwork.ads)
@cindex Interfacing to VxWorks
@cindex VxWorks, interfacing

@noindent
This package provides a limited binding to the VxWorks API
In particular, it interfaces with the
VxWorks hardware interrupt facilities

@node System.Address_Image (s-addima.ads)
@section System.Address_Image (s-addima.ads)
@cindex System.Address_Image (s-addima.ads)
@cindex Address image
@cindex Image, of an address

@noindent
This function provides a useful debugging
function that gives an (implementation dependent)
string which identifies an address.

@node System.Assertions (s-assert.ads)
@section System.Assertions (s-assert.ads)
@cindex System.Assertions (s-assert.ads)
@cindex  Assertions
@cindex  Assert_Failure, exception

@noindent
This package provides the declaration of the exception raised
by an run-time assertion failure, as well as the routine that
is used internally to raise this assertion.

@node System.Partition_Interface (s-parint.ads)
@section System.Partition_Interface (s-parint.ads)
@cindex System.Partition_Interface (s-parint.ads)
@cindex Partition intefacing functions

@noindent
This package provides facilities for partition interfacing. It
is used primarily in a distribution context when using Annex E
with @code{GLADE}.

@node System.Task_Info (s-tasinf.ads)
@section System.Task_Info (s-tasinf.ads)
@cindex System.Task_Info (s-tasinf.ads)
@cindex  Task_Info pragma

@noindent
This package provides target dependent functionality that is used
to support the @code{Task_Info} pragma

@node System.Wch_Cnv (s-wchcnv.ads)
@section System.Wch_Cnv (s-wchcnv.ads)
@cindex System.Wch_Cnv (s-wchcnv.ads)
@cindex Wide Character, Representation
@cindex Wide String, Conversion
@cindex Representation of wide characters

@noindent
This package provides routines for converting between
wide characters and a representation as a value of type
@code{Standard.String}, using a specified wide character
encoding method. Uses definitions in
package @code{System.Wch_Con}

@node System.Wch_Con (s-wchcon.ads)
@section System.Wch_Con (s-wchcon.ads)
@cindex System.Wch_Con (s-wchcon.ads)

@noindent
This package provides definitions and descriptions of
the various methods used for encoding wide characters
in ordinary strings. These definitions are used by
the package @code{System.Wch_Cnv}.

@node Interfacing to Other Languages
@chapter Interfacing to Other Languages
@noindent
The facilities in annex B of the Ada 95 Reference Manual are fully
implemented in GNAT, and in addition, a full interface to C++ is
provided.

@menu
* Interfacing to C::          
* Interfacing to C++::          
* Interfacing to COBOL::        
* Interfacing to Fortran::      
* Interfacing to non-GNAT Ada code::
@end menu

@node Interfacing to C
@section Interfacing to C

@noindent
Interfacing to C with GNAT can use one of two approaches:

@enumerate
@item
The types in the package @code{Interfaces.C} may be used.
@item
Standard Ada types may be used directly. This may be less portable to
other compilers, but will work on all GNAT compilers, which guarantee
correspondence between the C and Ada types.
@end enumerate

@noindent
Pragma @code{Convention C} maybe applied to Ada types, but mostly has no
effect, since this is the default. The following table shows the
correspondence between Ada scalar types and the corresponding C types.

@table @code
@item Integer
@code{int}
@item Short_Integer
@code{short}
@item Short_Short_Integer
@code{signed char}
@item Long_Integer
@code{long}
@item Long_Long_Integer
@code{long long}
@item Short_Float
@code{float}
@item Float
@code{float}
@item Long_Float
@code{double}
@item Long_Long_Float
This is the longest floating-point type supported by the hardware.
@end table

@itemize @bullet
@item
Ada enumeration types map to C enumeration types directly if pragma
@code{Convention C} is specified, which causes them to have int
length. Without pragma @code{Convention C}, Ada enumeration types map to
8, 16, or 32 bits (i.e.@: C types signed char, short, int respectively)
depending on the number of values passed. This is the only case in which
pragma @code{Convention C} affects the representation of an Ada type.

@item
Ada access types map to C pointers, except for the case of pointers to
unconstrained types in Ada, which have no direct C equivalent.

@item
Ada arrays map directly to C arrays.

@item
Ada records map directly to C structures.

@item
Packed Ada records map to C structures where all members are bit fields
of the length corresponding to the @code{@var{type}'Size} value in Ada.
@end itemize

@node Interfacing to C++
@section Interfacing to C++

@noindent
The interface to C++ makes use of the following pragmas, which are
primarily intended to be constructed automatically using a binding generator
tool, although it is possible to construct them by hand. Ada Core 
Technologies does not currently supply a suitable binding generator tool.

Using these pragmas it is possible to achieve complete
inter-operability between Ada tagged types and C class definitions.
See @ref{Implementation Defined Pragmas} for more details.

@table @code
@item pragma CPP_Class ([Entity =>] @var{local_name})
The argument denotes an entity in the current declarative region that is
declared as a tagged or untagged record type. It indicates that the type
corresponds to an externally declared C++ class type, and is to be laid
out the same way that C++ would lay out the type.

@item pragma CPP_Constructor ([Entity =>] @var{local_name})
This pragma identifies an imported function (imported in the usual way
with pragma @code{Import}) as corresponding to a C++ constructor.

@item pragma CPP_Vtable @dots{}
One @code{CPP_Vtable} pragma can be present for each component of type
@code{CPP.Interfaces.Vtable_Ptr} in a record to which pragma @code{CPP_Class}
applies.
@end table

@node Interfacing to COBOL
@section Interfacing to COBOL

@noindent
Interfacing to COBOL is achieved as described in section B.4 of
the Ada 95 reference manual.

@node Interfacing to Fortran
@section Interfacing to Fortran

@noindent
Interfacing to Fortran is achieved as described in section B.5 of the
reference manual. The pragma @code{Convention Fortran}, applied to a
multi- dimensional array causes the array to be stored in column-major
order as required for convenient interface to Fortran.

@node Interfacing to non-GNAT Ada code
@section Interfacing to non-GNAT Ada code

It is possible to specify the convention Ada in a pragma Import or
pragma Export. However this refers to the calling conventions used
by GNAT, which may or may not be similar enough to those used by
some other Ada 83 or Ada 95 compiler to allow interoperation.

If arguments types are kept simple, and if the foreign compiler generally
follows system calling conventions, then it may be possible to integrate
files compiled by other Ada compilers, provided that the elaboration
issues are adequately addressed (for example by eliminating the 
need for any load time elaboration).

In particular, GNAT running on VMS is designed to
be highly compatible with the DEC Ada 83 compiler, so this is one
case in which it is possible to import foreign units of this type,
provided that the data items passed are restricted to simple scalar
values or simple record types without variants, or simple array
types with fixed bounds.

@node Machine Code Insertions
@chapter Machine Code Insertions

@noindent
Package @code{Machine_Code} provides machine code support as described
in the Ada 95 Reference Manual in two separate forms:
@itemize @bullet
@item
Machine code statements, consisting of qualified expressions that
fit the requirements of RM section 13.8.
@item
An intrinsic callable procedure, providing an alternative mechanism of
including machine instructions in a subprogram.
@end itemize

The two features are similar, and both closely related to the mechanism
provided by the asm instruction in the GNU C compiler. Full understanding
and use of the facilities in this package requires understanding the asm
instruction as described in @cite{Using and Porting GNU CC} by Richard
Stallman.  Calls to the function @code{Asm} and the procedure @code{Asm}
have identical semantic restrictions and effects as described below.
Both are provided so that the procedure call can be used as a statement,
and the function call can be used to form a code_statement.

The first example given in the GNU CC documentation is the C @code{asm}
instruction:
@smallexample
   asm ("fsinx %1 %0" : "=f" (result) : "f" (angle));
@end smallexample

@noindent
The equivalent can be written for GNAT as:

@smallexample
Asm ("fsinx %1 %0",
     My_Float'Asm_Output ("=f", result),
     My_Float'Asm_Input  ("f",  angle));
@end smallexample

The first argument to @code{Asm} is the assembler template, and is
identical to what is used in GNU CC@. This string must be a static
expression.  The second argument is the output operand list. It is
either a single @code{Asm_Output} attribute reference, or a list of such
references enclosed in parentheses (technically an array aggregate of
such references).

The @code{Asm_Output} attribute denotes a function that takes two
parameters.  The first is a string, the second is the name of a variable
of the type designated by the attribute prefix. The first (string)
argument is required to be a static expression and designates the
constraint for the parameter (e.g.@: what kind of register is
required). The second argument is the variable to be updated with the
result. The possible values for constraint are the same as those used in
the RTL, and are dependent on the configuration file used to build the
GCC back end.  If there are no output operands, then this argument may
either be omitted, or explicitly given as @code{No_Output_Operands}.

The second argument of @code{@var{my_float}'Asm_Output} functions as
though it were an @code{out} parameter, which is a little curious, but
all names have the form of expressions, so there is no syntactic
irregularity, even though normally functions would not be permitted
@code{out} parameters.  The third argument is the list of input
operands. It is either a single @code{Asm_Input} attribute reference, or
a list of such references enclosed in parentheses (technically an array
aggregate of such references).

The @code{Asm_Input} attribute denotes a function that takes two
parameters.  The first is a string, the second is an expression of the
type designated by the prefix. The first (string) argument is required
to be a static expression, and is the constraint for the parameter,
(e.g.@: what kind of register is required). The second argument is the
value to be used as the input argument. The possible values for the
constant are the same as those used in the RTL, and are dependent on
the configuration file used to built the GCC back end.

If there are no input operands, this argument may either be omitted, or
explicitly given as @code{No_Input_Operands}.  The fourth argument, not
present in the above example, is a list of register names, called the
@dfn{clobber} argument. This argument, if given, must be a static string
expression, and is a space or comma separated list of names of registers
that must be considered destroyed as a result of the @code{Asm} call. If
this argument is the null string (the default value), then the code
generator assumes that no additional registers are destroyed.

The fifth argument, not present in the above example, called the
@dfn{volatile} argument, is by default @code{False}. It can be set to
the literal value @code{True} to indicate to the code generator that all
optimizations with respect to the instruction specified should be
suppressed, and that in particular, for an instruction that has outputs,
the instruction will still be generated, even if none of the outputs are
used. See the full description in the GCC manual for further details.

The @code{Asm} subprograms may be used in two ways. First the procedure
forms can be used anywhere a procedure call would be valid, and
correspond to what the RM calls ``intrinsic'' routines. Such calls can
be used to intersperse machine instructions with other Ada statements.
Second, the function forms, which return a dummy value of the limited
private type @code{Asm_Insn}, can be used in code statements, and indeed
this is the only context where such calls are allowed.  Code statements
appear as aggregates of the form:

@smallexample
Asm_Insn'(Asm (@dots{}));
Asm_Insn'(Asm_Volatile (@dots{}));
@end smallexample

In accordance with RM rules, such code statements are allowed only
within subprograms whose entire body consists of such statements.  It is
not permissible to intermix such statements with other Ada statements.

Typically the form using intrinsic procedure calls is more convenient
and more flexible. The code statement form is provided to meet the RM
suggestion that such a facility should be made available.  The following
is the exact syntax of the call to @code{Asm} (of course if named notation is
used, the arguments may be given in arbitrary order, following the
normal rules for use of positional and named arguments)

@smallexample
ASM_CALL ::= Asm (
                 [Template =>] static_string_EXPRESSION
               [,[Outputs  =>] OUTPUT_OPERAND_LIST      ]
               [,[Inputs   =>] INPUT_OPERAND_LIST       ]
               [,[Clobber  =>] static_string_EXPRESSION ]
               [,[Volatile =>] static_boolean_EXPRESSION] )
OUTPUT_OPERAND_LIST ::=
  No_Output_Operands
| OUTPUT_OPERAND_ATTRIBUTE
| (OUTPUT_OPERAND_ATTRIBUTE @{,OUTPUT_OPERAND_ATTRIBUTE@})
OUTPUT_OPERAND_ATTRIBUTE ::=
  SUBTYPE_MARK'Asm_Output (static_string_EXPRESSION, NAME)
INPUT_OPERAND_LIST ::=
  No_Input_Operands
| INPUT_OPERAND_ATTRIBUTE
| (INPUT_OPERAND_ATTRIBUTE @{,INPUT_OPERAND_ATTRIBUTE@})
INPUT_OPERAND_ATTRIBUTE ::=
  SUBTYPE_MARK'Asm_Input (static_string_EXPRESSION, EXPRESSION)
@end smallexample

@node GNAT Implementation of Tasking
@chapter GNAT Implementation of Tasking
@menu
* Mapping Ada Tasks onto the Underlying Kernel Threads::
* Ensuring Compliance with the Real-Time Annex::
@end menu

@node Mapping Ada Tasks onto the Underlying Kernel Threads
@section Mapping Ada Tasks onto the Underlying Kernel Threads

GNAT run-time system comprises two layers:

@itemize @bullet
@item GNARL (GNAT Run-time  Layer)
@item GNULL (GNAT Low-level Library)
@end itemize

In GNAT, Ada's tasking services rely on a platform and OS independent
layer known as GNARL@. This code is responsible for implementing the
correct semantics of Ada's task creation, rendezvous, protected
operations etc.

GNARL decomposes Ada's tasking semantics into simpler lower level
operations such as create a thread, set the priority of a thread,
yield, create a lock, lock/unlock, etc. The spec for these low-level
operations constitutes GNULLI, the GNULL Interface. This interface is
directly inspired from the POSIX real-time API@.

If the underlying executive or OS implements the POSIX standard
faithfully, the GNULL Interface maps as is to the services offered by
the underlying kernel. Otherwise, some target dependent glue code maps
the services offered by the underlying kernel to the semantics expected
by GNARL@.

Whatever the underlying OS (VxWorks, UNIX, OS/2, Windows NT, etc.) the
key point is that each Ada task is mapped on a thread in the underlying 
kernel. For example, in the case of VxWorks

     1 Ada task = 1 VxWorks task

In addition Ada task priorities map onto the underlying thread priorities.
Mapping Ada tasks onto the underlying kernel threads has several advantages:

@enumerate

@item
The underlying scheduler is used to schedule the Ada tasks. This
makes Ada tasks as efficient as kernel threads from a scheduling
standpoint.  

@item
Interaction with code written in C containing threads is eased
since at the lowest level Ada tasks and C threads map onto the same
underlying kernel concept.

@item
When an Ada task is blocked during I/O the remaining Ada tasks are
able to proceed.

@item
On multi-processor systems Ada Tasks can execute in parallel.
@end enumerate

@node Ensuring Compliance with the Real-Time Annex
@section Ensuring Compliance with the Real-Time Annex

The reader will be quick to notice that while mapping Ada tasks onto
the underlying threads has significant advantages, it does create some
complications when it comes to respecting the scheduling semantics
specified in the real-time annex (Annex D).

For instance Annex D requires that for the FIFO_Within_Priorities
scheduling policy we have:

@smallexample
When the active priority of a ready task that is not running
changes, or the setting of its base priority takes effect, the
task is removed from the ready queue for its old active priority
and is added at the tail of the ready queue for its new active
priority, except in the case where the active priority is lowered
due to the loss of inherited priority, in which case the task is
added at the head of the ready queue for its new active priority.
@end smallexample

While most kernels do put tasks at the end of the priority queue when
a task changes its priority, (which respects the main
FIFO_Within_Priorities requirement), almost none keep a thread at the
beginning of its priority queue when its priority drops from the loss
of inherited priority.

As a result most vendors have provided incomplete Annex D implementations.

The GNAT run-time, has a nice cooperative solution to this problem
which ensures that accurate FIFO_Within_Priorities semantics are
respected.

The principle is as follows. When an Ada task T is about to start
running, it checks whether some other Ada task R with the same
priority as T has been suspended due to the loss of priority
inheritance. If this is the case, T yields and is placed at the end of
its priority queue. When R arrives at the front of the queue it
executes. 

Note that this simple scheme preserves the relative order of the tasks
that were ready to execute in the priority queue where R has been
placed at the end.

@node    Code generation for array aggregates
@chapter  Code generation for array aggregates

@menu
* Static constant aggregates with static bounds::
* Constant aggregates with an unconstrained nominal types::
* Aggregates with static bounds::
* Aggregates with non-static bounds::
* Aggregates in assignments statements::
@end menu
 
Aggregate have a rich syntax and allow the user to specify the values of
complex data structures by means of a single construct. As a result, the
code generated for aggregates can be quite complex and involve loops, case
statements and multiple assignments. In the simplest cases, however, the
compiler will recognize aggregates whose components and constraints are
fully static, and in those cases the compiler will generate little or no
executable code. The following is an outline of the code that GNAT generates
for various aggregate constructs. For further details, the user will find it
useful to examine the output produced by the -gnatG flag to see the expanded
source that is input to the code generator.  The user will also want to examine
the assembly code generated at various levels of optimization.

The code generated for aggregates depends on the context, the component values,
and the type. In the context of an object declaration the code generated is
generally simpler than in the case of an assignment. As a general rule, static
component values and static subtypes also lead to simpler code.
 
@node Static constant aggregates with static bounds
@section Static constant aggregates with static bounds
 
 For the declarations:
@smallexample 
    type One_Dim is array (1..10) of integer;
    ar0 : constant One_Dim := ( 1, 2, 3, 4, 5, 6, 7, 8, 9, 0);
@end smallexample 

GNAT generates no executable code: the constant ar0 is placed in static memory.
The same is true for constant aggregates with named associations:
 
@smallexample 
    Cr1 : constant One_Dim := (4 => 16, 2 => 4, 3 => 9, 1=> 1);
    Cr3 : constant One_Dim := (others => 7777);
@end smallexample 
 
 The same is true for multidimensional constant arrays such as:
 
@smallexample 
    type two_dim is array (1..3, 1..3) of integer;
    Unit : constant two_dim := ( (1,0,0), (0,1,0), (0,0,1));
@end smallexample 
 
The same is true for arrays of one-dimensional arrays: the following are
static:
 
@smallexample 
type ar1b is array (1..3) of boolean;
type ar_ar is array (1..3) of ar1b;
None : constant ar1b := (others => false);      --  fully static
None2 : constant ar_ar := (1..3 => None);       --  fully static
@end smallexample 
 
However, for multidimensional aggregates with named associations, GNAT will
generate assignments and loops, even if all associations are static. The
following two declarations generate a loop for the first dimension, and
individual component assignments for the second dimension:
 
@smallexample 
Zero1: constant two_dim := (1..3 => (1..3 => 0));
Zero2: constant two_dim := (others => (others => 0));     
@end smallexample 
 
@node Constant aggregates with an unconstrained nominal types
@section Constant aggregates with an unconstrained nominal types
 
In such cases the aggregate itself establishes the subtype, so that associations
with "others" cannot be used. GNAT determines the bounds for the actual
subtype of the aggregate, and allocates the aggregate statically as well. No
code is generated for the following:
 
@smallexample 
    type One_Unc is array (natural range <>) of integer;
    Cr_Unc : constant One_Unc := (12,24,36);
@end smallexample 
 
@node Aggregates with static bounds
@section Aggregates with static bounds
 
In all previous examples the aggregate was the initial (and immutable) value
of a constant. If the aggregate initializes a variable, then code is generated
for it as a combination of individual assignments and loops over the target
object. The declarations
 
@smallexample 
       Cr_Var1 : One_Dim := (2, 5, 7, 11);
       Cr_Var2 : One_Dim := (others > -1);
@end smallexample 
 
generate the equivalent of
 
@smallexample 
       Cr_Var1 (1) := 2;
       Cr_Var1 (2) := 3;
       Cr_Var1 (3) := 5;
       Cr_Var1 (4) := 11;
 
       for I in Cr_Var2'range loop
          Cr_Var2 (I) := =-1;
       end loop;
@end smallexample 
 
@node Aggregates with non-static bounds
@section Aggregates with non-static bounds
 
If the bounds of the aggregate are not statically compatible with the bounds
of the nominal subtype  of the target, then constraint checks have to be
generated on the bounds. For a multidimensional array, constraint checks may
have to be applied to sub-arrays individually, if they do not have statically
compatible subtypes.
 
@node Aggregates in assignments statements
@section Aggregates in assignments statements
 
In general, aggregate assignment requires the construction of a temporary,
and a copy from the temporary to the target of the assignment. This is because
it is not always possible to convert the assignment into a series of individual 
component assignments. For example, consider the simple case:
 
@smallexample 
@end smallexample 
        A := (A(2), A(1));
 
This cannot be converted into:
 
@smallexample 
        A(1) := A(2);
        A(2) := A(1);
@end smallexample 
 
So the aggregate has to be built first in a separate location, and then
copied into the target. GNAT recognizes simple cases where this intermediate
step is not required, and the assignments can be performed in place, directly
into the target. The following sufficient criteria are applied:

@enumerate 
@item The bounds of the aggregate are static, and the associations are static.
@item  The components of the aggregate are static constants, names of
    simple variables that are not renamings, or expressions not involving
    indexed components whose operands obey these rules.
@end enumerate
 
If any of these conditions are violated, the aggregate will be built in
a temporary (created either by the front-end or the code generator) and then
that temporary will be copied onto the target.
 
 
@node Specialized Needs Annexes
@chapter Specialized Needs Annexes

@noindent
Ada 95 defines a number of specialized needs annexes, which are not
required in all implementations. However, as described in this chapter,
GNAT implements all of these special needs annexes:

@table @asis
@item Systems Programming (Annex C)
The systems programming annex is fully implemented.

@item Real-Time Systems (Annex D)
The real-time systems annex is fully implemented.

@item Distributed Systems (Annex E)
Stub generation is fully implemented in the @code{GNAT} compiler. In addition,
a complete compatible PCS is available as part of the @code{GLADE} system,
a separate product available from Ada Core Technologies. When the two
products are used in conjunction, this annex is fully implemented.

@item Information Systems (Annex F)
The information systems annex is fully implemented.

@item Numerics (Annex G)
The numerics annex is fully implemented.

@item Safety and Security (Annex H)
The safety and security annex is fully implemented.

@end table

@node Compatibility Guide
@chapter Compatibility Guide

@noindent
This chapter contains sections that describe compatibility issues between
GNAT and other Ada 83 and Ada 95 compilation systems, to aid in porting
applications developed in other Ada environments.

@menu
* Compatibility with Ada 83::       
* Compatibility with DEC Ada 83::
* Compatibility with Other Ada 95 Systems::
* Representation Clauses::
@end menu

@node Compatibility with Ada 83
@section Compatibility with Ada 83

@noindent
Ada 95 is designed to be highly upwards compatible with Ada 83. In
particular, the design intention is that the difficulties associated
with moving from Ada 83 to Ada 95 should be no greater than those
that occur when moving from one Ada 83 system to another.

However, there are a number of points at which there are minor
incompatibilities. The Ada 95 Annotated Reference Manual contains
full details of these issues,
and should be consulted for a complete treatment.
In practice the
following are the most likely issues to be encountered.

@table @asis
@item Character range
The range of Standard.Character is now the full 256 characters of Latin-1,
whereas in most Ada 83 implementations it was restricted to 128 characters.
This may show up as compile time or runtime errors. The desirable fix is to
adapt the program to accommodate the full character set, but in some cases
it may be convenient to define a subtype or derived type of Character that
covers only the restricted range.

@item New reserved words
The identifiers @code{abstract}, @code{aliased}, @code{protected},
@code{requeue}, @code{tagged}, and @code{until} are reserved in Ada 95.
Existing Ada 83 code using any of these identifiers must be edited to
use some alternative name.

@item Freezing rules
The rules in Ada 95 are slightly different with regard to the point at
which entities are frozen, and representation pragmas and clauses are
not permitted past the freeze point. This shows up most typically in
the form of an error message complaining that a representation item
appears too late, and the appropriate corrective action is to move
the item nearer to the declaration of the entity to which it refers.

A particular case is that representation pragmas (including the
extended DEC Ada 83 compatibility pragmas such as Export_Procedure), cannot
be applied to a subprogram body. If necessary, a separate subprogram 
declaration must be introduced to which the pragma can be applied.

@item Optional bodies for library packages
In Ada 83, a package that did not require a package body was nevertheless
allowed to have one. This lead to certain surprises in compiling large
systems (situations in which the body could be unexpectedly ignored). In
Ada 95, if a package does not require a body then it is not permitted to
have a body. To fix this problem, simply remove a redundant body if it
is empty, or, if it is non-empty, introduce a dummy declaration into the
spec that makes the body required. One approach is to add a private part
to the package declaration (if necessary), and define a parameterless
procedure called Requires_Body, which must then be given a dummy
procedure body in the package body, which then becomes required.

@item Numeric_Error is now the same as Constraint_Error
In Ada 95, the exception Numeric_Error is a renaming of Constraint_Error.
This means that it is illegal to have separate exception handlers for
the two exceptions. The fix is simply to remove the handler for the
Numeric_Error case (since even in Ada 83, a compiler was free to raise
Constraint_Error in place of Numeric_Error in all cases).

@item Indefinite subtypes in generics
In Ada 83, it was permissible to pass an indefinite type (e.g.@: String) as
the actual for a generic formal private type, but then the instantiation
would be illegal if there were any instances of declarations of variables
of this type in the generic body. In Ada 95, to avoid this clear violation
of the contract model, the generic declaration clearly indicates whether
or not such instantiations are permitted. If a generic formal parameter
has explicit unknown discriminants, indicated by using (<>) after the
type name, then it can be instantiated with indefinite types, but no
variables can be declared of this type. Any attempt to declare a variable
will result in an illegality at the time the generic is declared. If the
(<>) notation is not used, then it is illegal to instantiate the generic
with an indefinite type. This will show up as a compile time error, and
the fix is usually simply to add the (<>) to the generic declaration.
@end table

All implementations of GNAT provide a switch that causes GNAT to operate
in Ada 83 mode. In this mode, some but not all compatibility problems
of the type described above are handled automatically. For example, the
new Ada 95 protected keywords are not recognized in this mode. However,
in practice, it is usually advisable to make the necessary modifications
to the program to remove the need for using this switch.

@node Compatibility with Other Ada 95 Systems
@section Compatibility with Other Ada 95 Systems

@noindent
Providing that programs avoid the use of implementation dependent and
implementation defined features of Ada 95, as documented in the Ada 95
reference manual, there should be a high degree of portability between
GNAT and other Ada 95 systems. The following are specific items which
have proved troublesome in moving GNAT programs to other Ada 95
compilers, but do not affect porting code to GNAT@.

@table @asis
@item Ada 83 Pragmas and Attributes
Ada 95 compilers are allowed, but not required, to implement the missing
Ada 83 pragmas and attributes that are no longer defined in Ada 95.
GNAT implements all such pragmas and attributes, eliminating this as
a compatibility concern, but some other Ada 95 compilers reject these
pragmas and attributes.

@item Special-needs Annexes
GNAT implements the full set of special needs annexes. At the
current time, it is the only Ada 95 compiler to do so. This means that
programs making use of these features may not be portable to other Ada
95 compilation systems.

@item Representation Clauses
Some other Ada 95 compilers implement only the minimal set of
representation clauses required by the Ada 95 reference manual. GNAT goes
far beyond this minimal set, as described in the next section.
@end table

@node Representation Clauses
@section Representation Clauses

@noindent
The Ada 83 reference manual was quite vague in describing both the minimal
required implementation of representation clauses, and also their precise
effects. The Ada 95 reference manual is much more explicit, but the minimal
set of capabilities required in Ada 95 is quite limited.

GNAT implements the full required set of capabilities described in the
Ada 95 reference manual, but also goes much beyond this, and in particular
an effort has been made to be compatible with existing Ada 83 usage to the
greatest extent possible.

A few cases exist in which Ada 83 compiler behavior is incompatible with
requirements in the Ada 95 reference manual. These are instances of
intentional or accidental dependence on specific implementation dependent
characteristics of these Ada 83 compilers. The following is a list of
the cases most likely to arise in existing legacy Ada 83 code.

@table @asis
@item Implicit Packing
Some Ada 83 compilers allowed a Size specification to cause implicit
packing of an array or record. This could cause expensive implicit
conversions for change of representation in the presence of derived
types, and the Ada design intends to avoid this possibility.
Subsequent AI's were issued to make it clear that such implicit
change of representation in response to a Size clause is inadvisable,
and this recommendation is represented explicitly in the Ada 95 RM
as implementation advice that is followed by GNAT@.
The problem will show up as an error
message rejecting the size clause. The fix is simply to provide
the explicit pragma Pack, or for more fine tuned control, provide
a Component_Size clause.

@item Meaning of Size Attribute
The Size attribute in Ada 95 for discrete types is defined as being the
minimal number of bits required to hold values of the type. For example,
on a 32-bit machine, the size of Natural will typically be 31 and not
32 (since no sign bit is required). Some Ada 83 compilers gave 31, and
some 32 in this situation. This problem will usually show up as a compile
time error, but not always. It is a good idea to check all uses of the
'Size attribute when porting Ada 83 code. The GNAT specific attribute
Object_Size can provide a useful way of duplicating the behavior of
some Ada 83 compiler systems.

@item Size of Access Types
A common assumption in Ada 83 code is that an access type is in fact a pointer,
and that therefore it will be the same size as a System.Address value. This
assumption is true for GNAT in most cases with one exception. For the case of
a pointer to an unconstrained array type (where the bounds may vary from one
value of the access type to another), the default is to use a "fat pointer",
which is represented as two separate pointers, one to the bounds, and one to
the array. This representation has a number of advantages, including improved
efficiency. However, it may cause some difficulties in porting existing Ada 83
code which makes the assumption that, for example, pointers fit in 32 bits on
a machine with 32-bit addressing.

To get around this problem, GNAT also permits the use of "thin pointers" for
access types in this case (where the designated type is an unconstrained array
type). These thin pointers are indeed the same size as a System.Address value.
To specify a thin pointer, use a size clause for the type, for example:

@smallexample
type X is access all String;
for X'Size use Standard'Address_Size;
@end smallexample

@noindent
which will cause the type X to be represented using a single pointer. When using
this representation, the bounds are right behind the array. This representation
is slightly less efficient, and does not allow quite such flexibility in the
use of foreign pointers or in using the Unrestricted_Access attribute to create
pointers to non-aliased objects. But for any standard portable use of the access
type it will work in a functionally correct manner and allow porting of existing
code. Note that another way of forcing a thin pointer representation is to use
a component size clause for the element size in an array, or a record 
representation clause for an access field in a record.
@end table

@node Compatibility with DEC Ada 83
@section Compatibility with DEC Ada 83

@noindent
The VMS version of GNAT fully implements all the pragmas and attributes
provided by DEC Ada 83, as well as providing the standard DEC Ada 83
libraries, including Starlet. In addition, data layouts and parameter
passing conventions are highly compatible. This means that porting
existing DEC Ada 83 code to GNAT in VMS systems should be easier than
most other porting efforts. The following are some of the most
significant differences between GNAT and DEC Ada 83.

@table @asis
@item Default floating-point representation
In GNAT, the default floating-point format is IEEE, whereas in DEC Ada 83,
it is VMS format. GNAT does implement the necessary pragmas
(Long_Float, Float_Representation) for changing this default.

@item System
The package System in GNAT exactly corresponds to the definition in the
Ada 95 reference manual, which means that it excludes many of the 
DEC Ada 83 extensions. However, a separate package Aux_DEC is provided
that contains the additional definitions, and a special pragma,
Extend_System allows this package to be treated transparently as an
extension of package System.

@item To_Address
The definitions provided by Aux_DEC are exactly compatible with those
in the DEC Ada 83 version of System, with one exception. DEC Ada provides
the following declarations:

@smallexample
TO_ADDRESS(INTEGER)
TO_ADDRESS(UNSIGNED_LONGWORD)
TO_ADDRESS(universal_integer)
@end smallexample

@noindent
The version of TO_ADDRESS taking a universal integer argument is in fact
an extension to Ada 83 not strictly compatible with the reference manual.
In GNAT, we are constrained to be exactly compatible with the standard,
and this means we cannot provide this capability. In DEC Ada 83, the
point of this definition is to deal with a call like:

@smallexample
   TO_ADDRESS (16#12777#);
@end smallexample

@noindent
Normally, according to the Ada 83 standard, one would expect this to be
ambiguous, since it matches both the INTEGER and UNSIGNED_LONGWORD forms
of TO_ADDRESS@. However, in DEC Ada 83, there is no ambiguity, since the
definition using universal_integer takes precedence.

In GNAT, since the version with universal_integer cannot be supplied, it is
not possible to be 100% compatible. Since there are many programs using
numeric constants for the argument to TO_ADDRESS, the decision in GNAT was
to change the name of the function in the UNSIGNED_LONGWORD case, so the
declarations provided in the GNAT version of AUX_Dec are:

@smallexample
function To_Address (X : Integer) return Address;
pragma Pure_Function (To_Address);

function To_Address_Long (X : Unsigned_Longword)
 return Address;
pragma Pure_Function (To_Address_Long);
@end smallexample

@noindent
This means that programs using TO_ADDRESS for UNSIGNED_LONGWORD must
change the name to TO_ADDRESS_LONG@.

@item Task_Id values
The Task_Id values assigned will be different in the two systems, and GNAT
does not provide a specified value for the Task_Id of the environment task,
which in GNAT is treated like any other declared task.
@end table

For full details on these and other less significant compatibility issues,
see appendix E of the Digital publication entitled "DEC Ada, Technical
Overview and Comparison on DIGITAL Platforms".

For GNAT running on other than VMS systems, all the DEC Ada 83 pragmas and
attributes are recognized, although only a subset of them can sensibly
be implemented. The description of pragmas in this reference manual
indicates whether or not they are applicable to non-VMS systems.

@include gfdl.texi
@c GNU Free Documentation License

@node Index,,GNU Free Documentation License, Top
@unnumbered Index

@printindex cp

@contents

@bye