gcc/ada/doc/gnat_rm/the_implementation_of_standard_i_o.rst

.. _The_Implementation_of_Standard_I/O:

**********************************
The Implementation of Standard I/O
**********************************

GNAT implements all the required input-output facilities described in
A.6 through A.14.  These sections of the Ada 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 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.

.. _Standard_I/O_Packages:

Standard I/O Packages
=====================

The Standard I/O packages described in Annex A for

* 
  Ada.Text_IO
* 
  Ada.Text_IO.Complex_IO
* 
  Ada.Text_IO.Text_Streams
* 
  Ada.Wide_Text_IO
* 
  Ada.Wide_Text_IO.Complex_IO
* 
  Ada.Wide_Text_IO.Text_Streams
* 
  Ada.Wide_Wide_Text_IO
* 
  Ada.Wide_Wide_Text_IO.Complex_IO
* 
  Ada.Wide_Wide_Text_IO.Text_Streams
* 
  Ada.Stream_IO
* 
  Ada.Sequential_IO
* 
  Ada.Direct_IO

are implemented using the C
library streams facility; where

* 
  All files are opened using `fopen`.
* 
  All input/output operations use `fread`/`fwrite`.

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
library routines that support streams. This facilitates shared use of these
streams by mixed language programs. Note though that system level buffering is
explicitly enabled at elaboration of the standard I/O packages and that can
have an impact on mixed language programs, in particular those using I/O before
calling the Ada elaboration routine (e.g., adainit). It is recommended to call
the Ada elaboration routine before performing any I/O or when impractical,
flush the common I/O streams and in particular Standard_Output before
elaborating the Ada code.

.. _FORM_Strings:

FORM Strings
============

The format of a FORM string in GNAT is:


::

  "keyword=value,keyword=value,...,keyword=value"
  

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
the following keywords defined.


::

  TEXT_TRANSLATION=[YES|NO|TEXT|BINARY|U8TEXT|WTEXT|U16TEXT]
  SHARED=[YES|NO]
  WCEM=[n|h|u|s|e|8|b]
  ENCODING=[UTF8|8BITS]
  

The use of these parameters is described later in this section. If an
unrecognized keyword appears in a form string, it is silently ignored
and not considered invalid.

.. _Direct_IO:

Direct_IO
=========

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 ``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 `K` starts at offset
(`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.

.. _Sequential_IO:

Sequential_IO
=============

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 `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, `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 `Data_Error`:


.. code-block:: ada

   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);

  

On some Ada implementations, this will print `hell`, but the program is
clearly incorrect, since there is only one element in the file, and that
element is the string `hello!`.

In Ada 95 and Ada 2005, 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.

.. _Text_IO:

Text_IO
=======

Text_IO files consist of a stream of characters containing the following
special control characters:


::

  LF (line feed, 16#0A#) Line Mark
  FF (form feed, 16#0C#) Page Mark
  

A canonical Text_IO file is defined as one in which the following
conditions are met:

* 
  The character `LF` is used only as a line mark, i.e., to mark the end
  of the line.

* 
  The character `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
  `LF` (line mark) character.

* 
  The file ends with either `LF` (line mark) or `LF`-`FF`
  (line mark, page mark).  In the former case, the page mark is implicitly
  assumed to be present.

A file written using Text_IO will be in canonical form provided that no
explicit `LF` or `FF` characters are written using `Put`
or `Put_Line`.  There will be no `FF` character at the end of
the file unless an explicit `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 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:

* 
  The file contains `FF` characters not immediately following a
  `LF` character.

* 
  The file contains `LF` or `FF` characters written by
  `Put` or `Put_Line`, which are not logically considered to be
  line marks or page marks.

* 
  The file ends in a character other than `LF` or `FF`,
  i.e., there is no explicit line mark or page mark at the end of the file.

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 `FF` character that does not follow a `LF`
character may or may not be treated as a page mark from the point of
view of page and line numbering.  Every `LF` character is considered
to end a line, and there is an implied `LF` character at the end of
the file.

.. _Stream_Pointer_Positioning:

Stream Pointer Positioning
--------------------------

`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:

* 
  After a call to `End_Of_Page` that returns `True`, the stream
  is positioned past the `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 `End_Of_Page` call.

* 
  After a call to `End_Of_File` that returns `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.

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 ``shared=yes`` in the form string)
between two Ada files, then the difference may be observable in some
situations.

.. _Reading_and_Writing_Non-Regular_Files:

Reading and Writing Non-Regular Files
-------------------------------------

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 `Ascii.FF` characters (the character normally used for a
page mark) appearing in the file are considered to be data
characters.  In particular:

* 
  `Get_Line` and `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.

* 
  This avoids the need to wait for an extra character to be typed or
  entered from the pipe to complete one of these operations.

* 
  `End_Of_Page` always returns `False`

* 
  `End_Of_File` will return `False` if there is a page mark at
  the end of the file.

Output to non-regular files is the same as for regular files.  Page marks
may be written to non-regular files using `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 :kbd:`EOT` key,
then end of file
is signaled once (i.e., the test `End_Of_File`
will yield `True`, or a read will
raise `End_Error`), but then reading can resume
to read data past that end of
file indication, until another end of file indication is entered.

.. _Get_Immediate:

Get_Immediate
-------------

.. index:: Get_Immediate

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.

.. _Treating_Text_IO_Files_as_Streams:

Treating Text_IO Files as Streams
---------------------------------

.. index:: Stream files

The package `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# (`LF`) or
16#0C# (`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 `LF` and `FF` characters may be
skipped and the effect is similar to that described above for
`Get_Immediate`.

.. _Text_IO_Extensions:

Text_IO Extensions
------------------

.. index:: Text_IO extensions

A package GNAT.IO_Aux in the GNAT library provides some useful extensions
to the standard `Text_IO` package:

* function File_Exists (Name : String) return Boolean;
  Determines if a file of the given name exists.

* 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.

* 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.


.. _Text_IO_Facilities_for_Unbounded_Strings:

Text_IO Facilities for Unbounded Strings
----------------------------------------

.. index:: Text_IO for unbounded strings

.. index:: Unbounded_String, Text_IO operations

The package `Ada.Strings.Unbounded.Text_IO`
in library files `a-suteio.ads/adb` contains some GNAT-specific
subprograms useful for Text_IO operations on unbounded strings:


* function Get_Line (File : File_Type) return Unbounded_String;
  Reads a line from the specified file
  and returns the result as an unbounded string.

* 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
  `Put (To_String (U))` except that an extra copy is avoided.

* procedure Put_Line (File : File_Type; U : Unbounded_String);
  Writes the value of the given unbounded string to the specified file,
  followed by a `New_Line`.
  Similar to the effect of `Put_Line (To_String (U))` except
  that an extra copy is avoided.

In the above procedures, `File` is of type `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 `Ada.Strings.Wide_Unbounded.Wide_Text_IO` in library
files :file:`a-swuwti.ads` and :file:`a-swuwti.adb` provides similar extended
`Wide_Text_IO` functionality for unbounded wide strings.

The package `Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO` in library
files :file:`a-szuzti.ads` and :file:`a-szuzti.adb` provides similar extended
`Wide_Wide_Text_IO` functionality for unbounded wide wide strings.

.. _Wide_Text_IO:

Wide_Text_IO
============

`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:


::

  WCEM=`x`
  

as part of the FORM string (WCEM = wide character encoding method),
where `x` is one of the following characters

========== ====================
Character  Encoding
========== ====================
*h*        Hex ESC encoding
*u*        Upper half encoding
*s*        Shift-JIS encoding
*e*        EUC Encoding
*8*        UTF-8 encoding
*b*        Brackets encoding
========== ====================

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).



*Hex Coding*
  In this encoding, a wide character is represented by a five character
  sequence:


::

    ESC a b c d
    
..

  where `a`, `b`, `c`, `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
  `Wide_Character` set.


*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.


*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.


*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.


*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:


::

    16#0000#-16#007f#: 2#0xxxxxxx#
    16#0080#-16#07ff#: 2#110xxxxx# 2#10xxxxxx#
    16#0800#-16#ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#
    
..

  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 invalid UTF-8 sequences.)


*Brackets Coding*
  In this encoding, a wide character is represented by the following eight
  character sequence:


::

    [ " a b c d " ]
    
..

  where `a`, `b`, `c`, `d` are the four hexadecimal
  characters (using uppercase letters) of the wide character code.  For
  example, `["A345"]` is used to represent the wide character with 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., `["C1"]` for lower case a.  However, on output, brackets notation
  is only used for wide characters with a code greater than `16#FF#`.

  Note that brackets coding is not normally used in the context of
  Wide_Text_IO or Wide_Wide_Text_IO, since it is really just designed as
  a portable way of encoding source files. In the context of Wide_Text_IO
  or Wide_Wide_Text_IO, it can only be used if the file does not contain
  any instance of the left bracket character other than to encode wide
  character values using the brackets encoding method. In practice it is
  expected that some standard wide character encoding method such
  as UTF-8 will be used for text input output.

  If brackets notation is used, then any occurrence of a left bracket
  in the input file which is not the start of a valid wide character
  sequence will cause Constraint_Error to be raised. It is possible to
  encode a left bracket as ["5B"] and Wide_Text_IO and Wide_Wide_Text_IO
  input will interpret this as a left bracket.

  However, when a left bracket is output, it will be output as a left bracket
  and not as ["5B"]. We make this decision because for normal use of
  Wide_Text_IO for outputting messages, it is unpleasant to clobber left
  brackets. For example, if we write:


  .. code-block:: ada

       Put_Line ("Start of output [first run]");


  we really do not want to have the left bracket in this message clobbered so
  that the output reads:


::

       Start of output ["5B"]first run]
    
..

  In practice brackets encoding is reasonably useful for normal Put_Line use
  since we won't get confused between left brackets and wide character
  sequences in the output. But for input, or when files are written out
  and read back in, it really makes better sense to use one of the standard
  encoding methods such as UTF-8.


For the coding schemes other than UTF-8, Hex, or 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.

.. _Stream_Pointer_Positioning_1:

Stream Pointer Positioning
--------------------------

`Ada.Wide_Text_IO` is similar to `Ada.Text_IO` in its handling
of stream pointer positioning (:ref:`Text_IO`).  There is one additional
case:

If `Ada.Wide_Text_IO.Look_Ahead` reads a character outside the
normal lower ASCII set (i.e., a character in the range:


.. code-block:: ada

  Wide_Character'Val (16#0080#) .. Wide_Character'Val (16#FFFF#)
  

then although the logical position of the file pointer is unchanged by
the `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 `Wide_Text_IO` routines check the internal
indication that this situation has occurred so that this is not visible
to a normal program using `Wide_Text_IO`.  However, this discrepancy
can be observed if the wide text file shares a stream with another file.

.. _Reading_and_Writing_Non-Regular_Files_1:

Reading and Writing Non-Regular Files
-------------------------------------

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 `End_Of_Page` always returns
`False`.  Similarly, the end of file indication is not sticky, so
it is possible to read beyond an end of file.

.. _Wide_Wide_Text_IO:

Wide_Wide_Text_IO
=================

`Wide_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 wide character values.  The encoding scheme for a given file may be
specified using a FORM parameter:


::

  WCEM=`x`
  

as part of the FORM string (WCEM = wide character encoding method),
where `x` is one of the following characters

========== ====================
Character  Encoding
========== ====================
*h*        Hex ESC encoding
*u*        Upper half encoding
*s*        Shift-JIS encoding
*e*        EUC Encoding
*8*        UTF-8 encoding
*b*        Brackets encoding
========== ====================


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).



*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, three, or four byte sequence:


::

    16#000000#-16#00007f#: 2#0xxxxxxx#
    16#000080#-16#0007ff#: 2#110xxxxx# 2#10xxxxxx#
    16#000800#-16#00ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#
    16#010000#-16#10ffff#: 2#11110xxx# 2#10xxxxxx# 2#10xxxxxx# 2#10xxxxxx#
    
..

  where the `xxx` bits correspond to the left-padded bits of the
  21-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
  characters.


*Brackets Coding*
  In this encoding, a wide wide character is represented by the following eight
  character sequence if is in wide character range


::

    [ " a b c d " ]
    
..

  and by the following ten character sequence if not


::

    [ " a b c d e f " ]
    
..

  where `a`, `b`, `c`, `d`, `e`, and `f`
  are the four or six hexadecimal
  characters (using uppercase letters) of the wide wide character code.  For
  example, `["01A345"]` is used to represent the wide wide character
  with code `16#01A345#`.

  This scheme is compatible with use of the full Wide_Wide_Character set.
  On input, brackets coding can also be used for upper half characters,
  e.g., `["C1"]` for lower case a.  However, on output, brackets notation
  is only used for wide characters with a code greater than `16#FF#`.


If is also possible to use the other Wide_Character encoding methods,
such as Shift-JIS, but the other schemes cannot support the full range
of wide wide characters.
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.

.. _Stream_Pointer_Positioning_2:

Stream Pointer Positioning
--------------------------

`Ada.Wide_Wide_Text_IO` is similar to `Ada.Text_IO` in its handling
of stream pointer positioning (:ref:`Text_IO`).  There is one additional
case:

If `Ada.Wide_Wide_Text_IO.Look_Ahead` reads a character outside the
normal lower ASCII set (i.e., a character in the range:


.. code-block:: ada

  Wide_Wide_Character'Val (16#0080#) .. Wide_Wide_Character'Val (16#10FFFF#)
  

then although the logical position of the file pointer is unchanged by
the `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 `Wide_Wide_Text_IO` routines check the internal
indication that this situation has occurred so that this is not visible
to a normal program using `Wide_Wide_Text_IO`.  However, this discrepancy
can be observed if the wide text file shares a stream with another file.

.. _Reading_and_Writing_Non-Regular_Files_2:

Reading and Writing Non-Regular Files
-------------------------------------

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 `End_Of_Page` always returns
`False`.  Similarly, the end of file indication is not sticky, so
it is possible to read beyond an end of file.

.. _Stream_IO:

Stream_IO
=========

A stream file is a sequence of bytes, where individual elements are
written to the file as described in the Ada Reference Manual.  The type
`Stream_Element` is simply a byte.  There are two ways to read or
write a stream file.

* 
  The operations `Read` and `Write` directly read or write a
  sequence of stream elements with no control information.

* 
  The stream attributes applied to a stream file transfer data in the
  manner described for stream attributes.

.. _Text_Translation:

Text Translation
================

``Text_Translation=xxx`` may be used as the Form parameter
passed to Text_IO.Create and Text_IO.Open. ``Text_Translation=xxx``
has no effect on Unix systems. Possible values are:


* 
  ``Yes`` or ``Text`` is the default, which means to
  translate LF to/from CR/LF on Windows systems.

  ``No`` disables this translation; i.e. it
  uses binary mode. For output files, ``Text_Translation=No``
  may be used to create Unix-style files on
  Windows.

* 
  ``wtext`` translation enabled in Unicode mode.
  (corresponds to _O_WTEXT).

* 
  ``u8text`` translation enabled in Unicode UTF-8 mode.
  (corresponds to O_U8TEXT).

* 
  ``u16text`` translation enabled in Unicode UTF-16
  mode. (corresponds to_O_U16TEXT).


.. _Shared_Files:

Shared Files
============

Section A.14 of the Ada 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:

* 
  In the absence of a ``shared=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 `Use_Error` will be
  raised.  Note that a file that is not explicitly closed by the program
  remains open until the program terminates.

* 
  If the form parameter ``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.

* 
  If the form parameter ``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 Reference Manual, Section A.14.

When a program that opens multiple files with the same name is ported
from another Ada compiler to GNAT, the effect will be that
`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 ``shared=xxx`` parameters added to `Open`
and `Create` calls as required.

When a program is ported from GNAT to some other Ada compiler, no
special attention is required unless the ``shared=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 ``shared=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
``shared=yes`` is specified, it is preferable in Ada to use Stream_IO
for this purpose (using the stream attributes)

.. _Filenames_encoding:

Filenames encoding
==================

An encoding form parameter can be used to specify the filename
encoding ``encoding=xxx``.

* 
  If the form parameter ``encoding=utf8`` appears in the form string, the
  filename must be encoded in UTF-8.

* 
  If the form parameter ``encoding=8bits`` appears in the form
  string, the filename must be a standard 8bits string.

In the absence of a ``encoding=xxx`` form parameter, the
encoding is controlled by the ``GNAT_CODE_PAGE`` environment
variable. And if not set ``utf8`` is assumed.



*CP_ACP*
  The current system Windows ANSI code page.

*CP_UTF8*
  UTF-8 encoding

This encoding form parameter is only supported on the Windows
platform. On the other Operating Systems the run-time is supporting
UTF-8 natively.

.. _File_content_encoding:

File content encoding
=====================

For text files it is possible to specify the encoding to use. This is
controlled by the by the ``GNAT_CCS_ENCODING`` environment
variable. And if not set ``TEXT`` is assumed.

The possible values are those supported on Windows:



*TEXT*
  Translated text mode

*WTEXT*
  Translated unicode encoding

*U16TEXT*
  Unicode 16-bit encoding

*U8TEXT*
  Unicode 8-bit encoding

This encoding is only supported on the Windows platform.

.. _Open_Modes:

Open Modes
==========

`Open` and `Create` calls result in a call to `fopen`
using the mode shown in the following table:

+----------------------------+---------------+------------------+
|           `Open` and `Create` Call Modes                      |
+----------------------------+---------------+------------------+
|                            |   **OPEN**    |     **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+"          |
+----------------------------+---------------+------------------+
  

If text file translation is required, then either ``b`` or ``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 ``r`` or ``w`` mode for the
`In_File` and `Out_File` cases.  If a `Set_Mode` operation
subsequently requires switching from reading to writing or vice-versa,
then the file is reopened in ``r+`` mode to permit the required operation.

.. _Operations_on_C_Streams:

Operations on C Streams
=======================

The package `Interfaces.C_Streams` provides an Ada program with direct
access to the C library functions for operations on C streams:


.. code-block:: ada

  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 UNIX (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;
  

.. _Interfacing_to_C_Streams:

Interfacing to C Streams
========================

The packages in this section permit interfacing Ada files to C Stream
operations.


.. code-block:: ada

   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.Wide_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_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;
  

In each of these six packages, the `C_Stream` function obtains the
`FILE` pointer from a currently opened Ada file.  It is then
possible to use the `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
`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 `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.