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\input texinfo @c -*-texinfo-*-
@c %**start of header
@set copyrights-gfortran 1999-2013
@include gcc-common.texi
@settitle The GNU Fortran Compiler
@c Create a separate index for command line options
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@c Use with @@smallbook.
@c %** start of document
@c Cause even numbered pages to be printed on the left hand side of
@c the page and odd numbered pages to be printed on the right hand
@c side of the page. Using this, you can print on both sides of a
@c sheet of paper and have the text on the same part of the sheet.
@c The text on right hand pages is pushed towards the right hand
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Copyright @copyright{} @value{copyrights-gfortran} Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``Funding Free Software'', the Front-Cover
Texts being (a) (see below), and with the Back-Cover Texts being (b)
(see below). A copy of the license is included in the section entitled
``GNU Free Documentation License''.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
@end copying
@dircategory Software development
* gfortran: (gfortran). The GNU Fortran Compiler.
@end direntry
This file documents the use and the internals of
the GNU Fortran compiler, (@command{gfortran}).
Published by the Free Software Foundation
51 Franklin Street, Fifth Floor
Boston, MA 02110-1301 USA
@end ifinfo
@setchapternewpage odd
@title Using GNU Fortran
@author The @t{gfortran} team
@vskip 0pt plus 1filll
Published by the Free Software Foundation@*
51 Franklin Street, Fifth Floor@*
Boston, MA 02110-1301, USA@*
@c Last printed ??ber, 19??.@*
@c Printed copies are available for $? each.@*
@c ISBN ???
@sp 1
@end titlepage
@c TODO: The following "Part" definitions are included here temporarily
@c until they are incorporated into the official Texinfo distribution.
@end tex
@end tex
@c ---------------------------------------------------------------------
@c TexInfo table of contents.
@c ---------------------------------------------------------------------
@node Top
@top Introduction
@cindex Introduction
This manual documents the use of @command{gfortran},
the GNU Fortran compiler. You can find in this manual how to invoke
@command{gfortran}, as well as its features and incompatibilities.
@emph{Warning:} This document, and the compiler it describes, are still
under development. While efforts are made to keep it up-to-date, it might
not accurately reflect the status of the most recent GNU Fortran compiler.
@end ifset
@comment When you add a new menu item, please keep the right hand
@comment aligned to the same column. Do not use tabs. This provides
@comment better formatting.
* Introduction::
Part I: Invoking GNU Fortran
* Invoking GNU Fortran:: Command options supported by @command{gfortran}.
* Runtime:: Influencing runtime behavior with environment variables.
Part II: Language Reference
* Fortran 2003 and 2008 status:: Fortran 2003 and 2008 features supported by GNU Fortran.
* Compiler Characteristics:: User-visible implementation details.
* Extensions:: Language extensions implemented by GNU Fortran.
* Mixed-Language Programming:: Interoperability with C
* Intrinsic Procedures:: Intrinsic procedures supported by GNU Fortran.
* Intrinsic Modules:: Intrinsic modules supported by GNU Fortran.
* Contributing:: How you can help.
* Copying:: GNU General Public License says
how you can copy and share GNU Fortran.
* GNU Free Documentation License::
How you can copy and share this manual.
* Funding:: How to help assure continued work for free software.
* Option Index:: Index of command line options
* Keyword Index:: Index of concepts
@end menu
@end ifnottex
@c ---------------------------------------------------------------------
@c Introduction
@c ---------------------------------------------------------------------
@node Introduction
@chapter Introduction
@c The following duplicates the text on the TexInfo table of contents.
This manual documents the use of @command{gfortran}, the GNU Fortran
compiler. You can find in this manual how to invoke @command{gfortran},
as well as its features and incompatibilities.
@emph{Warning:} This document, and the compiler it describes, are still
under development. While efforts are made to keep it up-to-date, it
might not accurately reflect the status of the most recent GNU Fortran
@end ifset
@end iftex
The GNU Fortran compiler front end was
designed initially as a free replacement for,
or alternative to, the Unix @command{f95} command;
@command{gfortran} is the command you will use to invoke the compiler.
* About GNU Fortran:: What you should know about the GNU Fortran compiler.
* GNU Fortran and GCC:: You can compile Fortran, C, or other programs.
* Preprocessing and conditional compilation:: The Fortran preprocessor
* GNU Fortran and G77:: Why we chose to start from scratch.
* Project Status:: Status of GNU Fortran, roadmap, proposed extensions.
* Standards:: Standards supported by GNU Fortran.
@end menu
@c ---------------------------------------------------------------------
@c About GNU Fortran
@c ---------------------------------------------------------------------
@node About GNU Fortran
@section About GNU Fortran
The GNU Fortran compiler supports the Fortran 77, 90 and 95 standards
completely, parts of the Fortran 2003 and Fortran 2008 standards, and
several vendor extensions. The development goal is to provide the
following features:
@itemize @bullet
Read a user's program,
stored in a file and containing instructions written
in Fortran 77, Fortran 90, Fortran 95, Fortran 2003 or Fortran 2008.
This file contains @dfn{source code}.
Translate the user's program into instructions a computer
can carry out more quickly than it takes to translate the
instructions in the first
place. The result after compilation of a program is
@dfn{machine code},
code designed to be efficiently translated and processed
by a machine such as your computer.
Humans usually are not as good writing machine code
as they are at writing Fortran (or C++, Ada, or Java),
because it is easy to make tiny mistakes writing machine code.
Provide the user with information about the reasons why
the compiler is unable to create a binary from the source code.
Usually this will be the case if the source code is flawed.
The Fortran 90 standard requires that the compiler can point out
mistakes to the user.
An incorrect usage of the language causes an @dfn{error message}.
The compiler will also attempt to diagnose cases where the
user's program contains a correct usage of the language,
but instructs the computer to do something questionable.
This kind of diagnostics message is called a @dfn{warning message}.
Provide optional information about the translation passes
from the source code to machine code.
This can help a user of the compiler to find the cause of
certain bugs which may not be obvious in the source code,
but may be more easily found at a lower level compiler output.
It also helps developers to find bugs in the compiler itself.
Provide information in the generated machine code that can
make it easier to find bugs in the program (using a debugging tool,
called a @dfn{debugger}, such as the GNU Debugger @command{gdb}).
Locate and gather machine code already generated to
perform actions requested by statements in the user's program.
This machine code is organized into @dfn{modules} and is located
and @dfn{linked} to the user program.
@end itemize
The GNU Fortran compiler consists of several components:
@itemize @bullet
A version of the @command{gcc} command
(which also might be installed as the system's @command{cc} command)
that also understands and accepts Fortran source code.
The @command{gcc} command is the @dfn{driver} program for
all the languages in the GNU Compiler Collection (GCC);
With @command{gcc},
you can compile the source code of any language for
which a front end is available in GCC.
The @command{gfortran} command itself,
which also might be installed as the
system's @command{f95} command.
@command{gfortran} is just another driver program,
but specifically for the Fortran compiler only.
The difference with @command{gcc} is that @command{gfortran}
will automatically link the correct libraries to your program.
A collection of run-time libraries.
These libraries contain the machine code needed to support
capabilities of the Fortran language that are not directly
provided by the machine code generated by the
@command{gfortran} compilation phase,
such as intrinsic functions and subroutines,
and routines for interaction with files and the operating system.
@c and mechanisms to spawn,
@c unleash and pause threads in parallelized code.
The Fortran compiler itself, (@command{f951}).
This is the GNU Fortran parser and code generator,
linked to and interfaced with the GCC backend library.
@command{f951} ``translates'' the source code to
assembler code. You would typically not use this
program directly;
instead, the @command{gcc} or @command{gfortran} driver
programs will call it for you.
@end itemize
@c ---------------------------------------------------------------------
@c GNU Fortran and GCC
@c ---------------------------------------------------------------------
@node GNU Fortran and GCC
@section GNU Fortran and GCC
@cindex GNU Compiler Collection
@cindex GCC
GNU Fortran is a part of GCC, the @dfn{GNU Compiler Collection}. GCC
consists of a collection of front ends for various languages, which
translate the source code into a language-independent form called
@dfn{GENERIC}. This is then processed by a common middle end which
provides optimization, and then passed to one of a collection of back
ends which generate code for different computer architectures and
operating systems.
Functionally, this is implemented with a driver program (@command{gcc})
which provides the command-line interface for the compiler. It calls
the relevant compiler front-end program (e.g., @command{f951} for
Fortran) for each file in the source code, and then calls the assembler
and linker as appropriate to produce the compiled output. In a copy of
GCC which has been compiled with Fortran language support enabled,
@command{gcc} will recognize files with @file{.f}, @file{.for}, @file{.ftn},
@file{.f90}, @file{.f95}, @file{.f03} and @file{.f08} extensions as
Fortran source code, and compile it accordingly. A @command{gfortran}
driver program is also provided, which is identical to @command{gcc}
except that it automatically links the Fortran runtime libraries into the
compiled program.
Source files with @file{.f}, @file{.for}, @file{.fpp}, @file{.ftn}, @file{.F},
@file{.FOR}, @file{.FPP}, and @file{.FTN} extensions are treated as fixed form.
Source files with @file{.f90}, @file{.f95}, @file{.f03}, @file{.f08},
@file{.F90}, @file{.F95}, @file{.F03} and @file{.F08} extensions are
treated as free form. The capitalized versions of either form are run
through preprocessing. Source files with the lower case @file{.fpp}
extension are also run through preprocessing.
This manual specifically documents the Fortran front end, which handles
the programming language's syntax and semantics. The aspects of GCC
which relate to the optimization passes and the back-end code generation
are documented in the GCC manual; see
@ref{Top,,Introduction,gcc,Using the GNU Compiler Collection (GCC)}.
The two manuals together provide a complete reference for the GNU
Fortran compiler.
@c ---------------------------------------------------------------------
@c Preprocessing and conditional compilation
@c ---------------------------------------------------------------------
@node Preprocessing and conditional compilation
@section Preprocessing and conditional compilation
@cindex CPP
@cindex FPP
@cindex Conditional compilation
@cindex Preprocessing
@cindex preprocessor, include file handling
Many Fortran compilers including GNU Fortran allow passing the source code
through a C preprocessor (CPP; sometimes also called the Fortran preprocessor,
FPP) to allow for conditional compilation. In the case of GNU Fortran,
this is the GNU C Preprocessor in the traditional mode. On systems with
case-preserving file names, the preprocessor is automatically invoked if the
filename extension is @file{.F}, @file{.FOR}, @file{.FTN}, @file{.fpp},
@file{.FPP}, @file{.F90}, @file{.F95}, @file{.F03} or @file{.F08}. To manually
invoke the preprocessor on any file, use @option{-cpp}, to disable
preprocessing on files where the preprocessor is run automatically, use
If a preprocessed file includes another file with the Fortran @code{INCLUDE}
statement, the included file is not preprocessed. To preprocess included
files, use the equivalent preprocessor statement @code{#include}.
If GNU Fortran invokes the preprocessor, @code{__GFORTRAN__}
is defined and @code{__GNUC__}, @code{__GNUC_MINOR__} and
@code{__GNUC_PATCHLEVEL__} can be used to determine the version of the
compiler. See @ref{Top,,Overview,cpp,The C Preprocessor} for details.
While CPP is the de-facto standard for preprocessing Fortran code,
Part 3 of the Fortran 95 standard (ISO/IEC 1539-3:1998) defines
Conditional Compilation, which is not widely used and not directly
supported by the GNU Fortran compiler. You can use the program coco
to preprocess such files (@uref{}).
@c ---------------------------------------------------------------------
@c GNU Fortran and G77
@c ---------------------------------------------------------------------
@node GNU Fortran and G77
@section GNU Fortran and G77
@cindex Fortran 77
@cindex @command{g77}
The GNU Fortran compiler is the successor to @command{g77}, the Fortran
77 front end included in GCC prior to version 4. It is an entirely new
program that has been designed to provide Fortran 95 support and
extensibility for future Fortran language standards, as well as providing
backwards compatibility for Fortran 77 and nearly all of the GNU language
extensions supported by @command{g77}.
@c ---------------------------------------------------------------------
@c Project Status
@c ---------------------------------------------------------------------
@node Project Status
@section Project Status
As soon as @command{gfortran} can parse all of the statements correctly,
it will be in the ``larva'' state.
When we generate code, the ``puppa'' state.
When @command{gfortran} is done,
we'll see if it will be a beautiful butterfly,
or just a big bug....
--Andy Vaught, April 2000
@end quotation
The start of the GNU Fortran 95 project was announced on
the GCC homepage in March 18, 2000
(even though Andy had already been working on it for a while,
of course).
The GNU Fortran compiler is able to compile nearly all
standard-compliant Fortran 95, Fortran 90, and Fortran 77 programs,
including a number of standard and non-standard extensions, and can be
used on real-world programs. In particular, the supported extensions
include OpenMP, Cray-style pointers, and several Fortran 2003 and Fortran
2008 features, including TR 15581. However, it is still under
development and has a few remaining rough edges.
At present, the GNU Fortran compiler passes the
NIST Fortran 77 Test Suite}, and produces acceptable results on the
@uref{, LAPACK Test Suite}.
It also provides respectable performance on
the @uref{, Polyhedron Fortran
compiler benchmarks} and the
Livermore Fortran Kernels test}. It has been used to compile a number of
large real-world programs, including
@uref{, the HIRLAM
weather-forecasting code} and
@uref{, the Tonto quantum
chemistry package}; see @url{} for an
extended list.
Among other things, the GNU Fortran compiler is intended as a replacement
for G77. At this point, nearly all programs that could be compiled with
G77 can be compiled with GNU Fortran, although there are a few minor known
The primary work remaining to be done on GNU Fortran falls into three
categories: bug fixing (primarily regarding the treatment of invalid code
and providing useful error messages), improving the compiler optimizations
and the performance of compiled code, and extending the compiler to support
future standards---in particular, Fortran 2003 and Fortran 2008.
@c ---------------------------------------------------------------------
@c Standards
@c ---------------------------------------------------------------------
@node Standards
@section Standards
@cindex Standards
* Varying Length Character Strings::
@end menu
The GNU Fortran compiler implements
ISO/IEC 1539:1997 (Fortran 95). As such, it can also compile essentially all
standard-compliant Fortran 90 and Fortran 77 programs. It also supports
the ISO/IEC TR-15581 enhancements to allocatable arrays.
GNU Fortran also have a partial support for ISO/IEC 1539-1:2004 (Fortran
2003), ISO/IEC 1539-1:2010 (Fortran 2008), the Technical Specification
@code{Further Interoperability of Fortran with C} (ISO/IEC TS 29113:2012).
Full support of those standards and future Fortran standards is planned.
The current status of the support is can be found in the
@ref{Fortran 2003 status}, @ref{Fortran 2008 status} and
@ref{TS 29113 status} sections of the documentation.
Additionally, the GNU Fortran compilers supports the OpenMP specification
(version 3.1, @url{}).
@node Varying Length Character Strings
@subsection Varying Length Character Strings
@cindex Varying length character strings
@cindex Varying length strings
@cindex strings, varying length
The Fortran 95 standard specifies in Part 2 (ISO/IEC 1539-2:2000)
varying length character strings. While GNU Fortran currently does not
support such strings directly, there exist two Fortran implementations
for them, which work with GNU Fortran. They can be found at
@uref{} and at
Deferred-length character strings of Fortran 2003 supports part of
the features of @code{ISO_VARYING_STRING} and should be considered as
replacement. (Namely, allocatable or pointers of the type
@c =====================================================================
@c =====================================================================
\part{I}{Invoking GNU Fortran}
@end tex
@c ---------------------------------------------------------------------
@c Compiler Options
@c ---------------------------------------------------------------------
@include invoke.texi
@c ---------------------------------------------------------------------
@c Runtime
@c ---------------------------------------------------------------------
@node Runtime
@chapter Runtime: Influencing runtime behavior with environment variables
@cindex environment variable
The behavior of the @command{gfortran} can be influenced by
environment variables.
Malformed environment variables are silently ignored.
* TMPDIR:: Directory for scratch files
* GFORTRAN_STDIN_UNIT:: Unit number for standard input
* GFORTRAN_STDOUT_UNIT:: Unit number for standard output
* GFORTRAN_STDERR_UNIT:: Unit number for standard error
* GFORTRAN_UNBUFFERED_ALL:: Do not buffer I/O for all units.
* GFORTRAN_UNBUFFERED_PRECONNECTED:: Do not buffer I/O for preconnected units.
* GFORTRAN_SHOW_LOCUS:: Show location for runtime errors
* GFORTRAN_OPTIONAL_PLUS:: Print leading + where permitted
* GFORTRAN_DEFAULT_RECL:: Default record length for new files
* GFORTRAN_LIST_SEPARATOR:: Separator for list output
* GFORTRAN_CONVERT_UNIT:: Set endianness for unformatted I/O
* GFORTRAN_ERROR_BACKTRACE:: Show backtrace on run-time errors
@end menu
@node TMPDIR
@section @env{TMPDIR}---Directory for scratch files
When opening a file with @code{STATUS='SCRATCH'}, GNU Fortran tries to
create the file in one of the potential directories by testing each
directory in the order below.
The environment variable @env{TMPDIR}, if it exists.
On the MinGW target, the directory returned by the @code{GetTempPath}
function. Alternatively, on the Cygwin target, the @env{TMP} and
@env{TEMP} environment variables, if they exist, in that order.
The @code{P_tmpdir} macro if it is defined, otherwise the directory
@end enumerate
@section @env{GFORTRAN_STDIN_UNIT}---Unit number for standard input
This environment variable can be used to select the unit number
preconnected to standard input. This must be a positive integer.
The default value is 5.
@section @env{GFORTRAN_STDOUT_UNIT}---Unit number for standard output
This environment variable can be used to select the unit number
preconnected to standard output. This must be a positive integer.
The default value is 6.
@section @env{GFORTRAN_STDERR_UNIT}---Unit number for standard error
This environment variable can be used to select the unit number
preconnected to standard error. This must be a positive integer.
The default value is 0.
@section @env{GFORTRAN_UNBUFFERED_ALL}---Do not buffer I/O on all units
This environment variable controls whether all I/O is unbuffered. If
the first letter is @samp{y}, @samp{Y} or @samp{1}, all I/O is
unbuffered. This will slow down small sequential reads and writes. If
the first letter is @samp{n}, @samp{N} or @samp{0}, I/O is buffered.
This is the default.
@section @env{GFORTRAN_UNBUFFERED_PRECONNECTED}---Do not buffer I/O on preconnected units
The environment variable named @env{GFORTRAN_UNBUFFERED_PRECONNECTED} controls
whether I/O on a preconnected unit (i.e.@: STDOUT or STDERR) is unbuffered. If
the first letter is @samp{y}, @samp{Y} or @samp{1}, I/O is unbuffered. This
will slow down small sequential reads and writes. If the first letter
is @samp{n}, @samp{N} or @samp{0}, I/O is buffered. This is the default.
@section @env{GFORTRAN_SHOW_LOCUS}---Show location for runtime errors
If the first letter is @samp{y}, @samp{Y} or @samp{1}, filename and
line numbers for runtime errors are printed. If the first letter is
@samp{n}, @samp{N} or @samp{0}, do not print filename and line numbers
for runtime errors. The default is to print the location.
@section @env{GFORTRAN_OPTIONAL_PLUS}---Print leading + where permitted
If the first letter is @samp{y}, @samp{Y} or @samp{1},
a plus sign is printed
where permitted by the Fortran standard. If the first letter
is @samp{n}, @samp{N} or @samp{0}, a plus sign is not printed
in most cases. Default is not to print plus signs.
@section @env{GFORTRAN_DEFAULT_RECL}---Default record length for new files
This environment variable specifies the default record length, in
bytes, for files which are opened without a @code{RECL} tag in the
@code{OPEN} statement. This must be a positive integer. The
default value is 1073741824 bytes (1 GB).
@section @env{GFORTRAN_LIST_SEPARATOR}---Separator for list output
This environment variable specifies the separator when writing
list-directed output. It may contain any number of spaces and
at most one comma. If you specify this on the command line,
be sure to quote spaces, as in
@end smallexample
when @command{a.out} is the compiled Fortran program that you want to run.
Default is a single space.
@section @env{GFORTRAN_CONVERT_UNIT}---Set endianness for unformatted I/O
By setting the @env{GFORTRAN_CONVERT_UNIT} variable, it is possible
to change the representation of data for unformatted files.
The syntax for the @env{GFORTRAN_CONVERT_UNIT} variable is:
GFORTRAN_CONVERT_UNIT: mode | mode ';' exception | exception ;
mode: 'native' | 'swap' | 'big_endian' | 'little_endian' ;
exception: mode ':' unit_list | unit_list ;
unit_list: unit_spec | unit_list unit_spec ;
unit_spec: INTEGER | INTEGER '-' INTEGER ;
@end smallexample
The variable consists of an optional default mode, followed by
a list of optional exceptions, which are separated by semicolons
from the preceding default and each other. Each exception consists
of a format and a comma-separated list of units. Valid values for
the modes are the same as for the @code{CONVERT} specifier:
@itemize @w{}
@item @code{NATIVE} Use the native format. This is the default.
@item @code{SWAP} Swap between little- and big-endian.
@item @code{LITTLE_ENDIAN} Use the little-endian format
for unformatted files.
@item @code{BIG_ENDIAN} Use the big-endian format for unformatted files.
@end itemize
A missing mode for an exception is taken to mean @code{BIG_ENDIAN}.
Examples of values for @env{GFORTRAN_CONVERT_UNIT} are:
@itemize @w{}
@item @code{'big_endian'} Do all unformatted I/O in big_endian mode.
@item @code{'little_endian;native:10-20,25'} Do all unformatted I/O
in little_endian mode, except for units 10 to 20 and 25, which are in
native format.
@item @code{'10-20'} Units 10 to 20 are big-endian, the rest is native.
@end itemize
Setting the environment variables should be done on the command
line or via the @command{export}
command for @command{sh}-compatible shells and via @command{setenv}
for @command{csh}-compatible shells.
Example for @command{sh}:
$ gfortran foo.f90
$ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out
@end smallexample
Example code for @command{csh}:
% gfortran foo.f90
% setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
% ./a.out
@end smallexample
Using anything but the native representation for unformatted data
carries a significant speed overhead. If speed in this area matters
to you, it is best if you use this only for data that needs to be
@xref{CONVERT specifier}, for an alternative way to specify the
data representation for unformatted files. @xref{Runtime Options}, for
setting a default data representation for the whole program. The
@code{CONVERT} specifier overrides the @option{-fconvert} compile options.
@emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
environment variable will override the CONVERT specifier in the
open statement}. This is to give control over data formats to
users who do not have the source code of their program available.
@section @env{GFORTRAN_ERROR_BACKTRACE}---Show backtrace on run-time errors
If the @env{GFORTRAN_ERROR_BACKTRACE} variable is set to @samp{y},
@samp{Y} or @samp{1} (only the first letter is relevant) then a
backtrace is printed when a serious run-time error occurs. To disable
the backtracing, set the variable to @samp{n}, @samp{N}, @samp{0}.
Default is to print a backtrace unless the @option{-fno-backtrace}
compile option was used.
@c =====================================================================
@c =====================================================================
\part{II}{Language Reference}
@end tex
@c ---------------------------------------------------------------------
@c Fortran 2003 and 2008 Status
@c ---------------------------------------------------------------------
@node Fortran 2003 and 2008 status
@chapter Fortran 2003 and 2008 Status
* Fortran 2003 status::
* Fortran 2008 status::
* TS 29113 status::
@end menu
@node Fortran 2003 status
@section Fortran 2003 status
GNU Fortran supports several Fortran 2003 features; an incomplete
list can be found below. See also the
@uref{, wiki page} about Fortran 2003.
@item Procedure pointers including procedure-pointer components with
@code{PASS} attribute.
@item Procedures which are bound to a derived type (type-bound procedures)
including @code{PASS}, @code{PROCEDURE} and @code{GENERIC}, and
operators bound to a type.
@item Abstract interfaces and type extension with the possibility to
override type-bound procedures or to have deferred binding.
@item Polymorphic entities (``@code{CLASS}'') for derived types -- including
@code{SAME_TYPE_AS}, @code{EXTENDS_TYPE_OF} and @code{SELECT TYPE} for
scalars and arrays, including unlimited polymorphism.
@item Generic interface names, which have the same name as derived types,
are now supported. This allows one to write constructor functions. Note
that Fortran does not support static constructor functions. For static
variables, only default initialization or structure-constructor
initialization are available.
@item The @code{ASSOCIATE} construct.
@item Interoperability with C including enumerations,
@item In structure constructors the components with default values may be
@item Extensions to the @code{ALLOCATE} statement, allowing for a
type-specification with type parameter and for allocation and initialization
from a @code{SOURCE=} expression; @code{ALLOCATE} and @code{DEALLOCATE}
optionally return an error message string via @code{ERRMSG=}.
@item Reallocation on assignment: If an intrinsic assignment is
used, an allocatable variable on the left-hand side is automatically allocated
(if unallocated) or reallocated (if the shape is different). Currently, scalar
deferred character length left-hand sides are correctly handled but arrays
are not yet fully implemented.
@item Transferring of allocations via @code{MOVE_ALLOC}.
@item The @code{PRIVATE} and @code{PUBLIC} attributes may be given individually
to derived-type components.
@item In pointer assignments, the lower bound may be specified and
the remapping of elements is supported.
@item For pointers an @code{INTENT} may be specified which affect the
association status not the value of the pointer target.
@item Intrinsics @code{command_argument_count}, @code{get_command},
@code{get_command_argument}, and @code{get_environment_variable}.
@item Support for Unicode characters (ISO 10646) and UTF-8, including
the @code{SELECTED_CHAR_KIND} and @code{NEW_LINE} intrinsic functions.
@item Support for binary, octal and hexadecimal (BOZ) constants in the
intrinsic functions @code{INT}, @code{REAL}, @code{CMPLX} and @code{DBLE}.
@item Support for namelist variables with allocatable and pointer
attribute and nonconstant length type parameter.
@cindex array, constructors
@cindex @code{[...]}
Array constructors using square brackets. That is, @code{[...]} rather
than @code{(/.../)}. Type-specification for array constructors like
@code{(/ some-type :: ... /)}.
@item Extensions to the specification and initialization expressions,
including the support for intrinsics with real and complex arguments.
@item Support for the asynchronous input/output syntax; however, the
data transfer is currently always synchronously performed.
@cindex @code{FLUSH} statement
@cindex statement, @code{FLUSH}
@code{FLUSH} statement.
@cindex @code{IOMSG=} specifier
@code{IOMSG=} specifier for I/O statements.
@cindex @code{ENUM} statement
@cindex @code{ENUMERATOR} statement
@cindex statement, @code{ENUM}
@cindex statement, @code{ENUMERATOR}
@opindex @code{fshort-enums}
Support for the declaration of enumeration constants via the
@code{ENUM} and @code{ENUMERATOR} statements. Interoperability with
@command{gcc} is guaranteed also for the case where the
@command{-fshort-enums} command line option is given.
@cindex TR 15581
TR 15581:
@cindex @code{ALLOCATABLE} dummy arguments
@code{ALLOCATABLE} dummy arguments.
@cindex @code{ALLOCATABLE} function results
@code{ALLOCATABLE} function results
@cindex @code{ALLOCATABLE} components of derived types
@code{ALLOCATABLE} components of derived types
@end itemize
@cindex @code{STREAM} I/O
@cindex @code{ACCESS='STREAM'} I/O
The @code{OPEN} statement supports the @code{ACCESS='STREAM'} specifier,
allowing I/O without any record structure.
Namelist input/output for internal files.
@item Further I/O extensions: Rounding during formatted output, using of
a decimal comma instead of a decimal point, setting whether a plus sign
should appear for positive numbers.
@cindex @code{PROTECTED} statement
@cindex statement, @code{PROTECTED}
The @code{PROTECTED} statement and attribute.
@cindex @code{VALUE} statement
@cindex statement, @code{VALUE}
The @code{VALUE} statement and attribute.
@cindex @code{VOLATILE} statement
@cindex statement, @code{VOLATILE}
The @code{VOLATILE} statement and attribute.
@cindex @code{IMPORT} statement
@cindex statement, @code{IMPORT}
The @code{IMPORT} statement, allowing to import
host-associated derived types.
@item The intrinsic modules @code{ISO_FORTRAN_ENVIRONMENT} is supported,
which contains parameters of the I/O units, storage sizes. Additionally,
procedures for C interoperability are available in the @code{ISO_C_BINDING}
@cindex @code{USE, INTRINSIC} statement
@cindex statement, @code{USE, INTRINSIC}
@cindex @code{ISO_FORTRAN_ENV} statement
@cindex statement, @code{ISO_FORTRAN_ENV}
@code{USE} statement with @code{INTRINSIC} and @code{NON_INTRINSIC}
attribute; supported intrinsic modules: @code{ISO_FORTRAN_ENV},
@code{ISO_C_BINDING}, @code{OMP_LIB} and @code{OMP_LIB_KINDS}.
Renaming of operators in the @code{USE} statement.
@end itemize
@node Fortran 2008 status
@section Fortran 2008 status
The latest version of the Fortran standard is ISO/IEC 1539-1:2010, informally
known as Fortran 2008. The official version is available from International
Organization for Standardization (ISO) or its national member organizations.
The the final draft (FDIS) can be downloaded free of charge from
@url{}. Fortran is developed by the
Working Group 5 of Sub-Committee 22 of the Joint Technical Committee 1 of the
International Organization for Standardization and the International
Electrotechnical Commission (IEC). This group is known as
@uref{, WG5}.
The GNU Fortran compiler supports several of the new features of Fortran 2008;
the @uref{, wiki} has some information
about the current Fortran 2008 implementation status. In particular, the
following is implemented.
@item The @option{-std=f2008} option and support for the file extensions
@file{.f08} and @file{.F08}.
@item The @code{OPEN} statement now supports the @code{NEWUNIT=} option,
which returns a unique file unit, thus preventing inadvertent use of the
same unit in different parts of the program.
@item The @code{g0} format descriptor and unlimited format items.
@item The mathematical intrinsics @code{ASINH}, @code{ACOSH}, @code{ATANH},
@code{ERF}, @code{ERFC}, @code{GAMMA}, @code{LOG_GAMMA}, @code{BESSEL_J0},
@code{BESSEL_J1}, @code{BESSEL_JN}, @code{BESSEL_Y0}, @code{BESSEL_Y1},
@code{BESSEL_YN}, @code{HYPOT}, @code{NORM2}, and @code{ERFC_SCALED}.
@item Using complex arguments with @code{TAN}, @code{SINH}, @code{COSH},
@code{TANH}, @code{ASIN}, @code{ACOS}, and @code{ATAN} is now possible;
@code{ATAN}(@var{Y},@var{X}) is now an alias for @code{ATAN2}(@var{Y},@var{X}).
@item Support of the @code{PARITY} intrinsic functions.
@item The following bit intrinsics: @code{LEADZ} and @code{TRAILZ} for
counting the number of leading and trailing zero bits, @code{POPCNT} and
@code{POPPAR} for counting the number of one bits and returning the parity;
@code{BGE}, @code{BGT}, @code{BLE}, and @code{BLT} for bitwise comparisons;
@code{DSHIFTL} and @code{DSHIFTR} for combined left and right shifts,
@code{MASKL} and @code{MASKR} for simple left and right justified masks,
@code{MERGE_BITS} for a bitwise merge using a mask, @code{SHIFTA},
@code{SHIFTL} and @code{SHIFTR} for shift operations, and the
transformational bit intrinsics @code{IALL}, @code{IANY} and @code{IPARITY}.
@item Support of the @code{EXECUTE_COMMAND_LINE} intrinsic subroutine.
@item Support for the @code{STORAGE_SIZE} intrinsic inquiry function.
@item The @code{INT@{8,16,32@}} and @code{REAL@{32,64,128@}} kind type
parameters and the array-valued named constants @code{INTEGER_KINDS},
the intrinsic module @code{ISO_FORTRAN_ENV}.
@item The module procedures @code{C_SIZEOF} of the intrinsic module
of @code{ISO_FORTRAN_ENV}.
@item Coarray support for serial programs with @option{-fcoarray=single} flag
and experimental support for multiple images with the @option{-fcoarray=lib}
@item The @code{DO CONCURRENT} construct is supported.
@item The @code{BLOCK} construct is supported.
@item The @code{STOP} and the new @code{ERROR STOP} statements now
support all constant expressions.
@item Support for the @code{CONTIGUOUS} attribute.
@item Support for @code{ALLOCATE} with @code{MOLD}.
@item Support for the @code{IMPURE} attribute for procedures, which
allows for @code{ELEMENTAL} procedures without the restrictions of
@item Null pointers (including @code{NULL()}) and not-allocated variables
can be used as actual argument to optional non-pointer, non-allocatable
dummy arguments, denoting an absent argument.
@item Non-pointer variables with @code{TARGET} attribute can be used as
actual argument to @code{POINTER} dummies with @code{INTENT(IN)}.
@item Pointers including procedure pointers and those in a derived
type (pointer components) can now be initialized by a target instead
of only by @code{NULL}.
@item The @code{EXIT} statement (with construct-name) can be now be
used to leave not only the @code{DO} but also the @code{ASSOCIATE},
@code{BLOCK}, @code{IF}, @code{SELECT CASE} and @code{SELECT TYPE}
@item Internal procedures can now be used as actual argument.
@item Minor features: obsolesce diagnostics for @code{ENTRY} with
@option{-std=f2008}; a line may start with a semicolon; for internal
and module procedures @code{END} can be used instead of
now also takes a @code{RADIX} argument; intrinsic types are supported
for @code{TYPE}(@var{intrinsic-type-spec}); multiple type-bound procedures
can be declared in a single @code{PROCEDURE} statement; implied-shape
arrays are supported for named constants (@code{PARAMETER}).
@end itemize
@node TS 29113 status
@section Technical Specification 29113 Status
GNU Fortran supports some of the new features of the Technical
Specification (TS) 29113 on Further Interoperability of Fortran with C.
The @uref{, wiki} has some information
about the current TS 29113 implementation status. In particular, the
following is implemented.
See also @ref{Further Interoperability of Fortran with C}.
@item The @option{-std=f2008ts} option.
@item The @code{OPTIONAL} attribute is allowed for dummy arguments
of @code{BIND(C) procedures.}
@item The @code{RANK} intrinsic is supported.
@item GNU Fortran's implementation for variables with @code{ASYNCHRONOUS}
attribute is compatible with TS 29113.
@item Assumed types (@code{TYPE(*)}.
@item Assumed-rank (@code{DIMENSION(..)}). However, the array descriptor
of the TS is not yet supported.
@end itemize
@c ---------------------------------------------------------------------
@c Compiler Characteristics
@c ---------------------------------------------------------------------
@node Compiler Characteristics
@chapter Compiler Characteristics
This chapter describes certain characteristics of the GNU Fortran
compiler, that are not specified by the Fortran standard, but which
might in some way or another become visible to the programmer.
* KIND Type Parameters::
* Internal representation of LOGICAL variables::
* Thread-safety of the runtime library::
* Data consistency and durability::
@end menu
@node KIND Type Parameters
@section KIND Type Parameters
@cindex kind
The @code{KIND} type parameters supported by GNU Fortran for the primitive
data types are:
@table @code
1, 2, 4, 8*, 16*, default: 4 (1)
1, 2, 4, 8*, 16*, default: 4 (1)
@item REAL
4, 8, 10*, 16*, default: 4 (2)
4, 8, 10*, 16*, default: 4 (2)
1, 4, default: 1
@end table
* = not available on all systems @*
(1) Unless -fdefault-integer-8 is used @*
(2) Unless -fdefault-real-8 is used
The @code{KIND} value matches the storage size in bytes, except for
@code{COMPLEX} where the storage size is twice as much (or both real and
imaginary part are a real value of the given size). It is recommended to use
@code{SELECTED_REAL_KIND} intrinsics or the @code{INT8}, @code{INT16},
@code{INT32}, @code{INT64}, @code{REAL32}, @code{REAL64}, and @code{REAL128}
parameters of the @code{ISO_FORTRAN_ENV} module instead of the concrete values.
The available kind parameters can be found in the constant arrays
@code{REAL_KINDS} in the @code{ISO_FORTRAN_ENV} module
(see @ref{ISO_FORTRAN_ENV}).
@node Internal representation of LOGICAL variables
@section Internal representation of LOGICAL variables
@cindex logical, variable representation
The Fortran standard does not specify how variables of @code{LOGICAL}
type are represented, beyond requiring that @code{LOGICAL} variables
of default kind have the same storage size as default @code{INTEGER}
and @code{REAL} variables. The GNU Fortran internal representation is
as follows.
A @code{LOGICAL(KIND=N)} variable is represented as an
@code{INTEGER(KIND=N)} variable, however, with only two permissible
values: @code{1} for @code{.TRUE.} and @code{0} for
@code{.FALSE.}. Any other integer value results in undefined behavior.
Note that for mixed-language programming using the
@code{ISO_C_BINDING} feature, there is a @code{C_BOOL} kind that can
be used to create @code{LOGICAL(KIND=C_BOOL)} variables which are
interoperable with the C99 _Bool type. The C99 _Bool type has an
internal representation described in the C99 standard, which is
identical to the above description, i.e. with 1 for true and 0 for
false being the only permissible values. Thus the internal
representation of @code{LOGICAL} variables in GNU Fortran is identical
to C99 _Bool, except for a possible difference in storage size
depending on the kind.
@node Thread-safety of the runtime library
@section Thread-safety of the runtime library
@cindex thread-safety, threads
GNU Fortran can be used in programs with multiple threads, e.g.@: by
using OpenMP, by calling OS thread handling functions via the
@code{ISO_C_BINDING} facility, or by GNU Fortran compiled library code
being called from a multi-threaded program.
The GNU Fortran runtime library, (@code{libgfortran}), supports being
called concurrently from multiple threads with the following
During library initialization, the C @code{getenv} function is used,
which need not be thread-safe. Similarly, the @code{getenv}
function is used to implement the @code{GET_ENVIRONMENT_VARIABLE} and
@code{GETENV} intrinsics. It is the responsibility of the user to
ensure that the environment is not being updated concurrently when any
of these actions are taking place.
The @code{EXECUTE_COMMAND_LINE} and @code{SYSTEM} intrinsics are
implemented with the @code{system} function, which need not be
thread-safe. It is the responsibility of the user to ensure that
@code{system} is not called concurrently.
Finally, for platforms not supporting thread-safe POSIX functions,
further functionality might not be thread-safe. For details, please
consult the documentation for your operating system.
@node Data consistency and durability
@section Data consistency and durability
@cindex consistency, durability
This section contains a brief overview of data and metadata
consistency and durability issues when doing I/O.
With respect to durability, GNU Fortran makes no effort to ensure that
data is committed to stable storage. If this is required, the GNU
Fortran programmer can use the intrinsic @code{FNUM} to retrieve the
low level file descriptor corresponding to an open Fortran unit. Then,
using e.g. the @code{ISO_C_BINDING} feature, one can call the
underlying system call to flush dirty data to stable storage, such as
@code{fsync} on POSIX, @code{_commit} on MingW, or @code{fcntl(fd,
F_FULLSYNC, 0)} on Mac OS X. The following example shows how to call
! Declare the interface for POSIX fsync function
function fsync (fd) bind(c,name="fsync")
use iso_c_binding, only: c_int
integer(c_int), value :: fd
integer(c_int) :: fsync
end function fsync
end interface
! Variable declaration
integer :: ret
! Opening unit 10
open (10,file="foo")
! ...
! Perform I/O on unit 10
! ...
! Flush and sync
ret = fsync(fnum(10))
! Handle possible error
if (ret /= 0) stop "Error calling FSYNC"
@end smallexample
With respect to consistency, for regular files GNU Fortran uses
buffered I/O in order to improve performance. This buffer is flushed
automatically when full and in some other situations, e.g. when
closing a unit. It can also be explicitly flushed with the
@code{FLUSH} statement. Also, the buffering can be turned off with the
@code{GFORTRAN_UNBUFFERED_PRECONNECTED} environment variables. Special
files, such as terminals and pipes, are always unbuffered. Sometimes,
however, further things may need to be done in order to allow other
processes to see data that GNU Fortran has written, as follows.
The Windows platform supports a relaxed metadata consistency model,
where file metadata is written to the directory lazily. This means
that, for instance, the @code{dir} command can show a stale size for a
file. One can force a directory metadata update by closing the unit,
or by calling @code{_commit} on the file descriptor. Note, though,
that @code{_commit} will force all dirty data to stable storage, which
is often a very slow operation.
The Network File System (NFS) implements a relaxed consistency model
called open-to-close consistency. Closing a file forces dirty data and
metadata to be flushed to the server, and opening a file forces the
client to contact the server in order to revalidate cached
data. @code{fsync} will also force a flush of dirty data and metadata
to the server. Similar to @code{open} and @code{close}, acquiring and
releasing @code{fcntl} file locks, if the server supports them, will
also force cache validation and flushing dirty data and metadata.
@c ---------------------------------------------------------------------
@c Extensions
@c ---------------------------------------------------------------------
@c Maybe this chapter should be merged with the 'Standards' section,
@c whenever that is written :-)
@node Extensions
@chapter Extensions
@cindex extensions
The two sections below detail the extensions to standard Fortran that are
implemented in GNU Fortran, as well as some of the popular or
historically important extensions that are not (or not yet) implemented.
For the latter case, we explain the alternatives available to GNU Fortran
users, including replacement by standard-conforming code or GNU
* Extensions implemented in GNU Fortran::
* Extensions not implemented in GNU Fortran::
@end menu
@node Extensions implemented in GNU Fortran
@section Extensions implemented in GNU Fortran
@cindex extensions, implemented
GNU Fortran implements a number of extensions over standard
Fortran. This chapter contains information on their syntax and
meaning. There are currently two categories of GNU Fortran
extensions, those that provide functionality beyond that provided
by any standard, and those that are supported by GNU Fortran
purely for backward compatibility with legacy compilers. By default,
@option{-std=gnu} allows the compiler to accept both types of
extensions, but to warn about the use of the latter. Specifying
either @option{-std=f95}, @option{-std=f2003} or @option{-std=f2008}
disables both types of extensions, and @option{-std=legacy} allows both
without warning.
* Old-style kind specifications::
* Old-style variable initialization::
* Extensions to namelist::
* X format descriptor without count field::
* Commas in FORMAT specifications::
* Missing period in FORMAT specifications::
* I/O item lists::
* @code{Q} exponent-letter::
* BOZ literal constants::
* Real array indices::
* Unary operators::
* Implicitly convert LOGICAL and INTEGER values::
* Hollerith constants support::
* Cray pointers::
* CONVERT specifier::
* OpenMP::
* Argument list functions::
@end menu
@node Old-style kind specifications
@subsection Old-style kind specifications
@cindex kind, old-style
GNU Fortran allows old-style kind specifications in declarations. These
look like:
TYPESPEC*size x,y,z
@end smallexample
where @code{TYPESPEC} is a basic type (@code{INTEGER}, @code{REAL},
etc.), and where @code{size} is a byte count corresponding to the
storage size of a valid kind for that type. (For @code{COMPLEX}
variables, @code{size} is the total size of the real and imaginary
parts.) The statement then declares @code{x}, @code{y} and @code{z} to
be of type @code{TYPESPEC} with the appropriate kind. This is
equivalent to the standard-conforming declaration
TYPESPEC(k) x,y,z
@end smallexample
where @code{k} is the kind parameter suitable for the intended precision. As
kind parameters are implementation-dependent, use the @code{KIND},
@code{SELECTED_INT_KIND} and @code{SELECTED_REAL_KIND} intrinsics to retrieve
the correct value, for instance @code{REAL*8 x} can be replaced by:
REAL(KIND=dbl) :: x
@end smallexample
@node Old-style variable initialization
@subsection Old-style variable initialization
GNU Fortran allows old-style initialization of variables of the
INTEGER i/1/,j/2/
REAL x(2,2) /3*0.,1./
@end smallexample
The syntax for the initializers is as for the @code{DATA} statement, but
unlike in a @code{DATA} statement, an initializer only applies to the
variable immediately preceding the initialization. In other words,
something like @code{INTEGER I,J/2,3/} is not valid. This style of
initialization is only allowed in declarations without double colons
(@code{::}); the double colons were introduced in Fortran 90, which also
introduced a standard syntax for initializing variables in type
Examples of standard-conforming code equivalent to the above example
! Fortran 90
INTEGER :: i = 1, j = 2
REAL :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
! Fortran 77
REAL x(2,2)
DATA i/1/, j/2/, x/3*0.,1./
@end smallexample
Note that variables which are explicitly initialized in declarations
or in @code{DATA} statements automatically acquire the @code{SAVE}
@node Extensions to namelist
@subsection Extensions to namelist
@cindex Namelist
GNU Fortran fully supports the Fortran 95 standard for namelist I/O
including array qualifiers, substrings and fully qualified derived types.
The output from a namelist write is compatible with namelist read. The
output has all names in upper case and indentation to column 1 after the
namelist name. Two extensions are permitted:
Old-style use of @samp{$} instead of @samp{&}
X(:)%Y(2) = 1.0 2.0 3.0
CH(1:4) = "abcd"
@end smallexample
It should be noted that the default terminator is @samp{/} rather than
Querying of the namelist when inputting from stdin. After at least
one space, entering @samp{?} sends to stdout the namelist name and the names of
the variables in the namelist:
@end smallexample
Entering @samp{=?} outputs the namelist to stdout, as if
@code{WRITE(*,NML = mynml)} had been called:
X(1)%Y= 0.000000 , 1.000000 , 0.000000 ,
X(2)%Y= 0.000000 , 2.000000 , 0.000000 ,
X(3)%Y= 0.000000 , 3.000000 , 0.000000 ,
CH=abcd, /
@end smallexample
To aid this dialog, when input is from stdin, errors send their
messages to stderr and execution continues, even if @code{IOSTAT} is set.
@code{PRINT} namelist is permitted. This causes an error if
@option{-std=f95} is used.
PROGRAM test_print
REAL, dimension (4) :: x = (/1.0, 2.0, 3.0, 4.0/)
NAMELIST /mynml/ x
PRINT mynml
END PROGRAM test_print
@end smallexample
Expanded namelist reads are permitted. This causes an error if
@option{-std=f95} is used. In the following example, the first element
of the array will be given the value 0.00 and the two succeeding
elements will be given the values 1.00 and 2.00.
X(1,1) = 0.00 , 1.00 , 2.00
@end smallexample
@node X format descriptor without count field
@subsection @code{X} format descriptor without count field
To support legacy codes, GNU Fortran permits the count field of the
@code{X} edit descriptor in @code{FORMAT} statements to be omitted.
When omitted, the count is implicitly assumed to be one.
PRINT 10, 2, 3
10 FORMAT (I1, X, I1)
@end smallexample
@node Commas in FORMAT specifications
@subsection Commas in @code{FORMAT} specifications
To support legacy codes, GNU Fortran allows the comma separator
to be omitted immediately before and after character string edit
descriptors in @code{FORMAT} statements.
PRINT 10, 2, 3
10 FORMAT ('FOO='I1' BAR='I2)
@end smallexample
@node Missing period in FORMAT specifications
@subsection Missing period in @code{FORMAT} specifications
To support legacy codes, GNU Fortran allows missing periods in format
specifications if and only if @option{-std=legacy} is given on the
command line. This is considered non-conforming code and is
REAL :: value
READ(*,10) value
10 FORMAT ('F4')
@end smallexample
@node I/O item lists
@subsection I/O item lists
@cindex I/O item lists
To support legacy codes, GNU Fortran allows the input item list
of the @code{READ} statement, and the output item lists of the
@code{WRITE} and @code{PRINT} statements, to start with a comma.
@node @code{Q} exponent-letter
@subsection @code{Q} exponent-letter
@cindex @code{Q} exponent-letter
GNU Fortran accepts real literal constants with an exponent-letter
of @code{Q}, for example, @code{1.23Q45}. The constant is interpreted
as a @code{REAL(16)} entity on targets that support this type. If
the target does not support @code{REAL(16)} but has a @code{REAL(10)}
type, then the real-literal-constant will be interpreted as a
@code{REAL(10)} entity. In the absence of @code{REAL(16)} and
@code{REAL(10)}, an error will occur.
@node BOZ literal constants
@subsection BOZ literal constants
@cindex BOZ literal constants
Besides decimal constants, Fortran also supports binary (@code{b}),
octal (@code{o}) and hexadecimal (@code{z}) integer constants. The
syntax is: @samp{prefix quote digits quote}, were the prefix is
either @code{b}, @code{o} or @code{z}, quote is either @code{'} or
@code{"} and the digits are for binary @code{0} or @code{1}, for
octal between @code{0} and @code{7}, and for hexadecimal between
@code{0} and @code{F}. (Example: @code{b'01011101'}.)
Up to Fortran 95, BOZ literals were only allowed to initialize
integer variables in DATA statements. Since Fortran 2003 BOZ literals
are also allowed as argument of @code{REAL}, @code{DBLE}, @code{INT}
and @code{CMPLX}; the result is the same as if the integer BOZ
literal had been converted by @code{TRANSFER} to, respectively,
@code{real}, @code{double precision}, @code{integer} or @code{complex}.
As GNU Fortran extension the intrinsic procedures @code{FLOAT},
@code{DFLOAT}, @code{COMPLEX} and @code{DCMPLX} are treated alike.
As an extension, GNU Fortran allows hexadecimal BOZ literal constants to
be specified using the @code{X} prefix, in addition to the standard
@code{Z} prefix. The BOZ literal can also be specified by adding a
suffix to the string, for example, @code{Z'ABC'} and @code{'ABC'Z} are
Furthermore, GNU Fortran allows using BOZ literal constants outside
DATA statements and the four intrinsic functions allowed by Fortran 2003.
In DATA statements, in direct assignments, where the right-hand side
only contains a BOZ literal constant, and for old-style initializers of
the form @code{integer i /o'0173'/}, the constant is transferred
as if @code{TRANSFER} had been used; for @code{COMPLEX} numbers, only
the real part is initialized unless @code{CMPLX} is used. In all other
cases, the BOZ literal constant is converted to an @code{INTEGER} value with
the largest decimal representation. This value is then converted
numerically to the type and kind of the variable in question.
(For instance, @code{real :: r = b'0000001' + 1} initializes @code{r}
with @code{2.0}.) As different compilers implement the extension
differently, one should be careful when doing bitwise initialization
of non-integer variables.
Note that initializing an @code{INTEGER} variable with a statement such
as @code{DATA i/Z'FFFFFFFF'/} will give an integer overflow error rather
than the desired result of @math{-1} when @code{i} is a 32-bit integer
on a system that supports 64-bit integers. The @samp{-fno-range-check}
option can be used as a workaround for legacy code that initializes
integers in this manner.
@node Real array indices
@subsection Real array indices
@cindex array, indices of type real
As an extension, GNU Fortran allows the use of @code{REAL} expressions
or variables as array indices.
@node Unary operators
@subsection Unary operators
@cindex operators, unary
As an extension, GNU Fortran allows unary plus and unary minus operators
to appear as the second operand of binary arithmetic operators without
the need for parenthesis.
X = Y * -Z
@end smallexample
@node Implicitly convert LOGICAL and INTEGER values
@subsection Implicitly convert @code{LOGICAL} and @code{INTEGER} values
@cindex conversion, to integer
@cindex conversion, to logical
As an extension for backwards compatibility with other compilers, GNU
Fortran allows the implicit conversion of @code{LOGICAL} values to
@code{INTEGER} values and vice versa. When converting from a
@code{LOGICAL} to an @code{INTEGER}, @code{.FALSE.} is interpreted as
zero, and @code{.TRUE.} is interpreted as one. When converting from
@code{INTEGER} to @code{LOGICAL}, the value zero is interpreted as
@code{.FALSE.} and any nonzero value is interpreted as @code{.TRUE.}.
l = 1
@end smallexample
i = .TRUE.
@end smallexample
However, there is no implicit conversion of @code{INTEGER} values in
@code{if}-statements, nor of @code{LOGICAL} or @code{INTEGER} values
in I/O operations.
@node Hollerith constants support
@subsection Hollerith constants support
@cindex Hollerith constants
GNU Fortran supports Hollerith constants in assignments, function
arguments, and @code{DATA} and @code{ASSIGN} statements. A Hollerith
constant is written as a string of characters preceded by an integer
constant indicating the character count, and the letter @code{H} or
@code{h}, and stored in bytewise fashion in a numeric (@code{INTEGER},
@code{REAL}, or @code{complex}) or @code{LOGICAL} variable. The
constant will be padded or truncated to fit the size of the variable in
which it is stored.
Examples of valid uses of Hollerith constants:
complex*16 x(2)
data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/
call foo (4h abc)
@end smallexample
Invalid Hollerith constants examples:
integer*4 a
a = 8H12345678 ! Valid, but the Hollerith constant will be truncated.
a = 0H ! At least one character is needed.
@end smallexample
In general, Hollerith constants were used to provide a rudimentary
facility for handling character strings in early Fortran compilers,
prior to the introduction of @code{CHARACTER} variables in Fortran 77;
in those cases, the standard-compliant equivalent is to convert the
program to use proper character strings. On occasion, there may be a
case where the intent is specifically to initialize a numeric variable
with a given byte sequence. In these cases, the same result can be
obtained by using the @code{TRANSFER} statement, as in this example.
a = TRANSFER ("abcd", a) ! equivalent to: a = 4Habcd
@end smallexample
@node Cray pointers
@subsection Cray pointers
@cindex pointer, Cray
Cray pointers are part of a non-standard extension that provides a
C-like pointer in Fortran. This is accomplished through a pair of
variables: an integer "pointer" that holds a memory address, and a
"pointee" that is used to dereference the pointer.
Pointer/pointee pairs are declared in statements of the form:
pointer ( <pointer> , <pointee> )
@end smallexample
pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ...
@end smallexample
The pointer is an integer that is intended to hold a memory address.
The pointee may be an array or scalar. A pointee can be an assumed
size array---that is, the last dimension may be left unspecified by
using a @code{*} in place of a value---but a pointee cannot be an
assumed shape array. No space is allocated for the pointee.
The pointee may have its type declared before or after the pointer
statement, and its array specification (if any) may be declared
before, during, or after the pointer statement. The pointer may be
declared as an integer prior to the pointer statement. However, some
machines have default integer sizes that are different than the size
of a pointer, and so the following code is not portable:
integer ipt
pointer (ipt, iarr)
@end smallexample
If a pointer is declared with a kind that is too small, the compiler
will issue a warning; the resulting binary will probably not work
correctly, because the memory addresses stored in the pointers may be
truncated. It is safer to omit the first line of the above example;
if explicit declaration of ipt's type is omitted, then the compiler
will ensure that ipt is an integer variable large enough to hold a
Pointer arithmetic is valid with Cray pointers, but it is not the same
as C pointer arithmetic. Cray pointers are just ordinary integers, so
the user is responsible for determining how many bytes to add to a
pointer in order to increment it. Consider the following example:
real target(10)
real pointee(10)
pointer (ipt, pointee)
ipt = loc (target)
ipt = ipt + 1
@end smallexample
The last statement does not set @code{ipt} to the address of
@code{target(1)}, as it would in C pointer arithmetic. Adding @code{1}
to @code{ipt} just adds one byte to the address stored in @code{ipt}.
Any expression involving the pointee will be translated to use the
value stored in the pointer as the base address.
To get the address of elements, this extension provides an intrinsic
function @code{LOC()}. The @code{LOC()} function is equivalent to the
@code{&} operator in C, except the address is cast to an integer type:
real ar(10)
pointer(ipt, arpte(10))
real arpte
ipt = loc(ar) ! Makes arpte is an alias for ar
arpte(1) = 1.0 ! Sets ar(1) to 1.0
@end smallexample
The pointer can also be set by a call to the @code{MALLOC} intrinsic
(see @ref{MALLOC}).
Cray pointees often are used to alias an existing variable. For
integer target(10)
integer iarr(10)
pointer (ipt, iarr)
ipt = loc(target)
@end smallexample
As long as @code{ipt} remains unchanged, @code{iarr} is now an alias for
@code{target}. The optimizer, however, will not detect this aliasing, so
it is unsafe to use @code{iarr} and @code{target} simultaneously. Using
a pointee in any way that violates the Fortran aliasing rules or
assumptions is illegal. It is the user's responsibility to avoid doing
this; the compiler works under the assumption that no such aliasing
Cray pointers will work correctly when there is no aliasing (i.e., when
they are used to access a dynamically allocated block of memory), and
also in any routine where a pointee is used, but any variable with which
it shares storage is not used. Code that violates these rules may not
run as the user intends. This is not a bug in the optimizer; any code
that violates the aliasing rules is illegal. (Note that this is not
unique to GNU Fortran; any Fortran compiler that supports Cray pointers
will ``incorrectly'' optimize code with illegal aliasing.)
There are a number of restrictions on the attributes that can be applied
to Cray pointers and pointees. Pointees may not have the
@code{ALLOCATABLE}, @code{INTENT}, @code{OPTIONAL}, @code{DUMMY},
@code{TARGET}, @code{INTRINSIC}, or @code{POINTER} attributes. Pointers
may not have the @code{DIMENSION}, @code{POINTER}, @code{TARGET},
@code{ALLOCATABLE}, @code{EXTERNAL}, or @code{INTRINSIC} attributes, nor
may they be function results. Pointees may not occur in more than one
pointer statement. A pointee cannot be a pointer. Pointees cannot occur
in equivalence, common, or data statements.
A Cray pointer may also point to a function or a subroutine. For
example, the following excerpt is valid:
implicit none
external sub
pointer (subptr,subpte)
external subpte
subptr = loc(sub)
call subpte()
subroutine sub
end subroutine sub
@end smallexample
A pointer may be modified during the course of a program, and this
will change the location to which the pointee refers. However, when
pointees are passed as arguments, they are treated as ordinary
variables in the invoked function. Subsequent changes to the pointer
will not change the base address of the array that was passed.
@node CONVERT specifier
@subsection @code{CONVERT} specifier
@cindex @code{CONVERT} specifier
GNU Fortran allows the conversion of unformatted data between little-
and big-endian representation to facilitate moving of data
between different systems. The conversion can be indicated with
the @code{CONVERT} specifier on the @code{OPEN} statement.
@xref{GFORTRAN_CONVERT_UNIT}, for an alternative way of specifying
the data format via an environment variable.
Valid values for @code{CONVERT} are:
@itemize @w{}
@item @code{CONVERT='NATIVE'} Use the native format. This is the default.
@item @code{CONVERT='SWAP'} Swap between little- and big-endian.
@item @code{CONVERT='LITTLE_ENDIAN'} Use the little-endian representation
for unformatted files.
@item @code{CONVERT='BIG_ENDIAN'} Use the big-endian representation for
unformatted files.
@end itemize
Using the option could look like this:
open(file='big.dat',form='unformatted',access='sequential', &
@end smallexample
The value of the conversion can be queried by using
@code{INQUIRE(CONVERT=ch)}. The values returned are
@code{'BIG_ENDIAN'} and @code{'LITTLE_ENDIAN'}.
@code{CONVERT} works between big- and little-endian for
@code{INTEGER} values of all supported kinds and for @code{REAL}
on IEEE systems of kinds 4 and 8. Conversion between different
``extended double'' types on different architectures such as
m68k and x86_64, which GNU Fortran
supports as @code{REAL(KIND=10)} and @code{REAL(KIND=16)}, will
probably not work.
@emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
environment variable will override the CONVERT specifier in the
open statement}. This is to give control over data formats to
users who do not have the source code of their program available.
Using anything but the native representation for unformatted data
carries a significant speed overhead. If speed in this area matters
to you, it is best if you use this only for data that needs to be
@node OpenMP
@subsection OpenMP
@cindex OpenMP
OpenMP (Open Multi-Processing) is an application programming
interface (API) that supports multi-platform shared memory
multiprocessing programming in C/C++ and Fortran on many
architectures, including Unix and Microsoft Windows platforms.
It consists of a set of compiler directives, library routines,
and environment variables that influence run-time behavior.
GNU Fortran strives to be compatible to the
OpenMP Application Program Interface v3.1}.
To enable the processing of the OpenMP directive @code{!$omp} in
free-form source code; the @code{c$omp}, @code{*$omp} and @code{!$omp}
directives in fixed form; the @code{!$} conditional compilation sentinels
in free form; and the @code{c$}, @code{*$} and @code{!$} sentinels
in fixed form, @command{gfortran} needs to be invoked with the
@option{-fopenmp}. This also arranges for automatic linking of the
GNU OpenMP runtime library @ref{Top,,libgomp,libgomp,GNU OpenMP
runtime library}.
The OpenMP Fortran runtime library routines are provided both in a
form of a Fortran 90 module named @code{omp_lib} and in a form of
a Fortran @code{include} file named @file{omp_lib.h}.
An example of a parallelized loop taken from Appendix A.1 of
the OpenMP Application Program Interface v2.5:
!$OMP PARALLEL DO !I is private by default
DO I=2,N
B(I) = (A(I) + A(I-1)) / 2.0
@end smallexample
Please note:
@option{-fopenmp} implies @option{-frecursive}, i.e., all local arrays
will be allocated on the stack. When porting existing code to OpenMP,
this may lead to surprising results, especially to segmentation faults
if the stacksize is limited.
On glibc-based systems, OpenMP enabled applications cannot be statically
linked due to limitations of the underlying pthreads-implementation. It
might be possible to get a working solution if
@command{-Wl,--whole-archive -lpthread -Wl,--no-whole-archive} is added
to the command line. However, this is not supported by @command{gcc} and
thus not recommended.
@end itemize
@node Argument list functions
@subsection Argument list functions @code{%VAL}, @code{%REF} and @code{%LOC}
@cindex argument list functions
@cindex @code{%VAL}
@cindex @code{%REF}
@cindex @code{%LOC}
GNU Fortran supports argument list functions @code{%VAL}, @code{%REF}
and @code{%LOC} statements, for backward compatibility with g77.
It is recommended that these should be used only for code that is
accessing facilities outside of GNU Fortran, such as operating system
or windowing facilities. It is best to constrain such uses to isolated
portions of a program--portions that deal specifically and exclusively
with low-level, system-dependent facilities. Such portions might well
provide a portable interface for use by the program as a whole, but are
themselves not portable, and should be thoroughly tested each time they
are rebuilt using a new compiler or version of a compiler.
@code{%VAL} passes a scalar argument by value, @code{%REF} passes it by
reference and @code{%LOC} passes its memory location. Since gfortran
already passes scalar arguments by reference, @code{%REF} is in effect
a do-nothing. @code{%LOC} has the same effect as a Fortran pointer.
An example of passing an argument by value to a C subroutine foo.:
C prototype void foo_ (float x);
external foo
real*4 x
x = 3.14159
call foo (%VAL (x))
@end smallexample
For details refer to the g77 manual
Also, @code{c_by_val.f} and its partner @code{c_by_val.c} of the
GNU Fortran testsuite are worth a look.
@node Extensions not implemented in GNU Fortran
@section Extensions not implemented in GNU Fortran
@cindex extensions, not implemented
The long history of the Fortran language, its wide use and broad
userbase, the large number of different compiler vendors and the lack of
some features crucial to users in the first standards have lead to the
existence of a number of important extensions to the language. While
some of the most useful or popular extensions are supported by the GNU
Fortran compiler, not all existing extensions are supported. This section
aims at listing these extensions and offering advice on how best make
code that uses them running with the GNU Fortran compiler.
@c More can be found here:
@c --
@c -- the list of Fortran and libgfortran bugs closed as WONTFIX:
@c * UNION and MAP::
* ENCODE and DECODE statements::
* Variable FORMAT expressions::
@c * Q edit descriptor::
@c * AUTOMATIC statement::
@c * TYPE and ACCEPT I/O Statements::
@c * .XOR. operator::
@c * Omitted arguments in procedure call::
* Alternate complex function syntax::
@end menu
@subsection @code{STRUCTURE} and @code{RECORD}
@cindex @code{STRUCTURE}
@cindex @code{RECORD}
Record structures are a pre-Fortran-90 vendor extension to create
user-defined aggregate data types. GNU Fortran does not support
record structures, only Fortran 90's ``derived types'', which have
a different syntax.
In many cases, record structures can easily be converted to derived types.
To convert, replace @code{STRUCTURE /}@var{structure-name}@code{/}
by @code{TYPE} @var{type-name}. Additionally, replace
@code{RECORD /}@var{structure-name}@code{/} by
@code{TYPE(}@var{type-name}@code{)}. Finally, in the component access,
replace the period (@code{.}) by the percent sign (@code{%}).
Here is an example of code using the non portable record structure syntax:
! Declaring a structure named ``item'' and containing three fields:
! an integer ID, an description string and a floating-point price.
CHARACTER(LEN=200) description
REAL price
! Define two variables, an single record of type ``item''
! named ``pear'', and an array of items named ``store_catalog''
RECORD /item/ pear, store_catalog(100)
! We can directly access the fields of both variables = 92316
pear.description = "juicy D'Anjou pear"
pear.price = 0.15
store_catalog(7).id = 7831
store_catalog(7).description = "milk bottle"
store_catalog(7).price = 1.2
! We can also manipulate the whole structure
store_catalog(12) = pear
print *, store_catalog(12)
@end example
This code can easily be rewritten in the Fortran 90 syntax as following:
! ``STRUCTURE /name/ ... END STRUCTURE'' becomes
! ``TYPE name ... END TYPE''
TYPE item
CHARACTER(LEN=200) description
REAL price
! ``RECORD /name/ variable'' becomes ``TYPE(name) variable''
TYPE(item) pear, store_catalog(100)
! Instead of using a dot (.) to access fields of a record, the
! standard syntax uses a percent sign (%)
pear%id = 92316
pear%description = "juicy D'Anjou pear"
pear%price = 0.15
store_catalog(7)%id = 7831
store_catalog(7)%description = "milk bottle"
store_catalog(7)%price = 1.2
! Assignments of a whole variable do not change
store_catalog(12) = pear
print *, store_catalog(12)
@end example
@c @node UNION and MAP
@c @subsection @code{UNION} and @code{MAP}
@c @cindex @code{UNION}
@c @cindex @code{MAP}
@c For help writing this one, see
@c and
@node ENCODE and DECODE statements
@subsection @code{ENCODE} and @code{DECODE} statements
@cindex @code{ENCODE}
@cindex @code{DECODE}
GNU Fortran does not support the @code{ENCODE} and @code{DECODE}
statements. These statements are best replaced by @code{READ} and
@code{WRITE} statements involving internal files (@code{CHARACTER}
variables and arrays), which have been part of the Fortran standard since
Fortran 77. For example, replace a code fragment like
c ... Code that sets LINE
DECODE (80, 9000, LINE) A, B, C
9000 FORMAT (1X, 3(F10.5))
@end smallexample
with the following:
c ... Code that sets LINE
9000 FORMAT (1X, 3(F10.5))
@end smallexample
Similarly, replace a code fragment like
c ... Code that sets A, B and C
ENCODE (80, 9000, LINE) A, B, C
9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
@end smallexample
with the following:
c ... Code that sets A, B and C
9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
@end smallexample
@node Variable FORMAT expressions
@subsection Variable @code{FORMAT} expressions
@cindex @code{FORMAT}
A variable @code{FORMAT} expression is format statement which includes
angle brackets enclosing a Fortran expression: @code{FORMAT(I<N>)}. GNU
Fortran does not support this legacy extension. The effect of variable
format expressions can be reproduced by using the more powerful (and
standard) combination of internal output and string formats. For example,
replace a code fragment like this:
WRITE(6,20) INT1
20 FORMAT(I<N+1>)
@end smallexample
with the following:
c Variable declaration
c Other code here...
WRITE(FMT,'("(I", I0, ")")') N+1
@end smallexample
or with:
c Variable declaration
c Other code here...
WRITE(6,"(I" // ADJUSTL(FMT) // ")") INT1
@end smallexample
@node Alternate complex function syntax
@subsection Alternate complex function syntax
@cindex Complex function
Some Fortran compilers, including @command{g77}, let the user declare
complex functions with the syntax @code{COMPLEX FUNCTION name*16()}, as
well as @code{COMPLEX*16 FUNCTION name()}. Both are non-standard, legacy
extensions. @command{gfortran} accepts the latter form, which is more
common, but not the former.
@c ---------------------------------------------------------------------
@c Mixed-Language Programming
@c ---------------------------------------------------------------------
@node Mixed-Language Programming
@chapter Mixed-Language Programming
@cindex Interoperability
@cindex Mixed-language programming
* Interoperability with C::
* GNU Fortran Compiler Directives::
* Non-Fortran Main Program::
@end menu
This chapter is about mixed-language interoperability, but also applies
if one links Fortran code compiled by different compilers. In most cases,
use of the C Binding features of the Fortran 2003 standard is sufficient,
and their use is highly recommended.
@node Interoperability with C
@section Interoperability with C
* Intrinsic Types::
* Derived Types and struct::
* Interoperable Global Variables::
* Interoperable Subroutines and Functions::
* Working with Pointers::
* Further Interoperability of Fortran with C::
@end menu
Since Fortran 2003 (ISO/IEC 1539-1:2004(E)) there is a
standardized way to generate procedure and derived-type
declarations and global variables which are interoperable with C
(ISO/IEC 9899:1999). The @code{bind(C)} attribute has been added
to inform the compiler that a symbol shall be interoperable with C;
also, some constraints are added. Note, however, that not
all C features have a Fortran equivalent or vice versa. For instance,
neither C's unsigned integers nor C's functions with variable number
of arguments have an equivalent in Fortran.
Note that array dimensions are reversely ordered in C and that arrays in
C always start with index 0 while in Fortran they start by default with
1. Thus, an array declaration @code{A(n,m)} in Fortran matches
@code{A[m][n]} in C and accessing the element @code{A(i,j)} matches
@code{A[j-1][i-1]}. The element following @code{A(i,j)} (C: @code{A[j-1][i-1]};
assuming @math{i < n}) in memory is @code{A(i+1,j)} (C: @code{A[j-1][i]}).
@node Intrinsic Types
@subsection Intrinsic Types
In order to ensure that exactly the same variable type and kind is used
in C and Fortran, the named constants shall be used which are defined in the
@code{ISO_C_BINDING} intrinsic module. That module contains named constants
for kind parameters and character named constants for the escape sequences
in C. For a list of the constants, see @ref{ISO_C_BINDING}.
@node Derived Types and struct
@subsection Derived Types and struct
For compatibility of derived types with @code{struct}, one needs to use
the @code{BIND(C)} attribute in the type declaration. For instance, the
following type declaration
TYPE, BIND(C) :: myType
INTEGER(C_INT) :: i1, i2
@end smallexample
matches the following @code{struct} declaration in C
struct @{
int i1, i2;
/* Note: "char" might be signed or unsigned. */
signed char i3;
double d1;
float _Complex c1;
char str[5];
@} myType;
@end smallexample
Derived types with the C binding attribute shall not have the @code{sequence}
attribute, type parameters, the @code{extends} attribute, nor type-bound
procedures. Every component must be of interoperable type and kind and may not
have the @code{pointer} or @code{allocatable} attribute. The names of the
components are irrelevant for interoperability.
As there exist no direct Fortran equivalents, neither unions nor structs
with bit field or variable-length array members are interoperable.
@node Interoperable Global Variables
@subsection Interoperable Global Variables
Variables can be made accessible from C using the C binding attribute,
optionally together with specifying a binding name. Those variables
have to be declared in the declaration part of a @code{MODULE},
be of interoperable type, and have neither the @code{pointer} nor
the @code{allocatable} attribute.
USE myType_module
integer(C_INT), bind(C, name="_MyProject_flags") :: global_flag
type(myType), bind(C) :: tp
@end smallexample
Here, @code{_MyProject_flags} is the case-sensitive name of the variable
as seen from C programs while @code{global_flag} is the case-insensitive
name as seen from Fortran. If no binding name is specified, as for
@var{tp}, the C binding name is the (lowercase) Fortran binding name.
If a binding name is specified, only a single variable may be after the
double colon. Note of warning: You cannot use a global variable to
access @var{errno} of the C library as the C standard allows it to be
a macro. Use the @code{IERRNO} intrinsic (GNU extension) instead.
@node Interoperable Subroutines and Functions
@subsection Interoperable Subroutines and Functions
Subroutines and functions have to have the @code{BIND(C)} attribute to
be compatible with C. The dummy argument declaration is relatively
straightforward. However, one needs to be careful because C uses
call-by-value by default while Fortran behaves usually similar to
call-by-reference. Furthermore, strings and pointers are handled
differently. Note that in Fortran 2003 and 2008 only explicit size
and assumed-size arrays are supported but not assumed-shape or
deferred-shape (i.e. allocatable or pointer) arrays. However, those
are allowed since the Technical Specification 29113, see
@ref{Further Interoperability of Fortran with C}
To pass a variable by value, use the @code{VALUE} attribute.
Thus, the following C prototype
@code{int func(int i, int *j)}
@end smallexample
matches the Fortran declaration
integer(c_int) function func(i,j)
use iso_c_binding, only: c_int
integer(c_int), VALUE :: i
integer(c_int) :: j
@end smallexample
Note that pointer arguments also frequently need the @code{VALUE} attribute,
see @ref{Working with Pointers}.
Strings are handled quite differently in C and Fortran. In C a string
is a @code{NUL}-terminated array of characters while in Fortran each string
has a length associated with it and is thus not terminated (by e.g.
@code{NUL}). For example, if one wants to use the following C function,
#include <stdio.h>
void print_C(char *string) /* equivalent: char string[] */
printf("%s\n", string);
@end smallexample
to print ``Hello World'' from Fortran, one can call it using
use iso_c_binding, only: C_CHAR, C_NULL_CHAR
subroutine print_c(string) bind(C, name="print_C")
use iso_c_binding, only: c_char
character(kind=c_char) :: string(*)
end subroutine print_c
end interface
call print_c(C_CHAR_"Hello World"//C_NULL_CHAR)
@end smallexample
As the example shows, one needs to ensure that the
string is @code{NUL} terminated. Additionally, the dummy argument
@var{string} of @code{print_C} is a length-one assumed-size
array; using @code{character(len=*)} is not allowed. The example
above uses @code{c_char_"Hello World"} to ensure the string
literal has the right type; typically the default character
kind and @code{c_char} are the same and thus @code{"Hello World"}
is equivalent. However, the standard does not guarantee this.
The use of strings is now further illustrated using the C library
function @code{strncpy}, whose prototype is
char *strncpy(char *restrict s1, const char *restrict s2, size_t n);
@end smallexample
The function @code{strncpy} copies at most @var{n} characters from
string @var{s2} to @var{s1} and returns @var{s1}. In the following
example, we ignore the return value:
use iso_c_binding
implicit none
character(len=30) :: str,str2
! Ignore the return value of strncpy -> subroutine
! "restrict" is always assumed if we do not pass a pointer
subroutine strncpy(dest, src, n) bind(C)
character(kind=c_char), intent(out) :: dest(*)
character(kind=c_char), intent(in) :: src(*)
integer(c_size_t), value, intent(in) :: n
end subroutine strncpy
end interface
str = repeat('X',30) ! Initialize whole string with 'X'
call strncpy(str, c_char_"Hello World"//C_NULL_CHAR, &
len(c_char_"Hello World",kind=c_size_t))
print '(a)', str ! prints: "Hello WorldXXXXXXXXXXXXXXXXXXX"
@end smallexample
The intrinsic procedures are described in @ref{Intrinsic Procedures}.
@node Working with Pointers
@subsection Working with Pointers
C pointers are represented in Fortran via the special opaque derived type
@code{type(c_ptr)} (with private components). Thus one needs to
use intrinsic conversion procedures to convert from or to C pointers.
For some applications, using an assumed type (@code{TYPE(*)}) can be an
alternative to a C pointer; see
@ref{Further Interoperability of Fortran with C}.
For example,
use iso_c_binding
type(c_ptr) :: cptr1, cptr2
integer, target :: array(7), scalar
integer, pointer :: pa(:), ps
cptr1 = c_loc(array(1)) ! The programmer needs to ensure that the
! array is contiguous if required by the C
! procedure
cptr2 = c_loc(scalar)
call c_f_pointer(cptr2, ps)
call c_f_pointer(cptr2, pa, shape=[7])
@end smallexample
When converting C to Fortran arrays, the one-dimensional @code{SHAPE} argument
has to be passed.
If a pointer is a dummy-argument of an interoperable procedure, it usually
has to be declared using the @code{VALUE} attribute. @code{void*}
matches @code{TYPE(C_PTR), VALUE}, while @code{TYPE(C_PTR)} alone
matches @code{void**}.
Procedure pointers are handled analogously to pointers; the C type is
@code{TYPE(C_FUNPTR)} and the intrinsic conversion procedures are
@code{C_F_PROCPOINTER} and @code{C_FUNLOC}.
Let us consider two examples of actually passing a procedure pointer from
C to Fortran and vice versa. Note that these examples are also very
similar to passing ordinary pointers between both languages. First,
consider this code in C:
/* Procedure implemented in Fortran. */
void get_values (void (*)(double));
/* Call-back routine we want called from Fortran. */
print_it (double x)
printf ("Number is %f.\n", x);
/* Call Fortran routine and pass call-back to it. */
foobar ()
get_values (&print_it);
@end smallexample
A matching implementation for @code{get_values} in Fortran, that correctly
receives the procedure pointer from C and is able to call it, is given
in the following @code{MODULE}:
! Define interface of call-back routine.
SUBROUTINE callback (x)
! Define C-bound procedure.
SUBROUTINE get_values (cproc) BIND(C)
PROCEDURE(callback), POINTER :: proc
! Convert C to Fortran procedure pointer.
CALL C_F_PROCPOINTER (cproc, proc)
! Call it.
CALL proc (1.0_C_DOUBLE)
CALL proc (-42.0_C_DOUBLE)
CALL proc (18.12_C_DOUBLE)
@end smallexample
Next, we want to call a C routine that expects a procedure pointer argument
and pass it a Fortran procedure (which clearly must be interoperable!).
Again, the C function may be:
call_it (int (*func)(int), int arg)
return func (arg);
@end smallexample
It can be used as in the following Fortran code:
! Define interface of C function.
INTEGER(KIND=C_INT) FUNCTION call_it (func, arg) BIND(C)
! Define procedure passed to C function.
! It must be interoperable!
double_it = arg + arg
END FUNCTION double_it
! Call C function.
SUBROUTINE foobar ()
TYPE(C_FUNPTR) :: cproc
! Get C procedure pointer.
cproc = C_FUNLOC (double_it)
! Use it.
DO i = 1_C_INT, 10_C_INT
PRINT *, call_it (cproc, i)
@end smallexample
@node Further Interoperability of Fortran with C
@subsection Further Interoperability of Fortran with C
The Technical Specification ISO/IEC TS 29113:2012 on further
interoperability of Fortran with C extends the interoperability support
of Fortran 2003 and Fortran 2008. Besides removing some restrictions
and constraints, it adds assumed-type (@code{TYPE(*)}) and assumed-rank
(@code{dimension}) variables and allows for interoperability of
assumed-shape, assumed-rank and deferred-shape arrays, including
allocatables and pointers.
Note: Currently, GNU Fortran does not support the array descriptor
(dope vector) as specified in the Technical Specification, but uses
an array descriptor with different fields. The Chasm Language
Interoperability Tools, @url{},
provide an interface to GNU Fortran's array descriptor.
The Technical Specification adds the following new features, which
are supported by GNU Fortran:
@itemize @bullet
@item The @code{ASYNCHRONOUS} attribute has been clarified and
extended to allow its use with asynchronous communication in
user-provided libraries such as in implementations of the
Message Passing Interface specification.
@item Many constraints have been relaxed, in particular for
the @code{C_LOC} and @code{C_F_POINTER} intrinsics.
@item The @code{OPTIONAL} attribute is now allowed for dummy
arguments; an absent argument matches a @code{NULL} pointer.
@item Assumed types (@code{TYPE(*)}) have been added, which may
only be used for dummy arguments. They are unlimited polymorphic
but contrary to @code{CLASS(*)} they do not contain any type
information, similar to C's @code{void *} pointers. Expressions
of any type and kind can be passed; thus, it can be used as
replacement for @code{TYPE(C_PTR)}, avoiding the use of
@code{C_LOC} in the caller.
Note, however, that @code{TYPE(*)} only accepts scalar arguments,
unless the @code{DIMENSION} is explicitly specified. As
@code{DIMENSION(*)} only supports array (including array elements) but
no scalars, it is not a full replacement for @code{C_LOC}. On the
other hand, assumed-type assumed-rank dummy arguments
(@code{TYPE(*), DIMENSION(..)}) allow for both scalars and arrays, but
require special code on the callee side to handle the array descriptor.
@item Assumed-shape arrays (@code{DIMENSION(..)}) as dummy argument
allow that scalars and arrays of any rank can be passed as actual
argument. As the Technical Specification does not provide for direct
means to operate with them, they have to be used either from the C side
or be converted using @code{C_LOC} and @code{C_F_POINTER} to scalars
or arrays of a specific rank. The rank can be determined using the
@code{RANK} intrinisic.
@end itemize
Currently unimplemented:
@itemize @bullet
@item GNU Fortran always uses an array descriptor, which does not
match the one of the Technical Specification. The
@code{ISO_Fortran_binding.h} header file and the C functions it
specifies are not available.
@item Using assumed-shape, assumed-rank and deferred-shape arrays in
@code{BIND(C)} procedures is not fully supported. In particular,
C interoperable strings of other length than one are not supported
as this requires the new array descriptor.
@end itemize
@node GNU Fortran Compiler Directives
@section GNU Fortran Compiler Directives
The Fortran standard describes how a conforming program shall
behave; however, the exact implementation is not standardized. In order
to allow the user to choose specific implementation details, compiler
directives can be used to set attributes of variables and procedures
which are not part of the standard. Whether a given attribute is
supported and its exact effects depend on both the operating system and
on the processor; see
@ref{Top,,C Extensions,gcc,Using the GNU Compiler Collection (GCC)}
for details.
For procedures and procedure pointers, the following attributes can
be used to change the calling convention:
@item @code{CDECL} -- standard C calling convention
@item @code{STDCALL} -- convention where the called procedure pops the stack
@item @code{FASTCALL} -- part of the arguments are passed via registers
instead using the stack
@end itemize
Besides changing the calling convention, the attributes also influence
the decoration of the symbol name, e.g., by a leading underscore or by
a trailing at-sign followed by the number of bytes on the stack. When
assigning a procedure to a procedure pointer, both should use the same
calling convention.
On some systems, procedures and global variables (module variables and
@code{COMMON} blocks) need special handling to be accessible when they
are in a shared library. The following attributes are available:
@item @code{DLLEXPORT} -- provide a global pointer to a pointer in the DLL
@item @code{DLLIMPORT} -- reference the function or variable using a global pointer
@end itemize
The attributes are specified using the syntax
@code{!GCC$ ATTRIBUTES} @var{attribute-list} @code{::} @var{variable-list}
where in free-form source code only whitespace is allowed before @code{!GCC$}
and in fixed-form source code @code{!GCC$}, @code{cGCC$} or @code{*GCC$} shall
start in the first column.
For procedures, the compiler directives shall be placed into the body
of the procedure; for variables and procedure pointers, they shall be in
the same declaration part as the variable or procedure pointer.
@node Non-Fortran Main Program
@section Non-Fortran Main Program
* _gfortran_set_args:: Save command-line arguments
* _gfortran_set_options:: Set library option flags
* _gfortran_set_convert:: Set endian conversion
* _gfortran_set_record_marker:: Set length of record markers
* _gfortran_set_fpe:: Set when a Floating Point Exception should be raised
* _gfortran_set_max_subrecord_length:: Set subrecord length
@end menu
Even if you are doing mixed-language programming, it is very
likely that you do not need to know or use the information in this
section. Since it is about the internal structure of GNU Fortran,
it may also change in GCC minor releases.
When you compile a @code{PROGRAM} with GNU Fortran, a function
with the name @code{main} (in the symbol table of the object file)
is generated, which initializes the libgfortran library and then
calls the actual program which uses the name @code{MAIN__}, for
historic reasons. If you link GNU Fortran compiled procedures
to, e.g., a C or C++ program or to a Fortran program compiled by
a different compiler, the libgfortran library is not initialized
and thus a few intrinsic procedures do not work properly, e.g.
those for obtaining the command-line arguments.
Therefore, if your @code{PROGRAM} is not compiled with
GNU Fortran and the GNU Fortran compiled procedures require
intrinsics relying on the library initialization, you need to
initialize the library yourself. Using the default options,
gfortran calls @code{_gfortran_set_args} and
@code{_gfortran_set_options}. The initialization of the former
is needed if the called procedures access the command line
(and for backtracing); the latter sets some flags based on the
standard chosen or to enable backtracing. In typical programs,
it is not necessary to call any initialization function.
If your @code{PROGRAM} is compiled with GNU Fortran, you shall
not call any of the following functions. The libgfortran
initialization functions are shown in C syntax but using C
bindings they are also accessible from Fortran.
@node _gfortran_set_args
@subsection @code{_gfortran_set_args} --- Save command-line arguments
@fnindex _gfortran_set_args
@cindex libgfortran initialization, set_args
@table @asis
@item @emph{Description}:
@code{_gfortran_set_args} saves the command-line arguments; this
initialization is required if any of the command-line intrinsics
is called. Additionally, it shall be called if backtracing is
enabled (see @code{_gfortran_set_options}).
@item @emph{Syntax}:
@code{void _gfortran_set_args (int argc, char *argv[])}
@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{argc} @tab number of command line argument strings
@item @var{argv} @tab the command-line argument strings; argv[0]
is the pathname of the executable itself.
@end multitable
@item @emph{Example}:
int main (int argc, char *argv[])
/* Initialize libgfortran. */
_gfortran_set_args (argc, argv);
return 0;
@end smallexample
@end table
@node _gfortran_set_options
@subsection @code{_gfortran_set_options} --- Set library option flags
@fnindex _gfortran_set_options
@cindex libgfortran initialization, set_options
@table @asis
@item @emph{Description}:
@code{_gfortran_set_options} sets several flags related to the Fortran
standard to be used, whether backtracing should be enabled
and whether range checks should be performed. The syntax allows for
upward compatibility since the number of passed flags is specified; for
non-passed flags, the default value is used. See also
@pxref{Code Gen Options}. Please note that not all flags are actually
@item @emph{Syntax}:
@code{void _gfortran_set_options (int num, int options[])}
@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{num} @tab number of options passed
@item @var{argv} @tab The list of flag values
@end multitable
@item @emph{option flag list}:
@multitable @columnfractions .15 .70
@item @var{option}[0] @tab Allowed standard; can give run-time errors
if e.g. an input-output edit descriptor is invalid in a given standard.
Possible values are (bitwise or-ed) @code{GFC_STD_F77} (1),
@code{GFC_STD_F95_OBS} (2), @code{GFC_STD_F95_DEL} (4), @code{GFC_STD_F95}
(8), @code{GFC_STD_F2003} (16), @code{GFC_STD_GNU} (32),
@code{GFC_STD_LEGACY} (64), @code{GFC_STD_F2008} (128),
@code{GFC_STD_F2008_OBS} (256) and GFC_STD_F2008_TS (512). Default:
@code{GFC_STD_F95_OBS | GFC_STD_F95_DEL | GFC_STD_F95 | GFC_STD_F2003
| GFC_STD_F2008 | GFC_STD_F2008_TS | GFC_STD_F2008_OBS | GFC_STD_F77
@item @var{option}[1] @tab Standard-warning flag; prints a warning to
standard error. Default: @code{GFC_STD_F95_DEL | GFC_STD_LEGACY}.
@item @var{option}[2] @tab If non zero, enable pedantic checking.
Default: off.
@item @var{option}[3] @tab Unused.
@item @var{option}[4] @tab If non zero, enable backtracing on run-time
errors. Default: off.
Note: Installs a signal handler and requires command-line
initialization using @code{_gfortran_set_args}.
@item @var{option}[5] @tab If non zero, supports signed zeros.
Default: enabled.
@item @var{option}[6] @tab Enables run-time checking. Possible values
are (bitwise or-ed): GFC_RTCHECK_BOUNDS (1), GFC_RTCHECK_ARRAY_TEMPS (2),
Default: disabled.
@end multitable
@item @emph{Example}:
/* Use gfortran 4.8 default options. */
static int options[] = @{68, 511, 0, 0, 1, 1, 0@};
_gfortran_set_options (7, &options);
@end smallexample
@end table
@node _gfortran_set_convert
@subsection @code{_gfortran_set_convert} --- Set endian conversion
@fnindex _gfortran_set_convert
@cindex libgfortran initialization, set_convert
@table @asis
@item @emph{Description}:
@code{_gfortran_set_convert} set the representation of data for
unformatted files.
@item @emph{Syntax}:
@code{void _gfortran_set_convert (int conv)}
@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{conv} @tab Endian conversion, possible values:
@end multitable
@item @emph{Example}:
int main (int argc, char *argv[])
/* Initialize libgfortran. */
_gfortran_set_args (argc, argv);
_gfortran_set_convert (1);
return 0;
@end smallexample
@end table
@node _gfortran_set_record_marker
@subsection @code{_gfortran_set_record_marker} --- Set length of record markers
@fnindex _gfortran_set_record_marker
@cindex libgfortran initialization, set_record_marker
@table @asis
@item @emph{Description}:
@code{_gfortran_set_record_marker} sets the length of record markers
for unformatted files.
@item @emph{Syntax}:
@code{void _gfortran_set_record_marker (int val)}
@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{val} @tab Length of the record marker; valid values
are 4 and 8. Default is 4.
@end multitable
@item @emph{Example}:
int main (int argc, char *argv[])
/* Initialize libgfortran. */
_gfortran_set_args (argc, argv);
_gfortran_set_record_marker (8);
return 0;
@end smallexample
@end table
@node _gfortran_set_fpe
@subsection @code{_gfortran_set_fpe} --- Enable floating point exception traps
@fnindex _gfortran_set_fpe
@cindex libgfortran initialization, set_fpe
@table @asis
@item @emph{Description}:
@code{_gfortran_set_fpe} enables floating point exception traps for
the specified exceptions. On most systems, this will result in a
SIGFPE signal being sent and the program being aborted.
@item @emph{Syntax}:
@code{void _gfortran_set_fpe (int val)}
@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{option}[0] @tab IEEE exceptions. Possible values are
(bitwise or-ed) zero (0, default) no trapping,
@code{GFC_FPE_INVALID} (1), @code{GFC_FPE_DENORMAL} (2),
@code{GFC_FPE_ZERO} (4), @code{GFC_FPE_OVERFLOW} (8),
@code{GFC_FPE_UNDERFLOW} (16), and @code{GFC_FPE_INEXACT} (32).
@end multitable
@item @emph{Example}:
int main (int argc, char *argv[])
/* Initialize libgfortran. */
_gfortran_set_args (argc, argv);
/* FPE for invalid operations such as SQRT(-1.0). */
_gfortran_set_fpe (1);
return 0;
@end smallexample
@end table
@node _gfortran_set_max_subrecord_length
@subsection @code{_gfortran_set_max_subrecord_length} --- Set subrecord length
@fnindex _gfortran_set_max_subrecord_length
@cindex libgfortran initialization, set_max_subrecord_length
@table @asis
@item @emph{Description}:
@code{_gfortran_set_max_subrecord_length} set the maximum length
for a subrecord. This option only makes sense for testing and
debugging of unformatted I/O.
@item @emph{Syntax}:
@code{void _gfortran_set_max_subrecord_length (int val)}
@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{val} @tab the maximum length for a subrecord;
the maximum permitted value is 2147483639, which is also
the default.
@end multitable
@item @emph{Example}:
int main (int argc, char *argv[])
/* Initialize libgfortran. */
_gfortran_set_args (argc, argv);
_gfortran_set_max_subrecord_length (8);
return 0;
@end smallexample
@end table
@c Intrinsic Procedures
@c ---------------------------------------------------------------------
@include intrinsic.texi
@end tex
@c ---------------------------------------------------------------------
@c Contributing
@c ---------------------------------------------------------------------
@node Contributing
@unnumbered Contributing
@cindex Contributing
Free software is only possible if people contribute to efforts
to create it.
We're always in need of more people helping out with ideas
and comments, writing documentation and contributing code.
If you want to contribute to GNU Fortran,
have a look at the long lists of projects you can take on.
Some of these projects are small,
some of them are large;
some are completely orthogonal to the rest of what is
happening on GNU Fortran,
but others are ``mainstream'' projects in need of enthusiastic hackers.
All of these projects are important!
We will eventually get around to the things here,
but they are also things doable by someone who is willing and able.
* Contributors::
* Projects::
* Proposed Extensions::
@end menu
@node Contributors
@section Contributors to GNU Fortran
@cindex Contributors
@cindex Credits
@cindex Authors
Most of the parser was hand-crafted by @emph{Andy Vaught}, who is
also the initiator of the whole project. Thanks Andy!
Most of the interface with GCC was written by @emph{Paul Brook}.
The following individuals have contributed code and/or
ideas and significant help to the GNU Fortran project
(in alphabetical order):
@itemize @minus
@item Janne Blomqvist
@item Steven Bosscher
@item Paul Brook
@item Tobias Burnus
@item Fran@,{c}ois-Xavier Coudert
@item Bud Davis
@item Jerry DeLisle
@item Erik Edelmann
@item Bernhard Fischer
@item Daniel Franke
@item Richard Guenther
@item Richard Henderson
@item Katherine Holcomb
@item Jakub Jelinek
@item Niels Kristian Bech Jensen
@item Steven Johnson
@item Steven G. Kargl
@item Thomas Koenig
@item Asher Langton
@item H. J. Lu
@item Toon Moene
@item Brooks Moses
@item Andrew Pinski
@item Tim Prince
@item Christopher D. Rickett
@item Richard Sandiford
@item Tobias Schl@"uter
@item Roger Sayle
@item Paul Thomas
@item Andy Vaught
@item Feng Wang
@item Janus Weil
@item Daniel Kraft
@end itemize
The following people have contributed bug reports,
smaller or larger patches,
and much needed feedback and encouragement for the
GNU Fortran project:
@itemize @minus
@item Bill Clodius
@item Dominique d'Humi@`eres
@item Kate Hedstrom
@item Erik Schnetter
@item Joost VandeVondele
@end itemize
Many other individuals have helped debug,
test and improve the GNU Fortran compiler over the past few years,
and we welcome you to do the same!
If you already have done so,
and you would like to see your name listed in the
list above, please contact us.
@node Projects
@section Projects
@table @emph
@item Help build the test suite
Solicit more code for donation to the test suite: the more extensive the
testsuite, the smaller the risk of breaking things in the future! We can
keep code private on request.
@item Bug hunting/squishing
Find bugs and write more test cases! Test cases are especially very
welcome, because it allows us to concentrate on fixing bugs instead of
isolating them. Going through the bugzilla database at
@url{} to reduce testcases posted there and
add more information (for example, for which version does the testcase
work, for which versions does it fail?) is also very helpful.
@end table
@node Proposed Extensions
@section Proposed Extensions
Here's a list of proposed extensions for the GNU Fortran compiler, in no particular
order. Most of these are necessary to be fully compatible with
existing Fortran compilers, but they are not part of the official
J3 Fortran 95 standard.
@subsection Compiler extensions:
@itemize @bullet
User-specified alignment rules for structures.
Automatically extend single precision constants to double.
Compile code that conserves memory by dynamically allocating common and
module storage either on stack or heap.
Compile flag to generate code for array conformance checking (suggest -CC).
User control of symbol names (underscores, etc).
Compile setting for maximum size of stack frame size before spilling
parts to static or heap.
Flag to force local variables into static space.
Flag to force local variables onto stack.
@end itemize
@subsection Environment Options
@itemize @bullet
Pluggable library modules for random numbers, linear algebra.
LA should use BLAS calling conventions.
Environment variables controlling actions on arithmetic exceptions like
overflow, underflow, precision loss---Generate NaN, abort, default.
Set precision for fp units that support it (i387).
Variable for setting fp rounding mode.
Variable to fill uninitialized variables with a user-defined bit
Environment variable controlling filename that is opened for that unit
Environment variable to clear/trash memory being freed.
Environment variable to control tracing of allocations and frees.
Environment variable to display allocated memory at normal program end.
Environment variable for filename for * IO-unit.
Environment variable for temporary file directory.
Environment variable forcing standard output to be line buffered (Unix).
@end itemize
@c ---------------------------------------------------------------------
@c GNU General Public License
@c ---------------------------------------------------------------------
@include gpl_v3.texi
@c ---------------------------------------------------------------------
@c GNU Free Documentation License
@c ---------------------------------------------------------------------
@include fdl.texi
@c ---------------------------------------------------------------------
@c Funding Free Software
@c ---------------------------------------------------------------------
@include funding.texi
@c ---------------------------------------------------------------------
@c Indices
@c ---------------------------------------------------------------------
@node Option Index
@unnumbered Option Index
@command{gfortran}'s command line options are indexed here without any
initial @samp{-} or @samp{--}. Where an option has both positive and
negative forms (such as -foption and -fno-option), relevant entries in
the manual are indexed under the most appropriate form; it may sometimes
be useful to look up both forms.
@printindex op
@node Keyword Index
@unnumbered Keyword Index
@printindex cp