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\input texinfo @c -*-texinfo-*-
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@set copyrights-gfortran 1999-2021
@include gcc-common.texi
@settitle The GNU Fortran Compiler
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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
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@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 standards status:: Fortran 2003, 2008 and 2018 features supported by GNU Fortran.
* Compiler Characteristics:: User-visible implementation details.
* Extensions:: Language extensions implemented by GNU Fortran.
* Mixed-Language Programming:: Interoperability with C
* Coarray Programming::
* 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, 2008 and 2018 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, Fortran
2008 or Fortran 2018. 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. The macros @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.
GNU Fortran supports a number of @code{INTEGER} and @code{REAL} kind types
in additional to the kind types required by the Fortran standard.
The availability of any given kind type is architecture dependent. The
following pre-defined preprocessor macros can be used to conditionally
include code for these additional kind types: @code{__GFC_INT_1__},
@code{__GFC_INT_2__}, @code{__GFC_INT_8__}, @code{__GFC_INT_16__},
@code{__GFC_REAL_10__}, and @code{__GFC_REAL_16__}.
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, some old vendor extensions, and several
Fortran 2003 and Fortran 2008 features, including TR 15581. However, it is
still under development and has a few remaining rough edges.
There also is initial support for OpenACC.
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 HARMONIE and HIRLAM weather forecasting code} and
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,
Fortran 2008 and Fortran 2018.
@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{Fortran 2018 status} sections of the documentation.
Additionally, the GNU Fortran compilers supports the OpenMP specification
(version 4.5 and partial support of the features of the 5.0 version,
There also is support for the OpenACC specification (targeting
version 2.6, @uref{}). See
@uref{} for more information.
@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_LIST_SEPARATOR:: Separator for list output
* GFORTRAN_CONVERT_UNIT:: Set endianness for unformatted I/O
* GFORTRAN_ERROR_BACKTRACE:: Show backtrace on run-time errors
* GFORTRAN_FORMATTED_BUFFER_SIZE:: Buffer size for formatted files
* GFORTRAN_UNFORMATTED_BUFFER_SIZE:: Buffer size for unformatted files
@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_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.
@section @env{GFORTRAN_FORMATTED_BUFFER_SIZE}---Set buffer size for formatted I/O
The @env{GFORTRAN_FORMATTED_BUFFER_SIZE} environment variable
specifies buffer size in bytes to be used for formatted output.
The default value is 8192.
@section @env{GFORTRAN_UNFORMATTED_BUFFER_SIZE}---Set buffer size for unformatted I/O
The @env{GFORTRAN_UNFORMATTED_BUFFER_SIZE} environment variable
specifies buffer size in bytes to be used for unformatted output.
The default value is 131072.
@c =====================================================================
@c =====================================================================
\part{II}{Language Reference}
@end tex
@c ---------------------------------------------------------------------
@c Fortran standards status
@c ---------------------------------------------------------------------
@node Fortran standards status
@chapter Fortran standards status
* Fortran 2003 status::
* Fortran 2008 status::
* Fortran 2018 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 and unlimited
polymorphism (``@code{CLASS(*)}'') -- including @code{SAME_TYPE_AS},
@code{EXTENDS_TYPE_OF} and @code{SELECT TYPE} for scalars and arrays and
@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 Deferred-length character variables and scalar deferred-length character
components of derived types are supported. (Note that array-valued components
are not yet 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.
@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 Minor I/O features: Rounding during formatted output, using of
a decimal comma instead of a decimal point, setting whether a plus sign
should appear for positive numbers. On systems where @code{strtod} honours
the rounding mode, the rounding mode is also supported for input.
@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},
and @code{OPENACC}.
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 Submodules are supported. It should noted that @code{MODULEs} do not
produce the smod file needed by the descendent @code{SUBMODULEs} unless they
contain at least one @code{MODULE PROCEDURE} interface. The reason for this is
that @code{SUBMODULEs} are useless without @code{MODULE PROCEDUREs}. See for a discussion and a draft
interpretation. Adopting this interpretation has the advantage that code that
does not use submodules does not generate smod files.
@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. Both show the signals which were signaling
at termination.
@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 Fortran 2018 status
@section Status of Fortran 2018 support
@item ERROR STOP in a PURE procedure
An @code{ERROR STOP} statement is permitted in a @code{PURE}
@item IMPLICIT NONE with a spec-list
Support the @code{IMPLICIT NONE} statement with an
@item Behavior of INQUIRE with the RECL= specifier
The behavior of the @code{INQUIRE} statement with the @code{RECL=}
specifier now conforms to Fortran 2018.
@end itemize
@subsection TS 29113 Status (Further Interoperability with C)
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 @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(..)}).
@item ISO_Fortran_binding (now in Fortran 2018 18.4) is implemented such that
conversion of the array descriptor for assumed type or assumed rank arrays is
done in the library. The include file ISO_Fortran_binding.h is can be found in
@end itemize
@subsection TS 18508 Status (Additional Parallel Features)
GNU Fortran supports the following new features of the Technical
Specification 18508 on Additional Parallel Features in Fortran:
@item The new atomic ADD, CAS, FETCH and ADD/OR/XOR, OR and XOR intrinsics.
@item The @code{CO_MIN} and @code{CO_MAX} and @code{SUM} reduction intrinsics.
And the @code{CO_BROADCAST} and @code{CO_REDUCE} intrinsic, except that those
do not support polymorphic types or types with allocatable, pointer or
polymorphic components.
@item Events (@code{EVENT POST}, @code{EVENT WAIT}, @code{EVENT_QUERY})
@item Failed images (@code{FAIL IMAGE}, @code{IMAGE_STATUS},
@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::
* Evaluation of logical expressions::
* MAX and MIN intrinsics with REAL NaN arguments::
* Thread-safety of the runtime library::
* Data consistency and durability::
* Files opened without an explicit ACTION= specifier::
* File operations on symbolic links::
* File format of unformatted sequential files::
* Asynchronous I/O::
@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, 2, 4, 8*, 16*, default: 4**
@item REAL
4, 8, 10*, 16*, default: 4***
4, 8, 10*, 16*, default: 4***
4, 8, 10*, 16*, default: 8***
1, 4, default: 1
@end table
* not available on all systems @*
** unless @option{-fdefault-integer-8} is used @*
*** unless @option{-fdefault-real-8} is used (see @ref{Fortran Dialect Options})
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
@ref{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 @ref{ISO_FORTRAN_ENV} module. For C interoperability,
the kind parameters of the @ref{ISO_C_BINDING} module should be used.
@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.
See also @ref{Argument passing conventions} and @ref{Interoperability with C}.
@node Evaluation of logical expressions
@section Evaluation of logical expressions
The Fortran standard does not require the compiler to evaluate all parts of an
expression, if they do not contribute to the final result. For logical
expressions with @code{.AND.} or @code{.OR.} operators, in particular, GNU
Fortran will optimize out function calls (even to impure functions) if the
result of the expression can be established without them. However, since not
all compilers do that, and such an optimization can potentially modify the
program flow and subsequent results, GNU Fortran throws warnings for such
situations with the @option{-Wfunction-elimination} flag.
@node MAX and MIN intrinsics with REAL NaN arguments
@section MAX and MIN intrinsics with REAL NaN arguments
@cindex MAX, MIN, NaN
The Fortran standard does not specify what the result of the
@code{MAX} and @code{MIN} intrinsics are if one of the arguments is a
@code{NaN}. Accordingly, the GNU Fortran compiler does not specify
that either, as this allows for faster and more compact code to be
generated. If the programmer wishes to take some specific action in
case one of the arguments is a @code{NaN}, it is necessary to
explicitly test the arguments before calling @code{MAX} or @code{MIN},
e.g. with the @code{IEEE_IS_NAN} function from the intrinsic module
@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.
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.
The GNU Fortran runtime library uses various C library functions that
depend on the locale, such as @code{strtod} and @code{snprintf}. In
order to work correctly in locale-aware programs that set the locale
using @code{setlocale}, the locale is reset to the default ``C''
locale while executing a formatted @code{READ} or @code{WRITE}
statement. On targets supporting the POSIX 2008 per-thread locale
functions (e.g. @code{newlocale}, @code{uselocale},
@code{freelocale}), these are used and thus the global locale set
using @code{setlocale} or the per-thread locales in other threads are
not affected. However, on targets lacking this functionality, the
global LC_NUMERIC locale is set to ``C'' during the formatted I/O.
Thus, on such targets it's not safe to call @code{setlocale}
concurrently from another thread while a Fortran formatted I/O
operation is in progress. Also, other threads doing something
dependent on the LC_NUMERIC locale might not work correctly if a
formatted I/O operation is in progress in another thread.
@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.
@node Files opened without an explicit ACTION= specifier
@section Files opened without an explicit ACTION= specifier
@cindex open, action
The Fortran standard says that if an @code{OPEN} statement is executed
without an explicit @code{ACTION=} specifier, the default value is
processor dependent. GNU Fortran behaves as follows:
@item Attempt to open the file with @code{ACTION='READWRITE'}
@item If that fails, try to open with @code{ACTION='READ'}
@item If that fails, try to open with @code{ACTION='WRITE'}
@item If that fails, generate an error
@end enumerate
@node File operations on symbolic links
@section File operations on symbolic links
@cindex file, symbolic link
This section documents the behavior of GNU Fortran for file operations on
symbolic links, on systems that support them.
@item Results of INQUIRE statements of the ``inquire by file'' form will
relate to the target of the symbolic link. For example,
@code{INQUIRE(FILE="foo",EXIST=ex)} will set @var{ex} to @var{.true.} if
@var{foo} is a symbolic link pointing to an existing file, and @var{.false.}
if @var{foo} points to an non-existing file (``dangling'' symbolic link).
@item Using the @code{OPEN} statement with a @code{STATUS="NEW"} specifier
on a symbolic link will result in an error condition, whether the symbolic
link points to an existing target or is dangling.
@item If a symbolic link was connected, using the @code{CLOSE} statement
with a @code{STATUS="DELETE"} specifier will cause the symbolic link itself
to be deleted, not its target.
@end itemize
@node File format of unformatted sequential files
@section File format of unformatted sequential files
@cindex file, unformatted sequential
@cindex unformatted sequential
@cindex sequential, unformatted
@cindex record marker
@cindex subrecord
Unformatted sequential files are stored as logical records using
record markers. Each logical record consists of one of more
Each subrecord consists of a leading record marker, the data written
by the user program, and a trailing record marker. The record markers
are four-byte integers by default, and eight-byte integers if the
@option{-fmax-subrecord-length=8} option (which exists for backwards
compability only) is in effect.
The representation of the record markers is that of unformatted files
given with the @option{-fconvert} option, the @ref{CONVERT specifier}
in an open statement or the @ref{GFORTRAN_CONVERT_UNIT} environment
The maximum number of bytes of user data in a subrecord is 2147483639
(2 GiB - 9) for a four-byte record marker. This limit can be lowered
with the @option{-fmax-subrecord-length} option, altough this is
rarely useful. If the length of a logical record exceeds this limit,
the data is distributed among several subrecords.
The absolute of the number stored in the record markers is the number
of bytes of user data in the corresponding subrecord. If the leading
record marker of a subrecord contains a negative number, another
subrecord follows the current one. If the trailing record marker
contains a negative number, then there is a preceding subrecord.
In the most simple case, with only one subrecord per logical record,
both record markers contain the number of bytes of user data in the
The format for unformatted sequential data can be duplicated using
unformatted stream, as shown in the example program for an unformatted
record containing a single subrecord:
program main
use iso_fortran_env, only: int32
implicit none
integer(int32) :: i
real, dimension(10) :: a, b
call random_number(a)
open (10,file='test.dat',form='unformatted',access='stream')
inquire (iolength=i) a
write (10) i, a, i
close (10)
open (10,file='test.dat',form='unformatted')
read (10) b
if (all (a == b)) print *,'success!'
end program main
@end smallexample
@node Asynchronous I/O
@section Asynchronous I/O
@cindex input/output, asynchronous
@cindex asynchronous I/O
Asynchronous I/O is supported if the program is linked against the
POSIX thread library. If that is not the case, all I/O is performed
as synchronous. On systems which do not support pthread condition
variables, such as AIX, I/O is also performed as synchronous.
On some systems, such as Darwin or Solaris, the POSIX thread library
is always linked in, so asynchronous I/O is always performed. On other
sytems, such as Linux, it is necessary to specify @option{-pthread},
@option{-lpthread} or @option{-fopenmp} during the linking step.
@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}, @option{-std=f2008}, or @option{-std=f2018}
disables both types of extensions, and @option{-std=legacy} allows
both without warning. The special compile flag @option{-fdec} enables
additional compatibility extensions along with those enabled by
* 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::
* Default widths for F@comma{} G and I format descriptors::
* 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::
* Character conversion::
* Cray pointers::
* CONVERT specifier::
* OpenMP::
* OpenACC::
* Argument list functions::
* Read/Write after EOF marker::
* UNION and MAP::
* Type variants for integer intrinsics::
* AUTOMATIC and STATIC attributes::
* Extended math intrinsics::
* Form feed as whitespace::
* TYPE as an alias for PRINT::
* %LOC as an rvalue::
* .XOR. operator::
* Bitwise logical operators::
* Extended I/O specifiers::
* Legacy PARAMETER statements::
* Default exponents::
@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
When writing a namelist, if no @code{DELIM=} is specified, by default a
double quote is used to delimit character strings. If -std=F95, F2003,
or F2008, etc, the delim status is set to 'none'. Defaulting to
quotes ensures that namelists with character strings can be subsequently
read back in accurately.
@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. A comma with no following format
decriptor is permited if the @option{-fdec-blank-format-item} is given on
the command line. This is considered non-conforming code and is
PRINT 10, 2, 3
10 FORMAT ('FOO='I1' BAR='I2)
print 20, 5, 6
20 FORMAT (I3, I3,)
@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 Default widths for F@comma{} G and I format descriptors
@subsection Default widths for @code{F}, @code{G} and @code{I} format descriptors
To support legacy codes, GNU Fortran allows width to be omitted from format
specifications if and only if @option{-fdec-format-defaults} is given on the
command line. Default widths will be used. This is considered non-conforming
code and is discouraged.
REAL :: value1
INTEGER :: value2
WRITE(*,10) value1, value1, value2
10 FORMAT ('F, G, I')
@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}, where the prefix is
either @code{b}, @code{o} or @code{z}, quote is either @code{'} or
@code{"} and the digits are @code{0} or @code{1} for binary,
between @code{0} and @code{7} for octal, and between @code{0} and
@code{F} for hexadecimal. (Example: @code{b'01011101'}.)
Up to Fortran 95, BOZ literal constants were only allowed to initialize
integer variables in DATA statements. Since Fortran 2003 BOZ literal
constants are also allowed as actual arguments to the @code{REAL},
@code{DBLE}, @code{INT} and @code{CMPLX} intrinsic functions.
The BOZ literal constant is simply a string of bits, which is padded
or truncated as needed, during conversion to a numeric type. The
Fortran standard states that the treatment of the sign bit is processor
dependent. Gfortran interprets the sign bit as a user would expect.
As a deprecated extension, GNU Fortran allows hexadecimal BOZ literal
constants to be specified using the @code{X} prefix. That the BOZ literal
constant can also be specified by adding a suffix to the string, for
example, @code{Z'ABC'} and @code{'ABC'X} are equivalent. Additionally,
as extension, BOZ literals are permitted in some contexts outside of
@code{DATA} and the intrinsic functions listed in the Fortran standard.
Use @option{-fallow-invalid-boz} to enable the extension.
@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, @code{DATA}
statements, function and subroutine arguments. 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}), @code{LOGICAL} or @code{CHARACTER} variable.
The constant will be padded with spaces 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
Examples of Hollerith constants:
integer*4 a
a = 0H ! Invalid, at least one character is needed.
a = 4HAB12 ! Valid
a = 8H12345678 ! Valid, but the Hollerith constant will be truncated.
a = 3Hxyz ! Valid, but the Hollerith constant will be padded.
@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.
integer(kind=4) :: a
a = transfer ("abcd", a) ! equivalent to: a = 4Habcd
@end smallexample
The use of the @option{-fdec} option extends support of Hollerith constants
to comparisons:
integer*4 a
a = 4hABCD
if (a .ne. 4habcd) then
write(*,*) "no match"
end if
@end smallexample
Supported types are numeric (@code{INTEGER}, @code{REAL}, or @code{COMPLEX}),
and @code{CHARACTER}.
@node Character conversion
@subsection Character conversion
@cindex conversion, to character
Allowing character literals to be used in a similar way to Hollerith constants
is a non-standard extension. This feature is enabled using
-fdec-char-conversions and only applies to character literals of @code{kind=1}.
Character literals can be used in @code{DATA} statements and assignments with
numeric (@code{INTEGER}, @code{REAL}, or @code{COMPLEX}) or @code{LOGICAL}
variables. Like Hollerith constants they are copied byte-wise fashion. The
constant will be padded with spaces or truncated to fit the size of the
variable in which it is stored.
integer*4 x
data x / 'abcd' /
x = 'A' ! Will be padded.
x = 'ab1234' ! Will be truncated.
@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.
If an assumed-size array is permitted within the scoping unit, 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. 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 v4.5}.
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{!$} sentinels
in fixed form, @command{gfortran} needs to be invoked with the
@option{-fopenmp}. This also arranges for automatic linking of the
GNU Offloading and Multi Processing Runtime Library
@ref{Top,,libgomp,libgomp,GNU Offloading and Multi Processing Runtime
The OpenMP Fortran runtime library routines are provided both in a
form of a Fortran 90 module named @code{omp_lib} and in a form of
a Fortran @code{include} file named @file{omp_lib.h}.
An example of a parallelized loop taken from Appendix A.1 of
the OpenMP Application Program Interface v2.5:
!$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 OpenACC
@subsection OpenACC
@cindex OpenACC
OpenACC is an application programming interface (API) that supports
offloading of code to accelerator devices. 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
@uref{, OpenACC Application Programming
Interface v2.6}.
To enable the processing of the OpenACC directive @code{!$acc} in
free-form source code; the @code{c$acc}, @code{*$acc} and @code{!$acc}
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{-fopenacc}. This also arranges for automatic linking of
the GNU Offloading and Multi Processing Runtime Library
@ref{Top,,libgomp,libgomp,GNU Offloading and Multi Processing Runtime
The OpenACC Fortran runtime library routines are provided both in a
form of a Fortran 90 module named @code{openacc} and in a form of a
Fortran @code{include} file named @file{openacc_lib.h}.
@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 Read/Write after EOF marker
@subsection Read/Write after EOF marker
@cindex @code{EOF}
@cindex @code{BACKSPACE}
@cindex @code{REWIND}
Some legacy codes rely on allowing @code{READ} or @code{WRITE} after the
EOF file marker in order to find the end of a file. GNU Fortran normally
rejects these codes with a run-time error message and suggests the user
consider @code{BACKSPACE} or @code{REWIND} to properly position
the file before the EOF marker. As an extension, the run-time error may
be disabled using -std=legacy.
@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. Support for record structures in GNU
Fortran can be enabled with the @option{-fdec-structure} compile flag.
If you have a choice, you should instead use 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
GNU Fortran implements STRUCTURES like derived types with the following
rules and exceptions:
@itemize @bullet
@item Structures act like derived types with the @code{SEQUENCE} attribute.
Otherwise they may contain no specifiers.
@item Structures may contain a special field with the name @code{%FILL}.
This will create an anonymous component which cannot be accessed but occupies
space just as if a component of the same type was declared in its place, useful
for alignment purposes. As an example, the following structure will consist
of at least sixteen bytes:
structure /padded/
character(4) start
character(8) %FILL
character(4) end
end structure
@end smallexample
@item Structures may share names with other symbols. For example, the following
is invalid for derived types, but valid for structures:
structure /header/
! ...
end structure
record /header/ header
@end smallexample
@item Structure types may be declared nested within another parent structure.
The syntax is:
structure /type-name/
structure [/<type-name>/] <field-list>
@end smallexample
The type name may be ommitted, in which case the structure type itself is
anonymous, and other structures of the same type cannot be instantiated. The
following shows some examples:
structure /appointment/
! nested structure definition: app_time is an array of two 'time'
structure /time/ app_time (2)
integer(1) hour, minute
end structure
character(10) memo
end structure
! The 'time' structure is still usable
record /time/ now
now = time(5, 30)
structure /appointment/
! anonymous nested structure definition
structure start, end
integer(1) hour, minute
end structure
character(10) memo
end structure
@end example
@item Structures may contain @code{UNION} blocks. For more detail see the
section on @ref{UNION and MAP}.
@item Structures support old-style initialization of components, like
those described in @ref{Old-style variable initialization}. For array
initializers, an initializer may contain a repeat specification of the form
@code{<literal-integer> * <constant-initializer>}. The value of the integer
indicates the number of times to repeat the constant initializer when expanding
the initializer list.
@end itemize
@node UNION and MAP
@subsection @code{UNION} and @code{MAP}
@cindex @code{UNION}
@cindex @code{MAP}
Unions are an old vendor extension which were commonly used with the
non-standard @ref{STRUCTURE and RECORD} extensions. Use of @code{UNION} and
@code{MAP} is automatically enabled with @option{-fdec-structure}.
A @code{UNION} declaration occurs within a structure; within the definition of
each union is a number of @code{MAP} blocks. Each @code{MAP} shares storage
with its sibling maps (in the same union), and the size of the union is the
size of the largest map within it, just as with unions in C. The major
difference is that component references do not indicate which union or map the
component is in (the compiler gets to figure that out).
Here is a small example:
structure /myunion/
character(2) w0, w1, w2
end map
character(6) long
end map
end union
end structure
record /myunion/ rec
! After this assignment...
rec.long = 'hello!'
! The following is true:
! rec.w0 === 'he'
! rec.w1 === 'll'
! rec.w2 === 'o!'
@end smallexample
The two maps share memory, and the size of the union is ultimately six bytes:
0 1 2 3 4 5 6 Byte offset
| | | | | | |
^ W0 ^ W1 ^ W2 ^
\-------/ \-------/ \-------/
^ LONG ^
@end example
Following is an example mirroring the layout of an Intel x86_64 register:
structure /reg/
union ! U0 ! rax
character(16) rx
end map
character(8) rh ! rah
union ! U1
character(8) rl ! ral
end map
character(8) ex ! eax
end map
character(4) eh ! eah
union ! U2
character(4) el ! eal
end map
character(4) x ! ax
end map
character(2) h ! ah
character(2) l ! al
end map
end union
end map
end union
end map
end union
end structure
record /reg/ a
! After this assignment...
! The following is true:
a.rx === 'AAAAAAAA.BBB.C.D'
a.rh === 'AAAAAAAA'
a.rl === '.BBB.C.D'
a.ex === '.BBB.C.D' === '.BBB'
a.el === '.C.D'
a.x === '.C.D'
a.h === '.C'
a.l === '.D'
@end example
@node Type variants for integer intrinsics
@subsection Type variants for integer intrinsics
@cindex intrinsics, integer
Similar to the D/C prefixes to real functions to specify the input/output
types, GNU Fortran offers B/I/J/K prefixes to integer functions for
compatibility with DEC programs. The types implied by each are:
@code{B} - @code{INTEGER(kind=1)}
@code{I} - @code{INTEGER(kind=2)}
@code{J} - @code{INTEGER(kind=4)}
@code{K} - @code{INTEGER(kind=8)}
@end example
GNU Fortran supports these with the flag @option{-fdec-intrinsic-ints}.
Intrinsics for which prefixed versions are available and in what form are noted
in @ref{Intrinsic Procedures}. The complete list of supported intrinsics is
@multitable @columnfractions .2 .2 .2 .2 .2
@headitem Intrinsic @tab B @tab I @tab J @tab K
@item @code{@ref{ABS}}
@tab @code{BABS} @tab @code{IIABS} @tab @code{JIABS} @tab @code{KIABS}
@item @code{@ref{BTEST}}
@tab @code{BBTEST} @tab @code{BITEST} @tab @code{BJTEST} @tab @code{BKTEST}
@item @code{@ref{IAND}}
@tab @code{BIAND} @tab @code{IIAND} @tab @code{JIAND} @tab @code{KIAND}
@item @code{@ref{IBCLR}}
@tab @code{BBCLR} @tab @code{IIBCLR} @tab @code{JIBCLR} @tab @code{KIBCLR}
@item @code{@ref{IBITS}}
@tab @code{BBITS} @tab @code{IIBITS} @tab @code{JIBITS} @tab @code{KIBITS}
@item @code{@ref{IBSET}}
@tab @code{BBSET} @tab @code{IIBSET} @tab @code{JIBSET} @tab @code{KIBSET}
@item @code{@ref{IEOR}}
@tab @code{BIEOR} @tab @code{IIEOR} @tab @code{JIEOR} @tab @code{KIEOR}
@item @code{@ref{IOR}}
@tab @code{BIOR} @tab @code{IIOR} @tab @code{JIOR} @tab @code{KIOR}
@item @code{@ref{ISHFT}}
@tab @code{BSHFT} @tab @code{IISHFT} @tab @code{JISHFT} @tab @code{KISHFT}
@item @code{@ref{ISHFTC}}
@tab @code{BSHFTC} @tab @code{IISHFTC} @tab @code{JISHFTC} @tab @code{KISHFTC}
@item @code{@ref{MOD}}
@tab @code{BMOD} @tab @code{IMOD} @tab @code{JMOD} @tab @code{KMOD}
@item @code{@ref{NOT}}
@tab @code{BNOT} @tab @code{INOT} @tab @code{JNOT} @tab @code{KNOT}
@item @code{@ref{REAL}}
@tab @code{--} @tab @code{FLOATI} @tab @code{FLOATJ} @tab @code{FLOATK}
@end multitable
@node AUTOMATIC and STATIC attributes
@subsection @code{AUTOMATIC} and @code{STATIC} attributes
@cindex variable attributes
@cindex @code{AUTOMATIC}
@cindex @code{STATIC}
With @option{-fdec-static} GNU Fortran supports the DEC extended attributes
@code{STATIC} and @code{AUTOMATIC} to provide explicit specification of entity
storage. These follow the syntax of the Fortran standard @code{SAVE} attribute.
@code{STATIC} is exactly equivalent to @code{SAVE}, and specifies that
an entity should be allocated in static memory. As an example, @code{STATIC}
local variables will retain their values across multiple calls to a function.
Entities marked @code{AUTOMATIC} will be stack automatic whenever possible.
@code{AUTOMATIC} is the default for local variables smaller than
@option{-fmax-stack-var-size}, unless @option{-fno-automatic} is given. This
attribute overrides @option{-fno-automatic}, @option{-fmax-stack-var-size}, and
blanket @code{SAVE} statements.
subroutine f
integer, automatic :: i ! automatic variable
integer x, y ! static variables
@end example
subroutine f
integer a, b, c, x, y, z
static :: x
save y
automatic z, c
! a, b, c, and z are automatic
! x and y are static
@end example
! Compiled with -fno-automatic
subroutine f
integer a, b, c, d
automatic :: a
! a is automatic; b, c, and d are static
@end example
@node Extended math intrinsics
@subsection Extended math intrinsics
@cindex intrinsics, math
@cindex intrinsics, trigonometric functions
GNU Fortran supports an extended list of mathematical intrinsics with the
compile flag @option{-fdec-math} for compatability with legacy code.
These intrinsics are described fully in @ref{Intrinsic Procedures} where it is
noted that they are extensions and should be avoided whenever possible.
Specifically, @option{-fdec-math} enables the @ref{COTAN} intrinsic, and
trigonometric intrinsics which accept or produce values in degrees instead of
radians. Here is a summary of the new intrinsics:
@multitable @columnfractions .5 .5
@headitem Radians @tab Degrees
@item @code{@ref{ACOS}} @tab @code{@ref{ACOSD}}*
@item @code{@ref{ASIN}} @tab @code{@ref{ASIND}}*
@item @code{@ref{ATAN}} @tab @code{@ref{ATAND}}*
@item @code{@ref{ATAN2}} @tab @code{@ref{ATAN2D}}*
@item @code{@ref{COS}} @tab @code{@ref{COSD}}*
@item @code{@ref{COTAN}}* @tab @code{@ref{COTAND}}*
@item @code{@ref{SIN}} @tab @code{@ref{SIND}}*
@item @code{@ref{TAN}} @tab @code{@ref{TAND}}*
@end multitable
* Enabled with @option{-fdec-math}.
For advanced users, it may be important to know the implementation of these
functions. They are simply wrappers around the standard radian functions, which
have more accurate builtin versions. These functions convert their arguments
(or results) to degrees (or radians) by taking the value modulus 360 (or 2*pi)
and then multiplying it by a constant radian-to-degree (or degree-to-radian)
factor, as appropriate. The factor is computed at compile-time as 180/pi (or
@node Form feed as whitespace
@subsection Form feed as whitespace
@cindex form feed whitespace
Historically, legacy compilers allowed insertion of form feed characters ('\f',
ASCII 0xC) at the beginning of lines for formatted output to line printers,
though the Fortran standard does not mention this. GNU Fortran supports the
interpretation of form feed characters in source as whitespace for
@node TYPE as an alias for PRINT
@subsection TYPE as an alias for PRINT
@cindex type alias print
For compatibility, GNU Fortran will interpret @code{TYPE} statements as
@code{PRINT} statements with the flag @option{-fdec}. With this flag asserted,
the following two examples are equivalent:
TYPE *, 'hello world'
@end smallexample
PRINT *, 'hello world'
@end smallexample
@node %LOC as an rvalue
@subsection %LOC as an rvalue
@cindex LOC
Normally @code{%LOC} is allowed only in parameter lists. However the intrinsic
function @code{LOC} does the same thing, and is usable as the right-hand-side of
assignments. For compatibility, GNU Fortran supports the use of @code{%LOC} as
an alias for the builtin @code{LOC} with @option{-std=legacy}. With this
feature enabled the following two examples are equivalent:
integer :: i, l
l = %loc(i)
call sub(l)
@end smallexample
integer :: i
call sub(%loc(i))
@end smallexample
@node .XOR. operator
@subsection .XOR. operator
@cindex operators, xor
GNU Fortran supports @code{.XOR.} as a logical operator with @code{-std=legacy}
for compatibility with legacy code. @code{.XOR.} is equivalent to
@code{.NEQV.}. That is, the output is true if and only if the inputs differ.
@node Bitwise logical operators
@subsection Bitwise logical operators
@cindex logical, bitwise
With @option{-fdec}, GNU Fortran relaxes the type constraints on
logical operators to allow integer operands, and performs the corresponding
bitwise operation instead. This flag is for compatibility only, and should be
avoided in new code. Consider:
INTEGER :: i, j
i = z'33'
j = z'cc'
print *, i .AND. j
@end smallexample
In this example, compiled with @option{-fdec}, GNU Fortran will
replace the @code{.AND.} operation with a call to the intrinsic
@code{@ref{IAND}} function, yielding the bitwise-and of @code{i} and @code{j}.
Note that this conversion will occur if at least one operand is of integral
type. As a result, a logical operand will be converted to an integer when the
other operand is an integer in a logical operation. In this case,
@code{.TRUE.} is converted to @code{1} and @code{.FALSE.} to @code{0}.
Here is the mapping of logical operator to bitwise intrinsic used with
@multitable @columnfractions .25 .25 .5
@headitem Operator @tab Intrinsic @tab Bitwise operation
@item @code{.NOT.} @tab @code{@ref{NOT}} @tab complement
@item @code{.AND.} @tab @code{@ref{IAND}} @tab intersection
@item @code{.OR.} @tab @code{@ref{IOR}} @tab union
@item @code{.NEQV.} @tab @code{@ref{IEOR}} @tab exclusive or
@item @code{.EQV.} @tab @code{@ref{NOT}(@ref{IEOR})} @tab complement of exclusive or
@end multitable
@node Extended I/O specifiers
@subsection Extended I/O specifiers
@cindex @code{CARRIAGECONTROL}
@cindex @code{READONLY}
@cindex @code{SHARE}
@cindex @code{SHARED}
@cindex @code{NOSHARED}
@cindex I/O specifiers
GNU Fortran supports the additional legacy I/O specifiers
@code{CARRIAGECONTROL}, @code{READONLY}, and @code{SHARE} with the
compile flag @option{-fdec}, for compatibility.
@table @code
The @code{CARRIAGECONTROL} specifier allows a user to control line
termination settings between output records for an I/O unit. The specifier has
no meaning for readonly files. When @code{CARRAIGECONTROL} is specified upon
opening a unit for formatted writing, the exact @code{CARRIAGECONTROL} setting
determines what characters to write between output records. The syntax is:
@end smallexample
Where @emph{cc} is a character expression that evaluates to one of the
following values:
@multitable @columnfractions .2 .8
@item @code{'LIST'} @tab One line feed between records (default)
@item @code{'FORTRAN'} @tab Legacy interpretation of the first character (see below)
@item @code{'NONE'} @tab No separator between records
@end multitable
With @code{CARRIAGECONTROL='FORTRAN'}, when a record is written, the first
character of the input record is not written, and instead determines the output
record separator as follows:
@multitable @columnfractions .3 .3 .4
@headitem Leading character @tab Meaning @tab Output separating character(s)
@item @code{'+'} @tab Overprinting @tab Carriage return only
@item @code{'-'} @tab New line @tab Line feed and carriage return
@item @code{'0'} @tab Skip line @tab Two line feeds and carriage return
@item @code{'1'} @tab New page @tab Form feed and carriage return
@item @code{'$'} @tab Prompting @tab Line feed (no carriage return)
@item @code{CHAR(0)} @tab Overprinting (no advance) @tab None
@end multitable
The @code{READONLY} specifier may be given upon opening a unit, and is
equivalent to specifying @code{ACTION='READ'}, except that the file may not be
deleted on close (i.e. @code{CLOSE} with @code{STATUS="DELETE"}). The syntax
@code{OPEN(..., READONLY)}
@end smallexample
@item SHARE
The @code{SHARE} specifier allows system-level locking on a unit upon opening
it for controlled access from multiple processes/threads. The @code{SHARE}
specifier has several forms:
OPEN(..., SHARE=sh)
@end smallexample
Where @emph{sh} in the first form is a character expression that evaluates to
a value as seen in the table below. The latter two forms are aliases
for particular values of @emph{sh}:
@multitable @columnfractions .3 .3 .4
@headitem Explicit form @tab Short form @tab Meaning
@item @code{SHARE='DENYRW'} @tab @code{NOSHARED} @tab Exclusive (write) lock
@item @code{SHARE='DENYNONE'} @tab @code{SHARED} @tab Shared (read) lock
@end multitable
In general only one process may hold an exclusive (write) lock for a given file
at a time, whereas many processes may hold shared (read) locks for the same
The behavior of locking may vary with your operating system. On POSIX systems,
locking is implemented with @code{fcntl}. Consult your corresponding operating
system's manual pages for further details. Locking via @code{SHARE=} is not
supported on other systems.
@end table
@node Legacy PARAMETER statements
@subsection Legacy PARAMETER statements
For compatibility, GNU Fortran supports legacy PARAMETER statements without
parentheses with @option{-std=legacy}. A warning is emitted if used with
@option{-std=gnu}, and an error is acknowledged with a real Fortran standard
flag (@option{-std=f95}, etc...). These statements take the following form:
implicit real (E)
parameter e = 2.718282
real c
parameter c = 3.0e8
@end smallexample
@node Default exponents
@subsection Default exponents
@cindex exponent
For compatibility, GNU Fortran supports a default exponent of zero in real
constants with @option{-fdec}. For example, @code{9e} would be
interpreted as @code{9e0}, rather than an error.
@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:
* ENCODE and DECODE statements::
* Variable FORMAT expressions::
@c * TYPE and ACCEPT I/O Statements::
@c * Omitted arguments in procedure call::
* Alternate complex function syntax::
* Volatile COMMON blocks::
* OPEN( ... NAME=)::
* Q edit descriptor::
@end menu
@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.
@node Volatile COMMON blocks
@subsection Volatile @code{COMMON} blocks
@cindex @code{VOLATILE}
@cindex @code{COMMON}
Some Fortran compilers, including @command{g77}, let the user declare
@code{COMMON} with the @code{VOLATILE} attribute. This is
invalid standard Fortran syntax and is not supported by
@command{gfortran}. Note that @command{gfortran} accepts
@code{VOLATILE} variables in @code{COMMON} blocks since revision 4.3.
@node OPEN( ... NAME=)
@subsection @code{OPEN( ... NAME=)}
@cindex @code{NAME}
Some Fortran compilers, including @command{g77}, let the user declare
@code{OPEN( ... NAME=)}. This is
invalid standard Fortran syntax and is not supported by
@command{gfortran}. @code{OPEN( ... NAME=)} should be replaced
with @code{OPEN( ... FILE=)}.
@node Q edit descriptor
@subsection @code{Q} edit descriptor
@cindex @code{Q} edit descriptor
Some Fortran compilers provide the @code{Q} edit descriptor, which
transfers the number of characters left within an input record into an
integer variable.
A direct replacement of the @code{Q} edit descriptor is not available
in @command{gfortran}. How to replicate its functionality using
standard-conforming code depends on what the intent of the original
code is.
Options to replace @code{Q} may be to read the whole line into a
character variable and then counting the number of non-blank
characters left using @code{LEN_TRIM}. Another method may be to use
formatted stream, read the data up to the position where the @code{Q}
descriptor occurred, use @code{INQUIRE} to get the file position,
count the characters up to the next @code{NEW_LINE} and then start
reading from the position marked previously.
@c ---------------------------------------------------------------------
@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::
* Naming and argument-passing conventions::
@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}.
For logical types, please note that the Fortran standard only guarantees
interoperability between C99's @code{_Bool} and Fortran's @code{C_Bool}-kind
logicals and C99 defines that @code{true} has the value 1 and @code{false}
the value 0. Using any other integer value with GNU Fortran's @code{LOGICAL}
(with any kind parameter) gives an undefined result. (Passing other integer
values than 0 and 1 to GCC's @code{_Bool} is also undefined, unless the
integer is explicitly or implicitly casted to @code{_Bool}.)
@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 use internally the array descriptor
(dope vector) as specified in the Technical Specification, but uses
an array descriptor with different fields. Assumed type and assumed rank
formal arguments are converted in the library to the specified form. The
ISO_Fortran_binding API functions (also Fortran 2018 18.4) are implemented
in libgfortran. Alternatively, 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-rank 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
* ATTRIBUTES directive::
* UNROLL directive::
* BUILTIN directive::
* IVDEP directive::
* VECTOR directive::
* NOVECTOR directive::
@end menu
@node ATTRIBUTES directive
@subsection ATTRIBUTES directive
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
For dummy arguments, the @code{NO_ARG_CHECK} attribute can be used; in
other compilers, it is also known as @code{IGNORE_TKR}. For dummy arguments
with this attribute actual arguments of any type and kind (similar to
@code{TYPE(*)}), scalars and arrays of any rank (no equivalent
in Fortran standard) are accepted. As with @code{TYPE(*)}, the argument
is unlimited polymorphic and no type information is available.
Additionally, the argument may only be passed to dummy arguments
with the @code{NO_ARG_CHECK} attribute and as argument to the
@code{PRESENT} intrinsic function and to @code{C_LOC} of the
@code{ISO_C_BINDING} module.
Variables with @code{NO_ARG_CHECK} attribute shall be of assumed-type
(@code{TYPE(*)}; recommended) or of type @code{INTEGER}, @code{LOGICAL},
@code{REAL} or @code{COMPLEX}. They shall not have the @code{ALLOCATE},
@code{CODIMENSION}, @code{INTENT(OUT)}, @code{POINTER} or @code{VALUE}
attribute; furthermore, they shall be either scalar or of assumed-size
(@code{dimension(*)}). As @code{TYPE(*)}, the @code{NO_ARG_CHECK} attribute
requires an explicit interface.
@item @code{NO_ARG_CHECK} -- disable the type, kind and rank checking
@item @code{DEPRECATED} -- print a warning when using a such-tagged
deprecated procedure, variable or parameter; the warning can be suppressed
with @option{-Wno-deprecated-declarations}.
@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 UNROLL directive
@subsection UNROLL directive
The syntax of the directive is
@code{!GCC$ unroll N}
You can use this directive to control how many times a loop should be unrolled.
It must be placed immediately before a @code{DO} loop and applies only to the
loop that follows. N is an integer constant specifying the unrolling factor.
The values of 0 and 1 block any unrolling of the loop.
@node BUILTIN directive
@subsection BUILTIN directive
The syntax of the directive is
@code{!GCC$ BUILTIN (B) attributes simd FLAGS IF('target')}
You can use this directive to define which middle-end built-ins provide vector
implementations. @code{B} is name of the middle-end built-in. @code{FLAGS}
are optional and must be either "(inbranch)" or "(notinbranch)".
@code{IF} statement is optional and is used to filter multilib ABIs
for the built-in that should be vectorized. Example usage:
!GCC$ builtin (sinf) attributes simd (notinbranch) if('x86_64')
@end smallexample