\input texinfo @c -*-texinfo-*- @c %**start of header @setfilename gfortran.info @set copyrights-gfortran 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 @include gcc-common.texi @settitle The GNU Fortran Compiler @c Create a separate index for command line options @defcodeindex op @c Merge the standard indexes into a single one. @syncodeindex fn cp @syncodeindex vr cp @syncodeindex ky cp @syncodeindex pg cp @syncodeindex tp cp @c TODO: The following "Part" definitions are included here temporarily @c until they are incorporated into the official Texinfo distribution. @c They borrow heavily from Texinfo's \unnchapentry definitions. @tex \gdef\part#1#2{% \pchapsepmacro \gdef\thischapter{} \begingroup \vglue\titlepagetopglue \titlefonts \rm \leftline{Part #1:@* #2} \vskip4pt \hrule height 4pt width \hsize \vskip4pt \endgroup \writetocentry{part}{#2}{#1} } \gdef\blankpart{% \writetocentry{blankpart}{}{} } % Part TOC-entry definition for summary contents. \gdef\dosmallpartentry#1#2#3#4{% \vskip .5\baselineskip plus.2\baselineskip \begingroup \let\rm=\bf \rm \tocentry{Part #2: #1}{\doshortpageno\bgroup#4\egroup} \endgroup } \gdef\dosmallblankpartentry#1#2#3#4{% \vskip .5\baselineskip plus.2\baselineskip } % Part TOC-entry definition for regular contents. This has to be % equated to an existing entry to not cause problems when the PDF % outline is created. \gdef\dopartentry#1#2#3#4{% \unnchapentry{Part #2: #1}{}{#3}{#4} } \gdef\doblankpartentry#1#2#3#4{} @end tex @c %**end of header @c Use with @@smallbook. @c %** start of document @c Cause even numbered pages to be printed on the left hand side of @c the page and odd numbered pages to be printed on the right hand @c side of the page. Using this, you can print on both sides of a @c sheet of paper and have the text on the same part of the sheet. @c The text on right hand pages is pushed towards the right hand @c margin and the text on left hand pages is pushed toward the left @c hand margin. @c (To provide the reverse effect, set bindingoffset to -0.75in.) @c @tex @c \global\bindingoffset=0.75in @c \global\normaloffset =0.75in @c @end tex @copying 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 @ifinfo @dircategory Software development @direntry * 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 @insertcopying @end ifinfo @setchapternewpage odd @titlepage @title Using GNU Fortran @versionsubtitle @author The @t{gfortran} team @page @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 @insertcopying @end titlepage @c TODO: The following "Part" definitions are included here temporarily @c until they are incorporated into the official Texinfo distribution. @tex \global\let\partentry=\dosmallpartentry \global\let\blankpartentry=\dosmallblankpartentry @end tex @summarycontents @tex \global\let\partentry=\dopartentry \global\let\blankpartentry=\doblankpartentry @end tex @contents @page @c --------------------------------------------------------------------- @c TexInfo table of contents. @c --------------------------------------------------------------------- @ifnottex @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. @ifset DEVELOPMENT @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 @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. @comment @menu * Introduction:: Part I: Invoking GNU Fortran * Invoking GNU Fortran:: Command options supported by @command{gfortran}. * Runtime:: Influencing runtime behavior with environment variables. Part II: Language Reference * Fortran 2003 and 2008 status:: Fortran 2003 and 2008 features supported by GNU Fortran. * Compiler Characteristics:: User-visible implementation details. * Mixed-Language Programming:: Interoperability with C * Extensions:: Language extensions implemented by GNU Fortran. * 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. @iftex 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. @ifset DEVELOPMENT @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 @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'll use to invoke the compiler. @menu * About GNU Fortran:: What you should know about the GNU Fortran compiler. * GNU Fortran and GCC:: You can compile Fortran, C, or other programs. * Preprocessing and conditional compilation:: The Fortran preprocessor * GNU Fortran and G77:: Why we chose to start from scratch. * Project Status:: Status of GNU Fortran, roadmap, proposed extensions. * Standards:: Standards supported by GNU Fortran. @end menu @c --------------------------------------------------------------------- @c About GNU Fortran @c --------------------------------------------------------------------- @node About GNU Fortran @section About GNU Fortran The GNU Fortran compiler supports the Fortran 77, 90 and 95 standards completely, parts of the Fortran 2003 and Fortran 2008 standards, and several vendor extensions. The development goal is to provide the following features: @itemize @bullet @item Read a user's program, stored in a file and containing instructions written in Fortran 77, Fortran 90, Fortran 95, Fortran 2003 or Fortran 2008. This file contains @dfn{source code}. @item 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 aren't 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. @item 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}. @item 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. @item 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}). @item 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 @item 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. @item 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. @item 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. @item 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 @option{-nocpp}. If a preprocessed file includes another file with the Fortran @code{INCLUDE} statement, the included file is not preprocessed. To preprocess included files, use the equivalent preprocessor statement @code{#include}. If GNU Fortran invokes the preprocessor, @code{__GFORTRAN__} is defined and @code{__GNUC__}, @code{__GNUC_MINOR__} and @code{__GNUC_PATCHLEVEL__} can be used to determine the version of the compiler. See @ref{Top,,Overview,cpp,The C Preprocessor} for details. While CPP is the de-facto standard for preprocessing Fortran code, Part 3 of the Fortran 95 standard (ISO/IEC 1539-3:1998) defines Conditional Compilation, which is not widely used and not directly supported by the GNU Fortran compiler. You can use the program coco to preprocess such files (@uref{http://www.daniellnagle.com/coco.html}). @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 @quotation As soon as @command{gfortran} can parse all of the statements correctly, it will be in the ``larva'' state. When we generate code, the ``puppa'' state. When @command{gfortran} is done, we'll see if it will be a beautiful butterfly, or just a big bug.... --Andy Vaught, April 2000 @end quotation The start of the GNU Fortran 95 project was announced on the GCC homepage in March 18, 2000 (even though Andy had already been working on it for a while, of course). The GNU Fortran compiler is able to compile nearly all standard-compliant Fortran 95, Fortran 90, and Fortran 77 programs, including a number of standard and non-standard extensions, and can be used on real-world programs. In particular, the supported extensions include OpenMP, Cray-style pointers, and several Fortran 2003 and Fortran 2008 features, including TR 15581. However, it is still under development and has a few remaining rough edges. At present, the GNU Fortran compiler passes the @uref{http://www.fortran-2000.com/ArnaudRecipes/fcvs21_f95.html, NIST Fortran 77 Test Suite}, and produces acceptable results on the @uref{http://www.netlib.org/lapack/faq.html#1.21, LAPACK Test Suite}. It also provides respectable performance on the @uref{http://www.polyhedron.com/pb05.html, Polyhedron Fortran compiler benchmarks} and the @uref{http://www.llnl.gov/asci_benchmarks/asci/limited/lfk/README.html, Livermore Fortran Kernels test}. It has been used to compile a number of large real-world programs, including @uref{http://mysite.verizon.net/serveall/moene.pdf, the HIRLAM weather-forecasting code} and @uref{http://www.theochem.uwa.edu.au/tonto/, the Tonto quantum chemistry package}; see @url{http://gcc.gnu.org/@/wiki/@/GfortranApps} 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 regressions. The primary work remaining to be done on GNU Fortran falls into three categories: bug fixing (primarily regarding the treatment of invalid code and providing useful error messages), improving the compiler optimizations and the performance of compiled code, and extending the compiler to support future standards---in particular, Fortran 2003 and Fortran 2008. @c --------------------------------------------------------------------- @c Standards @c --------------------------------------------------------------------- @node Standards @section Standards @cindex Standards @menu * 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. In the future, the GNU Fortran compiler will also support ISO/IEC 1539-1:2004 (Fortran 2003), ISO/IEC 1539-1:2010 (Fortran 2008) and future Fortran standards. Partial support of the Fortran 2003 and Fortran 2008 standard is already provided; the current status of the support is reported in the @ref{Fortran 2003 status} and @ref{Fortran 2008 status} sections of the documentation. Additionally, the GNU Fortran compilers supports the OpenMP specification (version 3.1, @url{http://openmp.org/@/wp/@/openmp-specifications/}). @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{http://www.fortran.com/@/iso_varying_string.f95} and at @uref{ftp://ftp.nag.co.uk/@/sc22wg5/@/ISO_VARYING_STRING/}. @c ===================================================================== @c PART I: INVOCATION REFERENCE @c ===================================================================== @tex \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. @menu * 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_TMPDIR:: Directory for scratch files * GFORTRAN_UNBUFFERED_ALL:: Don't buffer I/O for all units. * GFORTRAN_UNBUFFERED_PRECONNECTED:: Don't buffer I/O for preconnected units. * GFORTRAN_SHOW_LOCUS:: Show location for runtime errors * GFORTRAN_OPTIONAL_PLUS:: Print leading + where permitted * GFORTRAN_DEFAULT_RECL:: Default record length for new files * GFORTRAN_LIST_SEPARATOR:: Separator for list output * GFORTRAN_CONVERT_UNIT:: Set endianness for unformatted I/O * GFORTRAN_ERROR_BACKTRACE:: Show backtrace on run-time errors @end menu @node GFORTRAN_STDIN_UNIT @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. @node GFORTRAN_STDOUT_UNIT @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. @node GFORTRAN_STDERR_UNIT @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. @node GFORTRAN_TMPDIR @section @env{GFORTRAN_TMPDIR}---Directory for scratch files This environment variable controls where scratch files are created. If this environment variable is missing, GNU Fortran searches for the environment variable @env{TMP}, then @env{TEMP}. If these are missing, the default is @file{/tmp}. @node GFORTRAN_UNBUFFERED_ALL @section @env{GFORTRAN_UNBUFFERED_ALL}---Don't 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. @node GFORTRAN_UNBUFFERED_PRECONNECTED @section @env{GFORTRAN_UNBUFFERED_PRECONNECTED}---Don't 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. @node GFORTRAN_SHOW_LOCUS @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}, don't print filename and line numbers for runtime errors. The default is to print the location. @node GFORTRAN_OPTIONAL_PLUS @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. @node GFORTRAN_DEFAULT_RECL @section @env{GFORTRAN_DEFAULT_RECL}---Default record length for new files This environment variable specifies the default record length, in bytes, for files which are opened without a @code{RECL} tag in the @code{OPEN} statement. This must be a positive integer. The default value is 1073741824 bytes (1 GB). @node GFORTRAN_LIST_SEPARATOR @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 @smallexample $ GFORTRAN_LIST_SEPARATOR=' , ' ./a.out @end smallexample when @command{a.out} is the compiled Fortran program that you want to run. Default is a single space. @node GFORTRAN_CONVERT_UNIT @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: @smallexample 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}: @smallexample $ gfortran foo.f90 $ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out @end smallexample Example code for @command{csh}: @smallexample % 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 portable. @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. @node GFORTRAN_ERROR_BACKTRACE @section @env{GFORTRAN_ERROR_BACKTRACE}---Show backtrace on run-time errors If the @env{GFORTRAN_ERROR_BACKTRACE} variable is set to @samp{y}, @samp{Y} or @samp{1} (only the first letter is relevant) then a backtrace is printed when a serious run-time error occurs. To disable the backtracing, set the variable to @samp{n}, @samp{N}, @samp{0}. Default is to print a backtrace unless the @option{-fno-backtrace} compile option was used. @c ===================================================================== @c PART II: LANGUAGE REFERENCE @c ===================================================================== @tex \part{II}{Language Reference} @end tex @c --------------------------------------------------------------------- @c Fortran 2003 and 2008 Status @c --------------------------------------------------------------------- @node Fortran 2003 and 2008 status @chapter Fortran 2003 and 2008 Status @menu * Fortran 2003 status:: * Fortran 2008 status:: * TS 29113 status:: @end menu @node Fortran 2003 status @section Fortran 2003 status GNU Fortran supports several Fortran 2003 features; an incomplete list can be found below. See also the @uref{http://gcc.gnu.org/wiki/Fortran2003, wiki page} about Fortran 2003. @itemize @item Procedure pointers including procedure-pointer components with @code{PASS} attribute. @item Procedures which are bound to a derived type (type-bound procedures) including @code{PASS}, @code{PROCEDURE} and @code{GENERIC}, and operators bound to a type. @item Abstract interfaces and type extension with the possibility to override type-bound procedures or to have deferred binding. @item Polymorphic entities (``@code{CLASS}'') for derived types -- including @code{SAME_TYPE_AS}, @code{EXTENDS_TYPE_OF} and @code{SELECT TYPE}. Note that the support for array-valued polymorphic entities is incomplete and unlimited polymophism is currently not supported. @item The @code{ASSOCIATE} construct. @item Interoperability with C including enumerations, @item In structure constructors the components with default values may be omitted. @item Extensions to the @code{ALLOCATE} statement, allowing for a type-specification with type parameter and for allocation and initialization from a @code{SOURCE=} expression; @code{ALLOCATE} and @code{DEALLOCATE} optionally return an error message string via @code{ERRMSG=}. @item Reallocation on assignment: If an intrinsic assignment is used, an allocatable variable on the left-hand side is automatically allocated (if unallocated) or reallocated (if the shape is different). Currently, scalar deferred character length left-hand sides are correctly handled but arrays are not yet fully implemented. @item Transferring of allocations via @code{MOVE_ALLOC}. @item The @code{PRIVATE} and @code{PUBLIC} attributes may be given individually to derived-type components. @item In pointer assignments, the lower bound may be specified and the remapping of elements is supported. @item For pointers an @code{INTENT} may be specified which affect the association status not the value of the pointer target. @item Intrinsics @code{command_argument_count}, @code{get_command}, @code{get_command_argument}, and @code{get_environment_variable}. @item Support for unicode characters (ISO 10646) and UTF-8, including the @code{SELECTED_CHAR_KIND} and @code{NEW_LINE} intrinsic functions. @item Support for binary, octal and hexadecimal (BOZ) constants in the intrinsic functions @code{INT}, @code{REAL}, @code{CMPLX} and @code{DBLE}. @item Support for namelist variables with allocatable and pointer attribute and nonconstant length type parameter. @item @cindex array, constructors @cindex @code{[...]} Array constructors using square brackets. That is, @code{[...]} rather than @code{(/.../)}. Type-specification for array constructors like @code{(/ some-type :: ... /)}. @item Extensions to the specification and initialization expressions, including the support for intrinsics with real and complex arguments. @item Support for the asynchronous input/output syntax; however, the data transfer is currently always synchronously performed. @item @cindex @code{FLUSH} statement @cindex statement, @code{FLUSH} @code{FLUSH} statement. @item @cindex @code{IOMSG=} specifier @code{IOMSG=} specifier for I/O statements. @item @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. @item @cindex TR 15581 TR 15581: @itemize @item @cindex @code{ALLOCATABLE} dummy arguments @code{ALLOCATABLE} dummy arguments. @item @cindex @code{ALLOCATABLE} function results @code{ALLOCATABLE} function results @item @cindex @code{ALLOCATABLE} components of derived types @code{ALLOCATABLE} components of derived types @end itemize @item @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. @item Namelist input/output for internal files. @item Further I/O extensions: Rounding during formatted output, using of a decimal comma instead of a decimal point, setting whether a plus sign should appear for positive numbers. @item @cindex @code{PROTECTED} statement @cindex statement, @code{PROTECTED} The @code{PROTECTED} statement and attribute. @item @cindex @code{VALUE} statement @cindex statement, @code{VALUE} The @code{VALUE} statement and attribute. @item @cindex @code{VOLATILE} statement @cindex statement, @code{VOLATILE} The @code{VOLATILE} statement and attribute. @item @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} module. @item @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}. @item 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{http://www.nag.co.uk/@/sc22wg5/@/links.html}. 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{http://www.nag.co.uk/sc22wg5/, WG5}. The GNU Fortran compiler supports several of the new features of Fortran 2008; the @uref{http://gcc.gnu.org/wiki/Fortran2008Status, wiki} has some information about the current Fortran 2008 implementation status. In particular, the following is implemented. @itemize @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}, @code{LOGICAL_KINDS}, @code{REAL_KINDS} and @code{CHARACTER_KINDS} of the intrinsic module @code{ISO_FORTRAN_ENV}. @item The module procedures @code{C_SIZEOF} of the intrinsic module @code{ISO_C_BINDINGS} and @code{COMPILER_VERSION} and @code{COMPILER_OPTIONS} 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} flag. @item The @code{DO CONCURRENT} construct is supported. @item The @code{BLOCK} construct is supported. @item The @code{STOP} and the new @code{ERROR STOP} statements now support all constant expressions. @item Support for the @code{CONTIGUOUS} attribute. @item Support for @code{ALLOCATE} with @code{MOLD}. @item Support for the @code{IMPURE} attribute for procedures, which allows for @code{ELEMENTAL} procedures without the restrictions of @code{PURE}. @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} constructs. @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 @code{END SUBROUTINE} and @code{END FUNCTION}; @code{SELECTED_REAL_KIND} now also takes a @code{RADIX} argument; intrinsic types are supported for @code{TYPE}(@var{intrinsic-type-spec}); multiple type-bound procedures can be declared in a single @code{PROCEDURE} statement; implied-shape arrays are supported for named constants (@code{PARAMETER}). @end itemize @node TS 29113 status @section Technical Specification 29113 Status GNU Fortran supports some of the new features of the Technical Specification (TS) 29113 on Further Interoperability of Fortran with C. The @uref{http://gcc.gnu.org/wiki/TS29113Status, wiki} has some information about the current TS 29113 implementation status. In particular, the following is implemented. @itemize @item The @option{-std=f2008ts} option. @item The @code{OPTIONAL} attribute is allowed for dummy arguments of @code{BIND(C) procedures.} @item The RANK intrinsic is supported. @item GNU Fortran's implementation for variables with @code{ASYNCHRONOUS} attribute is compatible with TS 29113. @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. @menu * KIND Type Parameters:: * Internal representation of LOGICAL variables:: * Thread-safety of the runtime library:: @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 @item INTEGER 1, 2, 4, 8*, 16*, default: 4 (1) @item LOGICAL 1, 2, 4, 8*, 16*, default: 4 (1) @item REAL 4, 8, 10*, 16*, default: 4 (2) @item COMPLEX 4, 8, 10*, 16*, default: 4 (2) @item CHARACTER 1, 4, default: 1 @end table @noindent * = not available on all systems @* (1) Unless -fdefault-integer-8 is used @* (2) Unless -fdefault-real-8 is used @noindent 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 the @code{SELECTED_CHAR_KIND}, @code{SELECTED_INT_KIND} and @code{SELECTED_REAL_KIND} intrinsics or the @code{INT8}, @code{INT16}, @code{INT32}, @code{INT64}, @code{REAL32}, @code{REAL64}, and @code{REAL128} parameters of the @code{ISO_FORTRAN_ENV} module instead of the concrete values. The available kind parameters can be found in the constant arrays @code{CHARACTER_KINDS}, @code{INTEGER_KINDS}, @code{LOGICAL_KINDS} and @code{REAL_KINDS} in the @code{ISO_FORTRAN_ENV} module (see @ref{ISO_FORTRAN_ENV}). @node Internal representation of LOGICAL variables @section Internal representation of LOGICAL variables @cindex logical, variable representation The Fortran standard does not specify how variables of @code{LOGICAL} type are represented, beyond requiring that @code{LOGICAL} variables of default kind have the same storage size as default @code{INTEGER} and @code{REAL} variables. The GNU Fortran internal representation is as follows. A @code{LOGICAL(KIND=N)} variable is represented as an @code{INTEGER(KIND=N)} variable, however, with only two permissible values: @code{1} for @code{.TRUE.} and @code{0} for @code{.FALSE.}. Any other integer value results in undefined behavior. Note that for mixed-language programming using the @code{ISO_C_BINDING} feature, there is a @code{C_BOOL} kind that can be used to create @code{LOGICAL(KIND=C_BOOL)} variables which are interoperable with the C99 _Bool type. The C99 _Bool type has an internal representation described in the C99 standard, which is identical to the above description, i.e. with 1 for true and 0 for false being the only permissible values. Thus the internal representation of @code{LOGICAL} variables in GNU Fortran is identical to C99 _Bool, except for a possible difference in storage size depending on the kind. @node Thread-safety of the runtime library @section Thread-safety of the runtime library @cindex thread-safety, threads GNU Fortran can be used in programs with multiple threads, e.g.@: by using OpenMP, by calling OS thread handling functions via the @code{ISO_C_BINDING} facility, or by GNU Fortran compiled library code being called from a multi-threaded program. The GNU Fortran runtime library, (@code{libgfortran}), supports being called concurrently from multiple threads with the following exceptions. During library initialization, the C @code{getenv} function is used, which need not be thread-safe. Similarly, the @code{getenv} function is used to implement the @code{GET_ENVIRONMENT_VARIABLE} and @code{GETENV} intrinsics. It is the responsibility of the user to ensure that the environment is not being updated concurrently when any of these actions are taking place. The @code{EXECUTE_COMMAND_LINE} and @code{SYSTEM} intrinsics are implemented with the @code{system} function, which need not be thread-safe. It is the responsibility of the user to ensure that @code{system} is not called concurrently. Finally, for platforms not supporting thread-safe POSIX functions, further functionality might not be thread-safe. For details, please consult the documentation for your operating system. @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. @menu * Extensions implemented in GNU Fortran:: * Extensions not implemented in GNU Fortran:: @end menu @node Extensions implemented in GNU Fortran @section Extensions implemented in GNU Fortran @cindex extensions, implemented GNU Fortran implements a number of extensions over standard Fortran. This chapter contains information on their syntax and meaning. There are currently two categories of GNU Fortran extensions, those that provide functionality beyond that provided by any standard, and those that are supported by GNU Fortran purely for backward compatibility with legacy compilers. By default, @option{-std=gnu} allows the compiler to accept both types of extensions, but to warn about the use of the latter. Specifying either @option{-std=f95}, @option{-std=f2003} or @option{-std=f2008} disables both types of extensions, and @option{-std=legacy} allows both without warning. @menu * Old-style kind specifications:: * Old-style variable initialization:: * Extensions to namelist:: * X format descriptor without count field:: * Commas in FORMAT specifications:: * Missing period in FORMAT specifications:: * I/O item lists:: * BOZ literal constants:: * @code{Q} exponent-letter:: * Real array indices:: * Unary operators:: * Implicitly convert LOGICAL and INTEGER values:: * Hollerith constants support:: * Cray pointers:: * CONVERT specifier:: * OpenMP:: * Argument list functions:: @end menu @node Old-style kind specifications @subsection Old-style kind specifications @cindex kind, old-style GNU Fortran allows old-style kind specifications in declarations. These look like: @smallexample TYPESPEC*size x,y,z @end smallexample @noindent 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 @smallexample TYPESPEC(k) x,y,z @end smallexample @noindent 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: @smallexample INTEGER, PARAMETER :: dbl = KIND(1.0d0) 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 form: @smallexample 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 declarations. Examples of standard-conforming code equivalent to the above example are: @smallexample ! Fortran 90 INTEGER :: i = 1, j = 2 REAL :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x)) ! Fortran 77 INTEGER i, j 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} attribute. @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{&} @smallexample $MYNML X(:)%Y(2) = 1.0 2.0 3.0 CH(1:4) = "abcd" $END @end smallexample It should be noted that the default terminator is @samp{/} rather than @samp{&END}. 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: @smallexample ? &mynml x x%y ch &end @end smallexample Entering @samp{=?} outputs the namelist to stdout, as if @code{WRITE(*,NML = mynml)} had been called: @smallexample =? &MYNML 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. @smallexample 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. @smallexample &MYNML X(1,1) = 0.00 , 1.00 , 2.00 / @end smallexample @node X format descriptor without count field @subsection @code{X} format descriptor without count field To support legacy codes, GNU Fortran permits the count field of the @code{X} edit descriptor in @code{FORMAT} statements to be omitted. When omitted, the count is implicitly assumed to be one. @smallexample 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. @smallexample PRINT 10, 2, 3 10 FORMAT ('FOO='I1' BAR='I2) @end smallexample @node Missing period in FORMAT specifications @subsection Missing period in @code{FORMAT} specifications To support legacy codes, GNU Fortran allows missing periods in format specifications if and only if @option{-std=legacy} is given on the command line. This is considered non-conforming code and is discouraged. @smallexample REAL :: value READ(*,10) value 10 FORMAT ('F4') @end smallexample @node I/O item lists @subsection I/O item lists @cindex I/O item lists To support legacy codes, GNU Fortran allows the input item list of the @code{READ} statement, and the output item lists of the @code{WRITE} and @code{PRINT} statements, to start with a comma. @node @code{Q} exponent-letter @subsection @code{Q} exponent-letter @cindex @code{Q} exponent-letter GNU Fortran accepts real literal constants with an exponent-letter of @code{Q}, for example, @code{1.23Q45}. The constant is interpreted as a @code{REAL(16)} entity on targets that suppports this type. If the target does not support @code{REAL(16)} but has a @code{REAL(10)} type, then the real-literal-constant will be interpreted as a @code{REAL(10)} entity. In the absence of @code{REAL(16)} and @code{REAL(10)}, an error will occur. @node BOZ literal constants @subsection BOZ literal constants @cindex BOZ literal constants Besides decimal constants, Fortran also supports binary (@code{b}), octal (@code{o}) and hexadecimal (@code{z}) integer constants. The syntax is: @samp{prefix quote digits quote}, were the prefix is either @code{b}, @code{o} or @code{z}, quote is either @code{'} or @code{"} and the digits are for binary @code{0} or @code{1}, for octal between @code{0} and @code{7}, and for hexadecimal between @code{0} and @code{F}. (Example: @code{b'01011101'}.) Up to Fortran 95, BOZ literals were only allowed to initialize integer variables in DATA statements. Since Fortran 2003 BOZ literals are also allowed as argument of @code{REAL}, @code{DBLE}, @code{INT} and @code{CMPLX}; the result is the same as if the integer BOZ literal had been converted by @code{TRANSFER} to, respectively, @code{real}, @code{double precision}, @code{integer} or @code{complex}. As GNU Fortran extension the intrinsic procedures @code{FLOAT}, @code{DFLOAT}, @code{COMPLEX} and @code{DCMPLX} are treated alike. As an extension, GNU Fortran allows hexadecimal BOZ literal constants to be specified using the @code{X} prefix, in addition to the standard @code{Z} prefix. The BOZ literal can also be specified by adding a suffix to the string, for example, @code{Z'ABC'} and @code{'ABC'Z} are equivalent. Furthermore, GNU Fortran allows using BOZ literal constants outside DATA statements and the four intrinsic functions allowed by Fortran 2003. In DATA statements, in direct assignments, where the right-hand side only contains a BOZ literal constant, and for old-style initializers of the form @code{integer i /o'0173'/}, the constant is transferred as if @code{TRANSFER} had been used; for @code{COMPLEX} numbers, only the real part is initialized unless @code{CMPLX} is used. In all other cases, the BOZ literal constant is converted to an @code{INTEGER} value with the largest decimal representation. This value is then converted numerically to the type and kind of the variable in question. (For instance, @code{real :: r = b'0000001' + 1} initializes @code{r} with @code{2.0}.) As different compilers implement the extension differently, one should be careful when doing bitwise initialization of non-integer variables. Note that initializing an @code{INTEGER} variable with a statement such as @code{DATA i/Z'FFFFFFFF'/} will give an integer overflow error rather than the desired result of @math{-1} when @code{i} is a 32-bit integer on a system that supports 64-bit integers. The @samp{-fno-range-check} option can be used as a workaround for legacy code that initializes integers in this manner. @node Real array indices @subsection Real array indices @cindex array, indices of type real As an extension, GNU Fortran allows the use of @code{REAL} expressions or variables as array indices. @node Unary operators @subsection Unary operators @cindex operators, unary As an extension, GNU Fortran allows unary plus and unary minus operators to appear as the second operand of binary arithmetic operators without the need for parenthesis. @smallexample 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.}. @smallexample LOGICAL :: l l = 1 @end smallexample @smallexample INTEGER :: i i = .TRUE. @end smallexample However, there is no implicit conversion of @code{INTEGER} values in @code{if}-statements, nor of @code{LOGICAL} or @code{INTEGER} values in I/O operations. @node Hollerith constants support @subsection Hollerith constants support @cindex Hollerith constants GNU Fortran supports Hollerith constants in assignments, function arguments, and @code{DATA} and @code{ASSIGN} statements. A Hollerith constant is written as a string of characters preceded by an integer constant indicating the character count, and the letter @code{H} or @code{h}, and stored in bytewise fashion in a numeric (@code{INTEGER}, @code{REAL}, or @code{complex}) or @code{LOGICAL} variable. The constant will be padded or truncated to fit the size of the variable in which it is stored. Examples of valid uses of Hollerith constants: @smallexample complex*16 x(2) data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/ x(1) = 16HABCDEFGHIJKLMNOP call foo (4h abc) @end smallexample Invalid Hollerith constants examples: @smallexample integer*4 a a = 8H12345678 ! Valid, but the Hollerith constant will be truncated. a = 0H ! At least one character is needed. @end smallexample In general, Hollerith constants were used to provide a rudimentary facility for handling character strings in early Fortran compilers, prior to the introduction of @code{CHARACTER} variables in Fortran 77; in those cases, the standard-compliant equivalent is to convert the program to use proper character strings. On occasion, there may be a case where the intent is specifically to initialize a numeric variable with a given byte sequence. In these cases, the same result can be obtained by using the @code{TRANSFER} statement, as in this example. @smallexample INTEGER(KIND=4) :: a a = TRANSFER ("abcd", a) ! equivalent to: a = 4Habcd @end smallexample @node Cray pointers @subsection Cray pointers @cindex pointer, Cray Cray pointers are part of a non-standard extension that provides a C-like pointer in Fortran. This is accomplished through a pair of variables: an integer "pointer" that holds a memory address, and a "pointee" that is used to dereference the pointer. Pointer/pointee pairs are declared in statements of the form: @smallexample pointer ( , ) @end smallexample or, @smallexample pointer ( , ), ( , ), ... @end smallexample The pointer is an integer that is intended to hold a memory address. The pointee may be an array or scalar. A pointee can be an assumed size array---that is, the last dimension may be left unspecified by using a @code{*} in place of a value---but a pointee cannot be an assumed shape array. No space is allocated for the pointee. The pointee may have its type declared before or after the pointer statement, and its array specification (if any) may be declared before, during, or after the pointer statement. The pointer may be declared as an integer prior to the pointer statement. However, some machines have default integer sizes that are different than the size of a pointer, and so the following code is not portable: @smallexample 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. 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: @smallexample 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: @smallexample 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 example: @smallexample 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 occurs. 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: @smallexample 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: @smallexample open(file='big.dat',form='unformatted',access='sequential', & convert='big_endian') @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 portable. @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 @uref{http://www.openmp.org/mp-documents/spec31.pdf, OpenMP Application Program Interface v3.1}. To enable the processing of the OpenMP directive @code{!$omp} in free-form source code; the @code{c$omp}, @code{*$omp} and @code{!$omp} directives in fixed form; the @code{!$} conditional compilation sentinels in free form; and the @code{c$}, @code{*$} and @code{!$} sentinels in fixed form, @command{gfortran} needs to be invoked with the @option{-fopenmp}. This also arranges for automatic linking of the GNU OpenMP runtime library @ref{Top,,libgomp,libgomp,GNU OpenMP runtime library}. The OpenMP Fortran runtime library routines are provided both in a form of a Fortran 90 module named @code{omp_lib} and in a form of a Fortran @code{include} file named @file{omp_lib.h}. An example of a parallelized loop taken from Appendix A.1 of the OpenMP Application Program Interface v2.5: @smallexample SUBROUTINE A1(N, A, B) INTEGER I, N REAL B(N), A(N) !$OMP PARALLEL DO !I is private by default DO I=2,N B(I) = (A(I) + A(I-1)) / 2.0 ENDDO !$OMP END PARALLEL DO END SUBROUTINE A1 @end smallexample Please note: @itemize @item @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. @item On glibc-based systems, OpenMP enabled applications cannot be statically linked due to limitations of the underlying pthreads-implementation. It might be possible to get a working solution if @command{-Wl,--whole-archive -lpthread -Wl,--no-whole-archive} is added to the command line. However, this is not supported by @command{gcc} and thus not recommended. @end itemize @node Argument list functions @subsection Argument list functions @code{%VAL}, @code{%REF} and @code{%LOC} @cindex argument list functions @cindex @code{%VAL} @cindex @code{%REF} @cindex @code{%LOC} GNU Fortran supports argument list functions @code{%VAL}, @code{%REF} and @code{%LOC} statements, for backward compatibility with g77. It is recommended that these should be used only for code that is accessing facilities outside of GNU Fortran, such as operating system or windowing facilities. It is best to constrain such uses to isolated portions of a program--portions that deal specifically and exclusively with low-level, system-dependent facilities. Such portions might well provide a portable interface for use by the program as a whole, but are themselves not portable, and should be thoroughly tested each time they are rebuilt using a new compiler or version of a compiler. @code{%VAL} passes a scalar argument by value, @code{%REF} passes it by reference and @code{%LOC} passes its memory location. Since gfortran already passes scalar arguments by reference, @code{%REF} is in effect a do-nothing. @code{%LOC} has the same effect as a Fortran pointer. An example of passing an argument by value to a C subroutine foo.: @smallexample C C prototype void foo_ (float x); C external foo real*4 x x = 3.14159 call foo (%VAL (x)) end @end smallexample For details refer to the g77 manual @uref{http://gcc.gnu.org/@/onlinedocs/@/gcc-3.4.6/@/g77/@/index.html#Top}. Also, @code{c_by_val.f} and its partner @code{c_by_val.c} of the GNU Fortran testsuite are worth a look. @node Extensions not implemented in GNU Fortran @section Extensions not implemented in GNU Fortran @cindex extensions, not implemented The long history of the Fortran language, its wide use and broad userbase, the large number of different compiler vendors and the lack of some features crucial to users in the first standards have lead to the existence of a number of important extensions to the language. While some of the most useful or popular extensions are supported by the GNU Fortran compiler, not all existing extensions are supported. This section aims at listing these extensions and offering advice on how best make code that uses them running with the GNU Fortran compiler. @c More can be found here: @c -- http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/Missing-Features.html @c -- the list of Fortran and libgfortran bugs closed as WONTFIX: @c http://tinyurl.com/2u4h5y @menu * STRUCTURE and RECORD:: @c * UNION and MAP:: * ENCODE and DECODE statements:: * Variable FORMAT expressions:: @c * Q edit descriptor:: @c * AUTOMATIC statement:: @c * TYPE and ACCEPT I/O Statements:: @c * .XOR. operator:: @c * CARRIAGECONTROL, DEFAULTFILE, DISPOSE and RECORDTYPE I/O specifiers:: @c * Omitted arguments in procedure call:: * Alternate complex function syntax:: @end menu @node STRUCTURE and RECORD @subsection @code{STRUCTURE} and @code{RECORD} @cindex @code{STRUCTURE} @cindex @code{RECORD} Structures are user-defined aggregate data types; this functionality was standardized in Fortran 90 with an different syntax, under the name of ``derived types''. Here is an example of code using the non portable structure syntax: @example ! Declaring a structure named ``item'' and containing three fields: ! an integer ID, an description string and a floating-point price. STRUCTURE /item/ INTEGER id CHARACTER(LEN=200) description REAL price END STRUCTURE ! 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 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 ! We can also manipulate the whole structure store_catalog(12) = pear print *, store_catalog(12) @end example @noindent This code can easily be rewritten in the Fortran 90 syntax as following: @example ! ``STRUCTURE /name/ ... END STRUCTURE'' becomes ! ``TYPE name ... END TYPE'' TYPE item INTEGER id CHARACTER(LEN=200) description REAL price END TYPE ! ``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 don't change store_catalog(12) = pear print *, store_catalog(12) @end example @c @node UNION and MAP @c @subsection @code{UNION} and @code{MAP} @c @cindex @code{UNION} @c @cindex @code{MAP} @c @c For help writing this one, see @c http://www.eng.umd.edu/~nsw/ench250/fortran1.htm#UNION and @c http://www.tacc.utexas.edu/services/userguides/pgi/pgiws_ug/pgi32u06.htm @node ENCODE and DECODE statements @subsection @code{ENCODE} and @code{DECODE} statements @cindex @code{ENCODE} @cindex @code{DECODE} GNU Fortran doesn't 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 @smallexample INTEGER*1 LINE(80) REAL A, B, C c ... Code that sets LINE DECODE (80, 9000, LINE) A, B, C 9000 FORMAT (1X, 3(F10.5)) @end smallexample @noindent with the following: @smallexample CHARACTER(LEN=80) LINE REAL A, B, C c ... Code that sets LINE READ (UNIT=LINE, FMT=9000) A, B, C 9000 FORMAT (1X, 3(F10.5)) @end smallexample Similarly, replace a code fragment like @smallexample INTEGER*1 LINE(80) REAL A, B, C 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 @noindent with the following: @smallexample CHARACTER(LEN=80) LINE REAL A, B, C c ... Code that sets A, B and C WRITE (UNIT=LINE, FMT=9000) A, B, 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)}. 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: @smallexample WRITE(6,20) INT1 20 FORMAT(I) @end smallexample @noindent with the following: @smallexample c Variable declaration CHARACTER(LEN=20) FMT c c Other code here... c WRITE(FMT,'("(I", I0, ")")') N+1 WRITE(6,FMT) INT1 @end smallexample @noindent or with: @smallexample c Variable declaration CHARACTER(LEN=20) FMT c c Other code here... c WRITE(FMT,*) N+1 WRITE(6,"(I" // ADJUSTL(FMT) // ")") INT1 @end smallexample @node Alternate complex function syntax @subsection Alternate complex function syntax @cindex Complex function Some Fortran compilers, including @command{g77}, let the user declare complex functions with the syntax @code{COMPLEX FUNCTION name*16()}, as well as @code{COMPLEX*16 FUNCTION name()}. Both are non-standard, legacy extensions. @command{gfortran} accepts the latter form, which is more common, but not the former. @c --------------------------------------------------------------------- @c Mixed-Language Programming @c --------------------------------------------------------------------- @node Mixed-Language Programming @chapter Mixed-Language Programming @cindex Interoperability @cindex Mixed-language programming @menu * Interoperability with C:: * GNU Fortran Compiler Directives:: * Non-Fortran Main Program:: @end menu This chapter is about mixed-language interoperability, but also applies if one links Fortran code compiled by different compilers. In most cases, use of the C Binding features of the Fortran 2003 standard is sufficient, and their use is highly recommended. @node Interoperability with C @section Interoperability with C @menu * Intrinsic Types:: * Derived Types and struct:: * Interoperable Global Variables:: * Interoperable Subroutines and Functions:: * Working with Pointers:: * Further Interoperability of Fortran with C:: @end menu Since Fortran 2003 (ISO/IEC 1539-1:2004(E)) there is a standardized way to generate procedure and derived-type declarations and global variables which are interoperable with C (ISO/IEC 9899:1999). The @code{bind(C)} attribute has been added to inform the compiler that a symbol shall be interoperable with C; also, some constraints are added. Note, however, that not all C features have a Fortran equivalent or vice versa. For instance, neither C's unsigned integers nor C's functions with variable number of arguments have an equivalent in Fortran. Note that array dimensions are reversely ordered in C and that arrays in C always start with index 0 while in Fortran they start by default with 1. Thus, an array declaration @code{A(n,m)} in Fortran matches @code{A[m][n]} in C and accessing the element @code{A(i,j)} matches @code{A[j-1][i-1]}. The element following @code{A(i,j)} (C: @code{A[j-1][i-1]}; assuming @math{i < n}) in memory is @code{A(i+1,j)} (C: @code{A[j-1][i]}). @node Intrinsic Types @subsection Intrinsic Types In order to ensure that exactly the same variable type and kind is used in C and Fortran, the named constants shall be used which are defined in the @code{ISO_C_BINDING} intrinsic module. That module contains named constants for kind parameters and character named constants for the escape sequences in C. For a list of the constants, see @ref{ISO_C_BINDING}. @node Derived Types and struct @subsection Derived Types and struct For compatibility of derived types with @code{struct}, one needs to use the @code{BIND(C)} attribute in the type declaration. For instance, the following type declaration @smallexample USE ISO_C_BINDING TYPE, BIND(C) :: myType INTEGER(C_INT) :: i1, i2 INTEGER(C_SIGNED_CHAR) :: i3 REAL(C_DOUBLE) :: d1 COMPLEX(C_FLOAT_COMPLEX) :: c1 CHARACTER(KIND=C_CHAR) :: str(5) END TYPE @end smallexample matches the following @code{struct} declaration in C @smallexample 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 variables 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. @smallexample MODULE m USE myType_module USE ISO_C_BINDING integer(C_INT), bind(C, name="_MyProject_flags") :: global_flag type(myType), bind(C) :: tp END MODULE @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 only explicit size and assumed-size arrays are supported but not assumed-shape or allocatable arrays. To pass a variable by value, use the @code{VALUE} attribute. Thus the following C prototype @smallexample @code{int func(int i, int *j)} @end smallexample matches the Fortran declaration @smallexample 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, @smallexample #include 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 @smallexample use iso_c_binding, only: C_CHAR, C_NULL_CHAR interface 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 @smallexample 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: @smallexample use iso_c_binding implicit none character(len=30) :: str,str2 interface ! 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) import 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 @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 example, @smallexample 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's 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: @smallexample /* Procedure implemented in Fortran. */ void get_values (void (*)(double)); /* Call-back routine we want called from Fortran. */ void print_it (double x) @{ printf ("Number is %f.\n", x); @} /* Call Fortran routine and pass call-back to it. */ void 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}: @smallexample MODULE m IMPLICIT NONE ! Define interface of call-back routine. ABSTRACT INTERFACE SUBROUTINE callback (x) USE, INTRINSIC :: ISO_C_BINDING REAL(KIND=C_DOUBLE), INTENT(IN), VALUE :: x END SUBROUTINE callback END INTERFACE CONTAINS ! Define C-bound procedure. SUBROUTINE get_values (cproc) BIND(C) USE, INTRINSIC :: ISO_C_BINDING TYPE(C_FUNPTR), INTENT(IN), VALUE :: cproc 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 SUBROUTINE get_values END MODULE m @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: @smallexample int call_it (int (*func)(int), int arg) @{ return func (arg); @} @end smallexample It can be used as in the following Fortran code: @smallexample MODULE m USE, INTRINSIC :: ISO_C_BINDING IMPLICIT NONE ! Define interface of C function. INTERFACE INTEGER(KIND=C_INT) FUNCTION call_it (func, arg) BIND(C) USE, INTRINSIC :: ISO_C_BINDING TYPE(C_FUNPTR), INTENT(IN), VALUE :: func INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg END FUNCTION call_it END INTERFACE CONTAINS ! Define procedure passed to C function. ! It must be interoperable! INTEGER(KIND=C_INT) FUNCTION double_it (arg) BIND(C) INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg double_it = arg + arg END FUNCTION double_it ! Call C function. SUBROUTINE foobar () TYPE(C_FUNPTR) :: cproc INTEGER(KIND=C_INT) :: i ! 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 DO END SUBROUTINE foobar END MODULE m @end smallexample @node Further Interoperability of Fortran with C @subsection Further Interoperability of Fortran with C Assumed-shape and allocatable arrays are passed using an array descriptor (dope vector). The internal structure of the array descriptor used by GNU Fortran is not yet documented and will change. There will also be a Technical Specification (TS 29113) which standardizes an interoperable array descriptor. Until then, you can use the Chasm Language Interoperability Tools, @url{http://chasm-interop.sourceforge.net/}, which provide an interface to GNU Fortran's array descriptor. GNU Fortran already supports the C-interoperable @code{OPTIONAL} attribute; for absent arguments, a @code{NULL} pointer is passed. @node GNU Fortran Compiler Directives @section GNU Fortran Compiler Directives The Fortran standard describes how a conforming program shall behave; however, the exact implementation is not standardized. In order to allow the user to choose specific implementation details, compiler directives can be used to set attributes of variables and procedures which are not part of the standard. Whether a given attribute is supported and its exact effects depend on both the operating system and on the processor; see @ref{Top,,C Extensions,gcc,Using the GNU Compiler Collection (GCC)} for details. For procedures and procedure pointers, the following attributes can be used to change the calling convention: @itemize @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: @itemize @item @code{DLLEXPORT} -- provide a global pointer to a pointer in the DLL @item @code{DLLIMPORT} -- reference the function or variable using a global pointer @end itemize The attributes are specified using the syntax @code{!GCC$ ATTRIBUTES} @var{attribute-list} @code{::} @var{variable-list} where in free-form source code only whitespace is allowed before @code{!GCC$} and in fixed-form source code @code{!GCC$}, @code{cGCC$} or @code{*GCC$} shall start in the first column. For procedures, the compiler directives shall be placed into the body of the procedure; for variables and procedure pointers, they shall be in the same declaration part as the variable or procedure pointer. @node Non-Fortran Main Program @section Non-Fortran Main Program @menu * _gfortran_set_args:: Save command-line arguments * _gfortran_set_options:: Set library option flags * _gfortran_set_convert:: Set endian conversion * _gfortran_set_record_marker:: Set length of record markers * _gfortran_set_max_subrecord_length:: Set subrecord length * _gfortran_set_fpe:: Set when a Floating Point Exception should be raised @end menu Even if you are doing mixed-language programming, it is very likely that you do not need to know or use the information in this section. Since it is about the internal structure of GNU Fortran, it may also change in GCC minor releases. When you compile a @code{PROGRAM} with GNU Fortran, a function with the name @code{main} (in the symbol table of the object file) is generated, which initializes the libgfortran library and then calls the actual program which uses the name @code{MAIN__}, for historic reasons. If you link GNU Fortran compiled procedures to, e.g., a C or C++ program or to a Fortran program compiled by a different compiler, the libgfortran library is not initialized and thus a few intrinsic procedures do not work properly, e.g. those for obtaining the command-line arguments. Therefore, if your @code{PROGRAM} is not compiled with GNU Fortran and the GNU Fortran compiled procedures require intrinsics relying on the library initialization, you need to initialize the library yourself. Using the default options, gfortran calls @code{_gfortran_set_args} and @code{_gfortran_set_options}. The initialization of the former is needed if the called procedures access the command line (and for backtracing); the latter sets some flags based on the standard chosen or to enable backtracing. In typical programs, it is not necessary to call any initialization function. If your @code{PROGRAM} is compiled with GNU Fortran, you shall not call any of the following functions. The libgfortran initialization functions are shown in C syntax but using C bindings they are also accessible from Fortran. @node _gfortran_set_args @subsection @code{_gfortran_set_args} --- Save command-line arguments @fnindex _gfortran_set_args @cindex libgfortran initialization, set_args @table @asis @item @emph{Description}: @code{_gfortran_set_args} saves the command-line arguments; this initialization is required if any of the command-line intrinsics is called. Additionally, it shall be called if backtracing is enabled (see @code{_gfortran_set_options}). @item @emph{Syntax}: @code{void _gfortran_set_args (int argc, char *argv[])} @item @emph{Arguments}: @multitable @columnfractions .15 .70 @item @var{argc} @tab number of command line argument strings @item @var{argv} @tab the command-line argument strings; argv[0] is the pathname of the executable itself. @end multitable @item @emph{Example}: @smallexample int main (int argc, char *argv[]) @{ /* Initialize libgfortran. */ _gfortran_set_args (argc, argv); return 0; @} @end smallexample @end table @node _gfortran_set_options @subsection @code{_gfortran_set_options} --- Set library option flags @fnindex _gfortran_set_options @cindex libgfortran initialization, set_options @table @asis @item @emph{Description}: @code{_gfortran_set_options} sets several flags related to the Fortran standard to be used, whether backtracing should be enabled and whether range checks should be performed. The syntax allows for upward compatibility since the number of passed flags is specified; for non-passed flags, the default value is used. See also @pxref{Code Gen Options}. Please note that not all flags are actually used. @item @emph{Syntax}: @code{void _gfortran_set_options (int num, int options[])} @item @emph{Arguments}: @multitable @columnfractions .15 .70 @item @var{num} @tab number of options passed @item @var{argv} @tab The list of flag values @end multitable @item @emph{option flag list}: @multitable @columnfractions .15 .70 @item @var{option}[0] @tab Allowed standard; can give run-time errors if e.g. an input-output edit descriptor is invalid in a given standard. Possible values are (bitwise or-ed) @code{GFC_STD_F77} (1), @code{GFC_STD_F95_OBS} (2), @code{GFC_STD_F95_DEL} (4), @code{GFC_STD_F95} (8), @code{GFC_STD_F2003} (16), @code{GFC_STD_GNU} (32), @code{GFC_STD_LEGACY} (64), @code{GFC_STD_F2008} (128), @code{GFC_STD_F2008_OBS} (256) and GFC_STD_F2008_TS (512). Default: @code{GFC_STD_F95_OBS | GFC_STD_F95_DEL | GFC_STD_F95 | GFC_STD_F2003 | GFC_STD_F2008 | GFC_STD_F2008_TS | GFC_STD_F2008_OBS | GFC_STD_F77 | GFC_STD_GNU | GFC_STD_LEGACY}. @item @var{option}[1] @tab Standard-warning flag; prints a warning to standard error. Default: @code{GFC_STD_F95_DEL | GFC_STD_LEGACY}. @item @var{option}[2] @tab If non zero, enable pedantic checking. Default: off. @item @var{option}[3] @tab Unused. @item @var{option}[4] @tab If non zero, enable backtracing on run-time errors. Default: off. Note: Installs a signal handler and requires command-line initialization using @code{_gfortran_set_args}. @item @var{option}[5] @tab If non zero, supports signed zeros. Default: enabled. @item @var{option}[6] @tab Enables run-time checking. Possible values are (bitwise or-ed): GFC_RTCHECK_BOUNDS (1), GFC_RTCHECK_ARRAY_TEMPS (2), GFC_RTCHECK_RECURSION (4), GFC_RTCHECK_DO (16), GFC_RTCHECK_POINTER (32). Default: disabled. @item @var{option}[7] @tab If non zero, range checking is enabled. Default: enabled. See -frange-check (@pxref{Code Gen Options}). @end multitable @item @emph{Example}: @smallexample /* Use gfortran 4.7 default options. */ static int options[] = @{68, 511, 0, 0, 1, 1, 0, 1@}; _gfortran_set_options (8, &options); @end smallexample @end table @node _gfortran_set_convert @subsection @code{_gfortran_set_convert} --- Set endian conversion @fnindex _gfortran_set_convert @cindex libgfortran initialization, set_convert @table @asis @item @emph{Description}: @code{_gfortran_set_convert} set the representation of data for unformatted files. @item @emph{Syntax}: @code{void _gfortran_set_convert (int conv)} @item @emph{Arguments}: @multitable @columnfractions .15 .70 @item @var{conv} @tab Endian conversion, possible values: GFC_CONVERT_NATIVE (0, default), GFC_CONVERT_SWAP (1), GFC_CONVERT_BIG (2), GFC_CONVERT_LITTLE (3). @end multitable @item @emph{Example}: @smallexample int main (int argc, char *argv[]) @{ /* Initialize libgfortran. */ _gfortran_set_args (argc, argv); _gfortran_set_convert (1); return 0; @} @end smallexample @end table @node _gfortran_set_record_marker @subsection @code{_gfortran_set_record_marker} --- Set length of record markers @fnindex _gfortran_set_record_marker @cindex libgfortran initialization, set_record_marker @table @asis @item @emph{Description}: @code{_gfortran_set_record_marker} sets the length of record markers for unformatted files. @item @emph{Syntax}: @code{void _gfortran_set_record_marker (int val)} @item @emph{Arguments}: @multitable @columnfractions .15 .70 @item @var{val} @tab Length of the record marker; valid values are 4 and 8. Default is 4. @end multitable @item @emph{Example}: @smallexample int main (int argc, char *argv[]) @{ /* Initialize libgfortran. */ _gfortran_set_args (argc, argv); _gfortran_set_record_marker (8); return 0; @} @end smallexample @end table @node _gfortran_set_fpe @subsection @code{_gfortran_set_fpe} --- Enable floating point exception traps @fnindex _gfortran_set_fpe @cindex libgfortran initialization, set_fpe @table @asis @item @emph{Description}: @code{_gfortran_set_fpe} enables floating point exception traps for the specified exceptions. On most systems, this will result in a SIGFPE signal being sent and the program being aborted. @item @emph{Syntax}: @code{void _gfortran_set_fpe (int val)} @item @emph{Arguments}: @multitable @columnfractions .15 .70 @item @var{option}[0] @tab IEEE exceptions. Possible values are (bitwise or-ed) zero (0, default) no trapping, @code{GFC_FPE_INVALID} (1), @code{GFC_FPE_DENORMAL} (2), @code{GFC_FPE_ZERO} (4), @code{GFC_FPE_OVERFLOW} (8), @code{GFC_FPE_UNDERFLOW} (16), and @code{GFC_FPE_INEXACT} (32). @end multitable @item @emph{Example}: @smallexample int main (int argc, char *argv[]) @{ /* Initialize libgfortran. */ _gfortran_set_args (argc, argv); /* FPE for invalid operations such as SQRT(-1.0). */ _gfortran_set_fpe (1); return 0; @} @end smallexample @end table @node _gfortran_set_max_subrecord_length @subsection @code{_gfortran_set_max_subrecord_length} --- Set subrecord length @fnindex _gfortran_set_max_subrecord_length @cindex libgfortran initialization, set_max_subrecord_length @table @asis @item @emph{Description}: @code{_gfortran_set_max_subrecord_length} set the maximum length for a subrecord. This option only makes sense for testing and debugging of unformatted I/O. @item @emph{Syntax}: @code{void _gfortran_set_max_subrecord_length (int val)} @item @emph{Arguments}: @multitable @columnfractions .15 .70 @item @var{val} @tab the maximum length for a subrecord; the maximum permitted value is 2147483639, which is also the default. @end multitable @item @emph{Example}: @smallexample int main (int argc, char *argv[]) @{ /* Initialize libgfortran. */ _gfortran_set_args (argc, argv); _gfortran_set_max_subrecord_length (8); return 0; @} @end smallexample @end table @c Intrinsic Procedures @c --------------------------------------------------------------------- @include intrinsic.texi @tex \blankpart @end tex @c --------------------------------------------------------------------- @c Contributing @c --------------------------------------------------------------------- @node Contributing @unnumbered Contributing @cindex Contributing Free software is only possible if people contribute to efforts to create it. We're always in need of more people helping out with ideas and comments, writing documentation and contributing code. If you want to contribute to GNU Fortran, have a look at the long lists of projects you can take on. Some of these projects are small, some of them are large; some are completely orthogonal to the rest of what is happening on GNU Fortran, but others are ``mainstream'' projects in need of enthusiastic hackers. All of these projects are important! We'll eventually get around to the things here, but they are also things doable by someone who is willing and able. @menu * Contributors:: * Projects:: * Proposed Extensions:: @end menu @node Contributors @section Contributors to GNU Fortran @cindex Contributors @cindex Credits @cindex Authors Most of the parser was hand-crafted by @emph{Andy Vaught}, who is also the initiator of the whole project. Thanks Andy! Most of the interface with GCC was written by @emph{Paul Brook}. The following individuals have contributed code and/or ideas and significant help to the GNU Fortran project (in alphabetical order): @itemize @minus @item Janne Blomqvist @item Steven Bosscher @item Paul Brook @item Tobias Burnus @item Fran@,{c}ois-Xavier Coudert @item Bud Davis @item Jerry DeLisle @item Erik Edelmann @item Bernhard Fischer @item Daniel Franke @item Richard Guenther @item Richard Henderson @item Katherine Holcomb @item Jakub Jelinek @item Niels Kristian Bech Jensen @item Steven Johnson @item Steven G. Kargl @item Thomas Koenig @item Asher Langton @item H. J. Lu @item Toon Moene @item Brooks Moses @item Andrew Pinski @item Tim Prince @item Christopher D. Rickett @item Richard Sandiford @item Tobias Schl@"uter @item Roger Sayle @item Paul Thomas @item Andy Vaught @item Feng Wang @item Janus Weil @item Daniel Kraft @end itemize The following people have contributed bug reports, smaller or larger patches, and much needed feedback and encouragement for the GNU Fortran project: @itemize @minus @item Bill Clodius @item Dominique d'Humi@`eres @item Kate Hedstrom @item Erik Schnetter @item Joost VandeVondele @end itemize Many other individuals have helped debug, test and improve the GNU Fortran compiler over the past few years, and we welcome you to do the same! If you already have done so, and you would like to see your name listed in the list above, please contact us. @node Projects @section Projects @table @emph @item Help build the test suite Solicit more code for donation to the test suite: the more extensive the testsuite, the smaller the risk of breaking things in the future! We can keep code private on request. @item Bug hunting/squishing Find bugs and write more test cases! Test cases are especially very welcome, because it allows us to concentrate on fixing bugs instead of isolating them. Going through the bugzilla database at @url{http://gcc.gnu.org/@/bugzilla/} to reduce testcases posted there and add more information (for example, for which version does the testcase work, for which versions does it fail?) is also very helpful. @end table @node Proposed Extensions @section Proposed Extensions Here's a list of proposed extensions for the GNU Fortran compiler, in no particular order. Most of these are necessary to be fully compatible with existing Fortran compilers, but they are not part of the official J3 Fortran 95 standard. @subsection Compiler extensions: @itemize @bullet @item User-specified alignment rules for structures. @item Automatically extend single precision constants to double. @item Compile code that conserves memory by dynamically allocating common and module storage either on stack or heap. @item Compile flag to generate code for array conformance checking (suggest -CC). @item User control of symbol names (underscores, etc). @item Compile setting for maximum size of stack frame size before spilling parts to static or heap. @item Flag to force local variables into static space. @item Flag to force local variables onto stack. @end itemize @subsection Environment Options @itemize @bullet @item Pluggable library modules for random numbers, linear algebra. LA should use BLAS calling conventions. @item Environment variables controlling actions on arithmetic exceptions like overflow, underflow, precision loss---Generate NaN, abort, default. action. @item Set precision for fp units that support it (i387). @item Variable for setting fp rounding mode. @item Variable to fill uninitialized variables with a user-defined bit pattern. @item Environment variable controlling filename that is opened for that unit number. @item Environment variable to clear/trash memory being freed. @item Environment variable to control tracing of allocations and frees. @item Environment variable to display allocated memory at normal program end. @item Environment variable for filename for * IO-unit. @item Environment variable for temporary file directory. @item Environment variable forcing standard output to be line buffered (unix). @end itemize @c --------------------------------------------------------------------- @c GNU General Public License @c --------------------------------------------------------------------- @include gpl_v3.texi @c --------------------------------------------------------------------- @c GNU Free Documentation License @c --------------------------------------------------------------------- @include fdl.texi @c --------------------------------------------------------------------- @c Funding Free Software @c --------------------------------------------------------------------- @include funding.texi @c --------------------------------------------------------------------- @c Indices @c --------------------------------------------------------------------- @node Option Index @unnumbered Option Index @command{gfortran}'s command line options are indexed here without any initial @samp{-} or @samp{--}. Where an option has both positive and negative forms (such as -foption and -fno-option), relevant entries in the manual are indexed under the most appropriate form; it may sometimes be useful to look up both forms. @printindex op @node Keyword Index @unnumbered Keyword Index @printindex cp @bye