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 \input texinfo @c -*-texinfo-*- @c %**start of header @setfilename gfortran.info @set copyrights-gfortran 1999-2021 @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 standards status:: Fortran 2003, 2008 and 2018 features supported by GNU Fortran. * Compiler Characteristics:: User-visible implementation details. * Extensions:: Language extensions implemented by GNU Fortran. * Mixed-Language Programming:: Interoperability with C * Coarray Programming:: * Intrinsic Procedures:: Intrinsic procedures supported by GNU Fortran. * Intrinsic Modules:: Intrinsic modules supported by GNU Fortran. * Contributing:: How you can help. * Copying:: GNU General Public License says how you can copy and share GNU Fortran. * GNU Free Documentation License:: How you can copy and share this manual. * Funding:: How to help assure continued work for free software. * Option Index:: Index of command line options * Keyword Index:: Index of concepts @end menu @end ifnottex @c --------------------------------------------------------------------- @c Introduction @c --------------------------------------------------------------------- @node Introduction @chapter Introduction @c The following duplicates the text on the TexInfo table of contents. @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 will 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, 2008 and 2018 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, Fortran 2008 or Fortran 2018. 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 are not as good writing machine code as they are at writing Fortran (or C++, Ada, or Java), because it is easy to make tiny mistakes writing machine code. @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. The macros @code{__GNUC__}, @code{__GNUC_MINOR__} and @code{__GNUC_PATCHLEVEL__} can be used to determine the version of the compiler. See @ref{Top,,Overview,cpp,The C Preprocessor} for details. GNU Fortran supports a number of @code{INTEGER} and @code{REAL} kind types in additional to the kind types required by the Fortran standard. The availability of any given kind type is architecture dependent. The following pre-defined preprocessor macros can be used to conditionally include code for these additional kind types: @code{__GFC_INT_1__}, @code{__GFC_INT_2__}, @code{__GFC_INT_8__}, @code{__GFC_INT_16__}, @code{__GFC_REAL_10__}, and @code{__GFC_REAL_16__}. While CPP is the de-facto standard for preprocessing Fortran code, Part 3 of the Fortran 95 standard (ISO/IEC 1539-3:1998) defines Conditional Compilation, which is not widely used and not directly supported by the GNU Fortran compiler. You can use the program coco to preprocess such files (@uref{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, some old vendor extensions, and several Fortran 2003 and Fortran 2008 features, including TR 15581. However, it is still under development and has a few remaining rough edges. There also is initial support for OpenACC. At present, the GNU Fortran compiler passes the @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/fortran-compiler-comparisons/polyhedron-benchmark-suite, Polyhedron Fortran compiler benchmarks} and the @uref{http://www.netlib.org/benchmark/livermore, Livermore Fortran Kernels test}. It has been used to compile a number of large real-world programs, including @uref{http://hirlam.org/, the HARMONIE and HIRLAM weather forecasting code} and @uref{http://physical-chemistry.scb.uwa.edu.au/tonto/wiki/index.php/Main_Page, the Tonto quantum chemistry package}; see @url{https://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, Fortran 2008 and Fortran 2018. @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. GNU Fortran also have a partial support for ISO/IEC 1539-1:2004 (Fortran 2003), ISO/IEC 1539-1:2010 (Fortran 2008), the Technical Specification @code{Further Interoperability of Fortran with C} (ISO/IEC TS 29113:2012). Full support of those standards and future Fortran standards is planned. The current status of the support is can be found in the @ref{Fortran 2003 status}, @ref{Fortran 2008 status} and @ref{Fortran 2018 status} sections of the documentation. Additionally, the GNU Fortran compilers supports the OpenMP specification (version 4.5 and partial support of the features of the 5.0 version, @url{http://openmp.org/@/openmp-specifications/}). There also is support for the OpenACC specification (targeting version 2.6, @uref{http://www.openacc.org/}). See @uref{https://gcc.gnu.org/wiki/OpenACC} for more information. @node Varying Length Character Strings @subsection Varying Length Character Strings @cindex Varying length character strings @cindex Varying length strings @cindex strings, varying length The Fortran 95 standard specifies in Part 2 (ISO/IEC 1539-2:2000) varying length character strings. While GNU Fortran currently does not support such strings directly, there exist two Fortran implementations for them, which work with GNU Fortran. They can be found at @uref{http://www.fortran.com/@/iso_varying_string.f95} and at @uref{ftp://ftp.nag.co.uk/@/sc22wg5/@/ISO_VARYING_STRING/}. Deferred-length character strings of Fortran 2003 supports part of the features of @code{ISO_VARYING_STRING} and should be considered as replacement. (Namely, allocatable or pointers of the type @code{character(len=:)}.) @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 * TMPDIR:: Directory for scratch files * GFORTRAN_STDIN_UNIT:: Unit number for standard input * GFORTRAN_STDOUT_UNIT:: Unit number for standard output * GFORTRAN_STDERR_UNIT:: Unit number for standard error * GFORTRAN_UNBUFFERED_ALL:: Do not buffer I/O for all units * GFORTRAN_UNBUFFERED_PRECONNECTED:: Do not buffer I/O for preconnected units. * GFORTRAN_SHOW_LOCUS:: Show location for runtime errors * GFORTRAN_OPTIONAL_PLUS:: Print leading + where permitted * GFORTRAN_LIST_SEPARATOR:: Separator for list output * GFORTRAN_CONVERT_UNIT:: Set endianness for unformatted I/O * GFORTRAN_ERROR_BACKTRACE:: Show backtrace on run-time errors * GFORTRAN_FORMATTED_BUFFER_SIZE:: Buffer size for formatted files * GFORTRAN_UNFORMATTED_BUFFER_SIZE:: Buffer size for unformatted files @end menu @node TMPDIR @section @env{TMPDIR}---Directory for scratch files When opening a file with @code{STATUS='SCRATCH'}, GNU Fortran tries to create the file in one of the potential directories by testing each directory in the order below. @enumerate @item The environment variable @env{TMPDIR}, if it exists. @item On the MinGW target, the directory returned by the @code{GetTempPath} function. Alternatively, on the Cygwin target, the @env{TMP} and @env{TEMP} environment variables, if they exist, in that order. @item The @code{P_tmpdir} macro if it is defined, otherwise the directory @file{/tmp}. @end enumerate @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_UNBUFFERED_ALL @section @env{GFORTRAN_UNBUFFERED_ALL}---Do not buffer I/O on all units This environment variable controls whether all I/O is unbuffered. If the first letter is @samp{y}, @samp{Y} or @samp{1}, all I/O is unbuffered. This will slow down small sequential reads and writes. If the first letter is @samp{n}, @samp{N} or @samp{0}, I/O is buffered. This is the default. @node GFORTRAN_UNBUFFERED_PRECONNECTED @section @env{GFORTRAN_UNBUFFERED_PRECONNECTED}---Do not buffer I/O on preconnected units The environment variable named @env{GFORTRAN_UNBUFFERED_PRECONNECTED} controls whether I/O on a preconnected unit (i.e.@: STDOUT or STDERR) is unbuffered. If the first letter is @samp{y}, @samp{Y} or @samp{1}, I/O is unbuffered. This will slow down small sequential reads and writes. If the first letter is @samp{n}, @samp{N} or @samp{0}, I/O is buffered. This is the default. @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}, do not 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_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. @node GFORTRAN_FORMATTED_BUFFER_SIZE @section @env{GFORTRAN_FORMATTED_BUFFER_SIZE}---Set buffer size for formatted I/O The @env{GFORTRAN_FORMATTED_BUFFER_SIZE} environment variable specifies buffer size in bytes to be used for formatted output. The default value is 8192. @node GFORTRAN_UNFORMATTED_BUFFER_SIZE @section @env{GFORTRAN_UNFORMATTED_BUFFER_SIZE}---Set buffer size for unformatted I/O The @env{GFORTRAN_UNFORMATTED_BUFFER_SIZE} environment variable specifies buffer size in bytes to be used for unformatted output. The default value is 131072. @c ===================================================================== @c PART II: LANGUAGE REFERENCE @c ===================================================================== @tex \part{II}{Language Reference} @end tex @c --------------------------------------------------------------------- @c Fortran standards status @c --------------------------------------------------------------------- @node Fortran standards status @chapter Fortran standards status @menu * Fortran 2003 status:: * Fortran 2008 status:: * Fortran 2018 status:: @end menu @node Fortran 2003 status @section Fortran 2003 status GNU Fortran supports several Fortran 2003 features; an incomplete list can be found below. See also the @uref{https://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 and unlimited polymorphism (@code{CLASS(*)}'') -- including @code{SAME_TYPE_AS}, @code{EXTENDS_TYPE_OF} and @code{SELECT TYPE} for scalars and arrays and finalization. @item Generic interface names, which have the same name as derived types, are now supported. This allows one to write constructor functions. Note that Fortran does not support static constructor functions. For static variables, only default initialization or structure-constructor initialization are available. @item The @code{ASSOCIATE} construct. @item Interoperability with C including enumerations, @item In structure constructors the components with default values may be 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 Deferred-length character variables and scalar deferred-length character components of derived types are supported. (Note that array-valued components are not yet implemented.) @item Transferring of allocations via @code{MOVE_ALLOC}. @item The @code{PRIVATE} and @code{PUBLIC} attributes may be given individually to derived-type components. @item In pointer assignments, the lower bound may be specified and the remapping of elements is supported. @item For pointers an @code{INTENT} may be specified which affect the association status not the value of the pointer target. @item Intrinsics @code{command_argument_count}, @code{get_command}, @code{get_command_argument}, and @code{get_environment_variable}. @item Support for Unicode characters (ISO 10646) and UTF-8, including the @code{SELECTED_CHAR_KIND} and @code{NEW_LINE} intrinsic functions. @item Support for binary, octal and hexadecimal (BOZ) constants in the intrinsic functions @code{INT}, @code{REAL}, @code{CMPLX} and @code{DBLE}. @item Support for namelist variables with allocatable and pointer attribute and nonconstant length type parameter. @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. @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 Minor I/O features: Rounding during formatted output, using of a decimal comma instead of a decimal point, setting whether a plus sign should appear for positive numbers. On systems where @code{strtod} honours the rounding mode, the rounding mode is also supported for input. @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}, and @code{OPENACC}. @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{https://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 Submodules are supported. It should noted that @code{MODULEs} do not produce the smod file needed by the descendent @code{SUBMODULEs} unless they contain at least one @code{MODULE PROCEDURE} interface. The reason for this is that @code{SUBMODULEs} are useless without @code{MODULE PROCEDUREs}. See http://j3-fortran.org/doc/meeting/207/15-209.txt for a discussion and a draft interpretation. Adopting this interpretation has the advantage that code that does not use submodules does not generate smod files. @item The @code{DO CONCURRENT} construct is supported. @item The @code{BLOCK} construct is supported. @item The @code{STOP} and the new @code{ERROR STOP} statements now support all constant expressions. Both show the signals which were signaling at termination. @item Support for the @code{CONTIGUOUS} attribute. @item Support for @code{ALLOCATE} with @code{MOLD}. @item Support for the @code{IMPURE} attribute for procedures, which allows for @code{ELEMENTAL} procedures without the restrictions of @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 Fortran 2018 status @section Status of Fortran 2018 support @itemize @item ERROR STOP in a PURE procedure An @code{ERROR STOP} statement is permitted in a @code{PURE} procedure. @item IMPLICIT NONE with a spec-list Support the @code{IMPLICIT NONE} statement with an @code{implicit-none-spec-list}. @item Behavior of INQUIRE with the RECL= specifier The behavior of the @code{INQUIRE} statement with the @code{RECL=} specifier now conforms to Fortran 2018. @end itemize @subsection TS 29113 Status (Further Interoperability with C) GNU Fortran supports some of the new features of the Technical Specification (TS) 29113 on Further Interoperability of Fortran with C. The @uref{https://gcc.gnu.org/wiki/TS29113Status, wiki} has some information about the current TS 29113 implementation status. In particular, the following is implemented. See also @ref{Further Interoperability of Fortran with C}. @itemize @item The @code{OPTIONAL} attribute is allowed for dummy arguments of @code{BIND(C) procedures.} @item The @code{RANK} intrinsic is supported. @item GNU Fortran's implementation for variables with @code{ASYNCHRONOUS} attribute is compatible with TS 29113. @item Assumed types (@code{TYPE(*)}). @item Assumed-rank (@code{DIMENSION(..)}). @item ISO_Fortran_binding (now in Fortran 2018 18.4) is implemented such that conversion of the array descriptor for assumed type or assumed rank arrays is done in the library. The include file ISO_Fortran_binding.h is can be found in @code{~prefix/lib/gcc/$target/$version}. @end itemize @subsection TS 18508 Status (Additional Parallel Features) GNU Fortran supports the following new features of the Technical Specification 18508 on Additional Parallel Features in Fortran: @itemize @item The new atomic ADD, CAS, FETCH and ADD/OR/XOR, OR and XOR intrinsics. @item The @code{CO_MIN} and @code{CO_MAX} and @code{SUM} reduction intrinsics. And the @code{CO_BROADCAST} and @code{CO_REDUCE} intrinsic, except that those do not support polymorphic types or types with allocatable, pointer or polymorphic components. @item Events (@code{EVENT POST}, @code{EVENT WAIT}, @code{EVENT_QUERY}) @item Failed images (@code{FAIL IMAGE}, @code{IMAGE_STATUS}, @code{FAILED_IMAGES}, @code{STOPPED_IMAGES}) @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:: * Evaluation of logical expressions:: * MAX and MIN intrinsics with REAL NaN arguments:: * Thread-safety of the runtime library:: * Data consistency and durability:: * Files opened without an explicit ACTION= specifier:: * File operations on symbolic links:: * File format of unformatted sequential files:: * Asynchronous I/O:: @end menu @node KIND Type Parameters @section KIND Type Parameters @cindex kind The @code{KIND} type parameters supported by GNU Fortran for the primitive data types are: @table @code @item INTEGER 1, 2, 4, 8*, 16*, default: 4** @item LOGICAL 1, 2, 4, 8*, 16*, default: 4** @item REAL 4, 8, 10*, 16*, default: 4*** @item COMPLEX 4, 8, 10*, 16*, default: 4*** @item DOUBLE PRECISION 4, 8, 10*, 16*, default: 8*** @item CHARACTER 1, 4, default: 1 @end table @noindent * not available on all systems @* ** unless @option{-fdefault-integer-8} is used @* *** unless @option{-fdefault-real-8} is used (see @ref{Fortran Dialect Options}) @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 @ref{SELECTED_CHAR_KIND}, @ref{SELECTED_INT_KIND} and @ref{SELECTED_REAL_KIND} intrinsics or the @code{INT8}, @code{INT16}, @code{INT32}, @code{INT64}, @code{REAL32}, @code{REAL64}, and @code{REAL128} parameters of the @code{ISO_FORTRAN_ENV} module instead of the concrete values. The available kind parameters can be found in the constant arrays @code{CHARACTER_KINDS}, @code{INTEGER_KINDS}, @code{LOGICAL_KINDS} and @code{REAL_KINDS} in the @ref{ISO_FORTRAN_ENV} module. For C interoperability, the kind parameters of the @ref{ISO_C_BINDING} module should be used. @node Internal representation of LOGICAL variables @section Internal representation of LOGICAL variables @cindex logical, variable representation The Fortran standard does not specify how variables of @code{LOGICAL} type are represented, beyond requiring that @code{LOGICAL} variables of default kind have the same storage size as default @code{INTEGER} and @code{REAL} variables. The GNU Fortran internal representation is as follows. A @code{LOGICAL(KIND=N)} variable is represented as an @code{INTEGER(KIND=N)} variable, however, with only two permissible values: @code{1} for @code{.TRUE.} and @code{0} for @code{.FALSE.}. Any other integer value results in undefined behavior. See also @ref{Argument passing conventions} and @ref{Interoperability with C}. @node Evaluation of logical expressions @section Evaluation of logical expressions The Fortran standard does not require the compiler to evaluate all parts of an expression, if they do not contribute to the final result. For logical expressions with @code{.AND.} or @code{.OR.} operators, in particular, GNU Fortran will optimize out function calls (even to impure functions) if the result of the expression can be established without them. However, since not all compilers do that, and such an optimization can potentially modify the program flow and subsequent results, GNU Fortran throws warnings for such situations with the @option{-Wfunction-elimination} flag. @node MAX and MIN intrinsics with REAL NaN arguments @section MAX and MIN intrinsics with REAL NaN arguments @cindex MAX, MIN, NaN The Fortran standard does not specify what the result of the @code{MAX} and @code{MIN} intrinsics are if one of the arguments is a @code{NaN}. Accordingly, the GNU Fortran compiler does not specify that either, as this allows for faster and more compact code to be generated. If the programmer wishes to take some specific action in case one of the arguments is a @code{NaN}, it is necessary to explicitly test the arguments before calling @code{MAX} or @code{MIN}, e.g. with the @code{IEEE_IS_NAN} function from the intrinsic module @code{IEEE_ARITHMETIC}. @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. For platforms not supporting thread-safe POSIX functions, further functionality might not be thread-safe. For details, please consult the documentation for your operating system. The GNU Fortran runtime library uses various C library functions that depend on the locale, such as @code{strtod} and @code{snprintf}. In order to work correctly in locale-aware programs that set the locale using @code{setlocale}, the locale is reset to the default C'' locale while executing a formatted @code{READ} or @code{WRITE} statement. On targets supporting the POSIX 2008 per-thread locale functions (e.g. @code{newlocale}, @code{uselocale}, @code{freelocale}), these are used and thus the global locale set using @code{setlocale} or the per-thread locales in other threads are not affected. However, on targets lacking this functionality, the global LC_NUMERIC locale is set to C'' during the formatted I/O. Thus, on such targets it's not safe to call @code{setlocale} concurrently from another thread while a Fortran formatted I/O operation is in progress. Also, other threads doing something dependent on the LC_NUMERIC locale might not work correctly if a formatted I/O operation is in progress in another thread. @node Data consistency and durability @section Data consistency and durability @cindex consistency, durability This section contains a brief overview of data and metadata consistency and durability issues when doing I/O. With respect to durability, GNU Fortran makes no effort to ensure that data is committed to stable storage. If this is required, the GNU Fortran programmer can use the intrinsic @code{FNUM} to retrieve the low level file descriptor corresponding to an open Fortran unit. Then, using e.g. the @code{ISO_C_BINDING} feature, one can call the underlying system call to flush dirty data to stable storage, such as @code{fsync} on POSIX, @code{_commit} on MingW, or @code{fcntl(fd, F_FULLSYNC, 0)} on Mac OS X. The following example shows how to call fsync: @smallexample ! Declare the interface for POSIX fsync function interface function fsync (fd) bind(c,name="fsync") use iso_c_binding, only: c_int integer(c_int), value :: fd integer(c_int) :: fsync end function fsync end interface ! Variable declaration integer :: ret ! Opening unit 10 open (10,file="foo") ! ... ! Perform I/O on unit 10 ! ... ! Flush and sync flush(10) ret = fsync(fnum(10)) ! Handle possible error if (ret /= 0) stop "Error calling FSYNC" @end smallexample With respect to consistency, for regular files GNU Fortran uses buffered I/O in order to improve performance. This buffer is flushed automatically when full and in some other situations, e.g. when closing a unit. It can also be explicitly flushed with the @code{FLUSH} statement. Also, the buffering can be turned off with the @code{GFORTRAN_UNBUFFERED_ALL} and @code{GFORTRAN_UNBUFFERED_PRECONNECTED} environment variables. Special files, such as terminals and pipes, are always unbuffered. Sometimes, however, further things may need to be done in order to allow other processes to see data that GNU Fortran has written, as follows. The Windows platform supports a relaxed metadata consistency model, where file metadata is written to the directory lazily. This means that, for instance, the @code{dir} command can show a stale size for a file. One can force a directory metadata update by closing the unit, or by calling @code{_commit} on the file descriptor. Note, though, that @code{_commit} will force all dirty data to stable storage, which is often a very slow operation. The Network File System (NFS) implements a relaxed consistency model called open-to-close consistency. Closing a file forces dirty data and metadata to be flushed to the server, and opening a file forces the client to contact the server in order to revalidate cached data. @code{fsync} will also force a flush of dirty data and metadata to the server. Similar to @code{open} and @code{close}, acquiring and releasing @code{fcntl} file locks, if the server supports them, will also force cache validation and flushing dirty data and metadata. @node Files opened without an explicit ACTION= specifier @section Files opened without an explicit ACTION= specifier @cindex open, action The Fortran standard says that if an @code{OPEN} statement is executed without an explicit @code{ACTION=} specifier, the default value is processor dependent. GNU Fortran behaves as follows: @enumerate @item Attempt to open the file with @code{ACTION='READWRITE'} @item If that fails, try to open with @code{ACTION='READ'} @item If that fails, try to open with @code{ACTION='WRITE'} @item If that fails, generate an error @end enumerate @node File operations on symbolic links @section File operations on symbolic links @cindex file, symbolic link This section documents the behavior of GNU Fortran for file operations on symbolic links, on systems that support them. @itemize @item Results of INQUIRE statements of the inquire by file'' form will relate to the target of the symbolic link. For example, @code{INQUIRE(FILE="foo",EXIST=ex)} will set @var{ex} to @var{.true.} if @var{foo} is a symbolic link pointing to an existing file, and @var{.false.} if @var{foo} points to an non-existing file (dangling'' symbolic link). @item Using the @code{OPEN} statement with a @code{STATUS="NEW"} specifier on a symbolic link will result in an error condition, whether the symbolic link points to an existing target or is dangling. @item If a symbolic link was connected, using the @code{CLOSE} statement with a @code{STATUS="DELETE"} specifier will cause the symbolic link itself to be deleted, not its target. @end itemize @node File format of unformatted sequential files @section File format of unformatted sequential files @cindex file, unformatted sequential @cindex unformatted sequential @cindex sequential, unformatted @cindex record marker @cindex subrecord Unformatted sequential files are stored as logical records using record markers. Each logical record consists of one of more subrecords. Each subrecord consists of a leading record marker, the data written by the user program, and a trailing record marker. The record markers are four-byte integers by default, and eight-byte integers if the @option{-fmax-subrecord-length=8} option (which exists for backwards compability only) is in effect. The representation of the record markers is that of unformatted files given with the @option{-fconvert} option, the @ref{CONVERT specifier} in an open statement or the @ref{GFORTRAN_CONVERT_UNIT} environment variable. The maximum number of bytes of user data in a subrecord is 2147483639 (2 GiB - 9) for a four-byte record marker. This limit can be lowered with the @option{-fmax-subrecord-length} option, altough this is rarely useful. If the length of a logical record exceeds this limit, the data is distributed among several subrecords. The absolute of the number stored in the record markers is the number of bytes of user data in the corresponding subrecord. If the leading record marker of a subrecord contains a negative number, another subrecord follows the current one. If the trailing record marker contains a negative number, then there is a preceding subrecord. In the most simple case, with only one subrecord per logical record, both record markers contain the number of bytes of user data in the record. The format for unformatted sequential data can be duplicated using unformatted stream, as shown in the example program for an unformatted record containing a single subrecord: @smallexample program main use iso_fortran_env, only: int32 implicit none integer(int32) :: i real, dimension(10) :: a, b call random_number(a) open (10,file='test.dat',form='unformatted',access='stream') inquire (iolength=i) a write (10) i, a, i close (10) open (10,file='test.dat',form='unformatted') read (10) b if (all (a == b)) print *,'success!' end program main @end smallexample @node Asynchronous I/O @section Asynchronous I/O @cindex input/output, asynchronous @cindex asynchronous I/O Asynchronous I/O is supported if the program is linked against the POSIX thread library. If that is not the case, all I/O is performed as synchronous. On systems which do not support pthread condition variables, such as AIX, I/O is also performed as synchronous. On some systems, such as Darwin or Solaris, the POSIX thread library is always linked in, so asynchronous I/O is always performed. On other sytems, such as Linux, it is necessary to specify @option{-pthread}, @option{-lpthread} or @option{-fopenmp} during the linking step. @c --------------------------------------------------------------------- @c Extensions @c --------------------------------------------------------------------- @c Maybe this chapter should be merged with the 'Standards' section, @c whenever that is written :-) @node Extensions @chapter Extensions @cindex extensions The two sections below detail the extensions to standard Fortran that are implemented in GNU Fortran, as well as some of the popular or historically important extensions that are not (or not yet) implemented. For the latter case, we explain the alternatives available to GNU Fortran users, including replacement by standard-conforming code or GNU extensions. @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}, @option{-std=f2008}, or @option{-std=f2018} disables both types of extensions, and @option{-std=legacy} allows both without warning. The special compile flag @option{-fdec} enables additional compatibility extensions along with those enabled by @option{-std=legacy}. @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:: * Default widths for F@comma{} G and I format descriptors:: * I/O item lists:: * @code{Q} exponent-letter:: * BOZ literal constants:: * Real array indices:: * Unary operators:: * Implicitly convert LOGICAL and INTEGER values:: * Hollerith constants support:: * Character conversion:: * Cray pointers:: * CONVERT specifier:: * OpenMP:: * OpenACC:: * Argument list functions:: * Read/Write after EOF marker:: * STRUCTURE and RECORD:: * UNION and MAP:: * Type variants for integer intrinsics:: * AUTOMATIC and STATIC attributes:: * Extended math intrinsics:: * Form feed as whitespace:: * TYPE as an alias for PRINT:: * %LOC as an rvalue:: * .XOR. operator:: * Bitwise logical operators:: * Extended I/O specifiers:: * Legacy PARAMETER statements:: * Default exponents:: @end menu @node Old-style kind specifications @subsection Old-style kind specifications @cindex kind, old-style GNU Fortran allows old-style kind specifications in declarations. These look like: @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 When writing a namelist, if no @code{DELIM=} is specified, by default a double quote is used to delimit character strings. If -std=F95, F2003, or F2008, etc, the delim status is set to 'none'. Defaulting to quotes ensures that namelists with character strings can be subsequently read back in accurately. @node X format descriptor without count field @subsection @code{X} format descriptor without count field To support legacy codes, GNU Fortran permits the count field of the @code{X} edit descriptor in @code{FORMAT} statements to be omitted. When omitted, the count is implicitly assumed to be one. @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. A comma with no following format decriptor is permited if the @option{-fdec-blank-format-item} is given on the command line. This is considered non-conforming code and is discouraged. @smallexample PRINT 10, 2, 3 10 FORMAT ('FOO='I1' BAR='I2) print 20, 5, 6 20 FORMAT (I3, I3,) @end smallexample @node Missing period in FORMAT specifications @subsection Missing period in @code{FORMAT} specifications To support legacy codes, GNU Fortran allows missing periods in format specifications if and only if @option{-std=legacy} is given on the command line. This is considered non-conforming code and is discouraged. @smallexample REAL :: value READ(*,10) value 10 FORMAT ('F4') @end smallexample @node Default widths for F@comma{} G and I format descriptors @subsection Default widths for @code{F}, @code{G} and @code{I} format descriptors To support legacy codes, GNU Fortran allows width to be omitted from format specifications if and only if @option{-fdec-format-defaults} is given on the command line. Default widths will be used. This is considered non-conforming code and is discouraged. @smallexample REAL :: value1 INTEGER :: value2 WRITE(*,10) value1, value1, value2 10 FORMAT ('F, G, I') @end smallexample @node I/O item lists @subsection I/O item lists @cindex I/O item lists To support legacy codes, GNU Fortran allows the input item list of the @code{READ} statement, and the output item lists of the @code{WRITE} and @code{PRINT} statements, to start with a comma. @node @code{Q} exponent-letter @subsection @code{Q} exponent-letter @cindex @code{Q} exponent-letter GNU Fortran accepts real literal constants with an exponent-letter of @code{Q}, for example, @code{1.23Q45}. The constant is interpreted as a @code{REAL(16)} entity on targets that support this type. If the target does not support @code{REAL(16)} but has a @code{REAL(10)} type, then the real-literal-constant will be interpreted as a @code{REAL(10)} entity. In the absence of @code{REAL(16)} and @code{REAL(10)}, an error will occur. @node BOZ literal constants @subsection BOZ literal constants @cindex BOZ literal constants Besides decimal constants, Fortran also supports binary (@code{b}), octal (@code{o}) and hexadecimal (@code{z}) integer constants. The syntax is: @samp{prefix quote digits quote}, where the prefix is either @code{b}, @code{o} or @code{z}, quote is either @code{'} or @code{"} and the digits are @code{0} or @code{1} for binary, between @code{0} and @code{7} for octal, and between @code{0} and @code{F} for hexadecimal. (Example: @code{b'01011101'}.) Up to Fortran 95, BOZ literal constants were only allowed to initialize integer variables in DATA statements. Since Fortran 2003 BOZ literal constants are also allowed as actual arguments to the @code{REAL}, @code{DBLE}, @code{INT} and @code{CMPLX} intrinsic functions. The BOZ literal constant is simply a string of bits, which is padded or truncated as needed, during conversion to a numeric type. The Fortran standard states that the treatment of the sign bit is processor dependent. Gfortran interprets the sign bit as a user would expect. As a deprecated extension, GNU Fortran allows hexadecimal BOZ literal constants to be specified using the @code{X} prefix. That the BOZ literal constant can also be specified by adding a suffix to the string, for example, @code{Z'ABC'} and @code{'ABC'X} are equivalent. Additionally, as extension, BOZ literals are permitted in some contexts outside of @code{DATA} and the intrinsic functions listed in the Fortran standard. Use @option{-fallow-invalid-boz} to enable the extension. @node Real array indices @subsection Real array indices @cindex array, indices of type real As an extension, GNU Fortran allows the use of @code{REAL} expressions or variables as array indices. @node Unary operators @subsection Unary operators @cindex operators, unary As an extension, GNU Fortran allows unary plus and unary minus operators to appear as the second operand of binary arithmetic operators without the need for parenthesis. @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, @code{DATA} statements, function and subroutine arguments. A Hollerith constant is written as a string of characters preceded by an integer constant indicating the character count, and the letter @code{H} or @code{h}, and stored in bytewise fashion in a numeric (@code{INTEGER}, @code{REAL}, or @code{COMPLEX}), @code{LOGICAL} or @code{CHARACTER} variable. The constant will be padded with spaces or truncated to fit the size of the variable in which it is stored. Examples of valid uses of Hollerith constants: @smallexample complex*16 x(2) data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/ x(1) = 16HABCDEFGHIJKLMNOP call foo (4h abc) @end smallexample Examples of Hollerith constants: @smallexample integer*4 a a = 0H ! Invalid, at least one character is needed. a = 4HAB12 ! Valid a = 8H12345678 ! Valid, but the Hollerith constant will be truncated. a = 3Hxyz ! Valid, but the Hollerith constant will be padded. @end smallexample In general, Hollerith constants were used to provide a rudimentary facility for handling character strings in early Fortran compilers, prior to the introduction of @code{CHARACTER} variables in Fortran 77; in those cases, the standard-compliant equivalent is to convert the program to use proper character strings. On occasion, there may be a case where the intent is specifically to initialize a numeric variable with a given byte sequence. In these cases, the same result can be obtained by using the @code{TRANSFER} statement, as in this example. @smallexample integer(kind=4) :: a a = transfer ("abcd", a) ! equivalent to: a = 4Habcd @end smallexample The use of the @option{-fdec} option extends support of Hollerith constants to comparisons: @smallexample integer*4 a a = 4hABCD if (a .ne. 4habcd) then write(*,*) "no match" end if @end smallexample Supported types are numeric (@code{INTEGER}, @code{REAL}, or @code{COMPLEX}), and @code{CHARACTER}. @node Character conversion @subsection Character conversion @cindex conversion, to character Allowing character literals to be used in a similar way to Hollerith constants is a non-standard extension. This feature is enabled using -fdec-char-conversions and only applies to character literals of @code{kind=1}. Character literals can be used in @code{DATA} statements and assignments with numeric (@code{INTEGER}, @code{REAL}, or @code{COMPLEX}) or @code{LOGICAL} variables. Like Hollerith constants they are copied byte-wise fashion. The constant will be padded with spaces or truncated to fit the size of the variable in which it is stored. Examples: @smallexample integer*4 x data x / 'abcd' / x = 'A' ! Will be padded. x = 'ab1234' ! Will be truncated. @end smallexample @node Cray pointers @subsection Cray pointers @cindex pointer, Cray Cray pointers are part of a non-standard extension that provides a C-like pointer in Fortran. This is accomplished through a pair of variables: an integer "pointer" that holds a memory address, and a "pointee" that is used to dereference the pointer. Pointer/pointee pairs are declared in statements of the form: @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. If an assumed-size array is permitted within the scoping unit, a pointee can be an assumed-size array. That is, the last dimension may be left unspecified by using a @code{*} in place of a value. A pointee cannot be an assumed shape array. No space is allocated for the pointee. The pointee may have its type declared before or after the pointer statement, and its array specification (if any) may be declared before, during, or after the pointer statement. The pointer may be declared as an integer prior to the pointer statement. However, some machines have default integer sizes that are different than the size of a pointer, and so the following code is not portable: @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://openmp.org/wp/openmp-specifications/, OpenMP Application Program Interface v4.5}. To enable the processing of the OpenMP directive @code{!$omp} in free-form source code; the @code{c$omp}, @code{*$omp} and @code{!$omp} directives in fixed form; the @code{!$} conditional compilation sentinels in free form; and the @code{c$}, @code{*$} and @code{!$} sentinels in fixed form, @command{gfortran} needs to be invoked with the @option{-fopenmp}. This also arranges for automatic linking of the GNU Offloading and Multi Processing Runtime Library @ref{Top,,libgomp,libgomp,GNU Offloading and Multi Processing Runtime Library}. The OpenMP Fortran runtime library routines are provided both in a form of a Fortran 90 module named @code{omp_lib} and in a form of a Fortran @code{include} file named @file{omp_lib.h}. An example of a parallelized loop taken from Appendix A.1 of the OpenMP Application Program Interface v2.5: @smallexample SUBROUTINE A1(N, A, B) INTEGER I, N REAL B(N), A(N) !$OMP PARALLEL DO !I is private by default DO I=2,N B(I) = (A(I) + A(I-1)) / 2.0 ENDDO !$OMP END PARALLEL DO END SUBROUTINE A1 @end smallexample Please note: @itemize @item @option{-fopenmp} implies @option{-frecursive}, i.e., all local arrays will be allocated on the stack. When porting existing code to OpenMP, this may lead to surprising results, especially to segmentation faults if the stacksize is limited. @item On glibc-based systems, OpenMP enabled applications cannot be statically linked due to limitations of the underlying pthreads-implementation. It might be possible to get a working solution if @command{-Wl,--whole-archive -lpthread -Wl,--no-whole-archive} is added to the command line. However, this is not supported by @command{gcc} and thus not recommended. @end itemize @node OpenACC @subsection OpenACC @cindex OpenACC OpenACC is an application programming interface (API) that supports offloading of code to accelerator devices. It consists of a set of compiler directives, library routines, and environment variables that influence run-time behavior. GNU Fortran strives to be compatible to the @uref{http://www.openacc.org/, OpenACC Application Programming Interface v2.6}. To enable the processing of the OpenACC directive @code{!$acc} in free-form source code; the @code{c$acc}, @code{*$acc} and @code{!$acc} directives in fixed form; the @code{!$} conditional compilation sentinels in free form; and the @code{c$}, @code{*$} and @code{!} sentinels in fixed form, @command{gfortran} needs to be invoked with the @option{-fopenacc}. This also arranges for automatic linking of the GNU Offloading and Multi Processing Runtime Library @ref{Top,,libgomp,libgomp,GNU Offloading and Multi Processing Runtime Library}. The OpenACC Fortran runtime library routines are provided both in a form of a Fortran 90 module named @code{openacc} and in a form of a Fortran @code{include} file named @file{openacc_lib.h}. @node Argument list functions @subsection Argument list functions @code{%VAL}, @code{%REF} and @code{%LOC} @cindex argument list functions @cindex @code{%VAL} @cindex @code{%REF} @cindex @code{%LOC} GNU Fortran supports argument list functions @code{%VAL}, @code{%REF} and @code{%LOC} statements, for backward compatibility with g77. It is recommended that these should be used only for code that is accessing facilities outside of GNU Fortran, such as operating system or windowing facilities. It is best to constrain such uses to isolated portions of a program--portions that deal specifically and exclusively with low-level, system-dependent facilities. Such portions might well provide a portable interface for use by the program as a whole, but are themselves not portable, and should be thoroughly tested each time they are rebuilt using a new compiler or version of a compiler. @code{%VAL} passes a scalar argument by value, @code{%REF} passes it by reference and @code{%LOC} passes its memory location. Since gfortran already passes scalar arguments by reference, @code{%REF} is in effect a do-nothing. @code{%LOC} has the same effect as a Fortran pointer. An example of passing an argument by value to a C subroutine foo.: @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{https://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 Read/Write after EOF marker @subsection Read/Write after EOF marker @cindex @code{EOF} @cindex @code{BACKSPACE} @cindex @code{REWIND} Some legacy codes rely on allowing @code{READ} or @code{WRITE} after the EOF file marker in order to find the end of a file. GNU Fortran normally rejects these codes with a run-time error message and suggests the user consider @code{BACKSPACE} or @code{REWIND} to properly position the file before the EOF marker. As an extension, the run-time error may be disabled using -std=legacy. @node STRUCTURE and RECORD @subsection @code{STRUCTURE} and @code{RECORD} @cindex @code{STRUCTURE} @cindex @code{RECORD} Record structures are a pre-Fortran-90 vendor extension to create user-defined aggregate data types. Support for record structures in GNU Fortran can be enabled with the @option{-fdec-structure} compile flag. If you have a choice, you should instead use Fortran 90's derived types'', which have a different syntax. In many cases, record structures can easily be converted to derived types. To convert, replace @code{STRUCTURE /}@var{structure-name}@code{/} by @code{TYPE} @var{type-name}. Additionally, replace @code{RECORD /}@var{structure-name}@code{/} by @code{TYPE(}@var{type-name}@code{)}. Finally, in the component access, replace the period (@code{.}) by the percent sign (@code{%}). Here is an example of code using the non portable record structure syntax: @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 do not change store_catalog(12) = pear print *, store_catalog(12) @end example @noindent GNU Fortran implements STRUCTURES like derived types with the following rules and exceptions: @itemize @bullet @item Structures act like derived types with the @code{SEQUENCE} attribute. Otherwise they may contain no specifiers. @item Structures may contain a special field with the name @code{%FILL}. This will create an anonymous component which cannot be accessed but occupies space just as if a component of the same type was declared in its place, useful for alignment purposes. As an example, the following structure will consist of at least sixteen bytes: @smallexample structure /padded/ character(4) start character(8) %FILL character(4) end end structure @end smallexample @item Structures may share names with other symbols. For example, the following is invalid for derived types, but valid for structures: @smallexample structure /header/ ! ... end structure record /header/ header @end smallexample @item Structure types may be declared nested within another parent structure. The syntax is: @smallexample structure /type-name/ ... structure [//] ... @end smallexample The type name may be ommitted, in which case the structure type itself is anonymous, and other structures of the same type cannot be instantiated. The following shows some examples: @example structure /appointment/ ! nested structure definition: app_time is an array of two 'time' structure /time/ app_time (2) integer(1) hour, minute end structure character(10) memo end structure ! The 'time' structure is still usable record /time/ now now = time(5, 30) ... structure /appointment/ ! anonymous nested structure definition structure start, end integer(1) hour, minute end structure character(10) memo end structure @end example @item Structures may contain @code{UNION} blocks. For more detail see the section on @ref{UNION and MAP}. @item Structures support old-style initialization of components, like those described in @ref{Old-style variable initialization}. For array initializers, an initializer may contain a repeat specification of the form @code{ * }. The value of the integer indicates the number of times to repeat the constant initializer when expanding the initializer list. @end itemize @node UNION and MAP @subsection @code{UNION} and @code{MAP} @cindex @code{UNION} @cindex @code{MAP} Unions are an old vendor extension which were commonly used with the non-standard @ref{STRUCTURE and RECORD} extensions. Use of @code{UNION} and @code{MAP} is automatically enabled with @option{-fdec-structure}. A @code{UNION} declaration occurs within a structure; within the definition of each union is a number of @code{MAP} blocks. Each @code{MAP} shares storage with its sibling maps (in the same union), and the size of the union is the size of the largest map within it, just as with unions in C. The major difference is that component references do not indicate which union or map the component is in (the compiler gets to figure that out). Here is a small example: @smallexample structure /myunion/ union map character(2) w0, w1, w2 end map map character(6) long end map end union end structure record /myunion/ rec ! After this assignment... rec.long = 'hello!' ! The following is true: ! rec.w0 === 'he' ! rec.w1 === 'll' ! rec.w2 === 'o!' @end smallexample The two maps share memory, and the size of the union is ultimately six bytes: @example 0 1 2 3 4 5 6 Byte offset ------------------------------- | | | | | | | ------------------------------- ^ W0 ^ W1 ^ W2 ^ \-------/ \-------/ \-------/ ^ LONG ^ \---------------------------/ @end example Following is an example mirroring the layout of an Intel x86_64 register: @example structure /reg/ union ! U0 ! rax map character(16) rx end map map character(8) rh ! rah union ! U1 map character(8) rl ! ral end map map character(8) ex ! eax end map map character(4) eh ! eah union ! U2 map character(4) el ! eal end map map character(4) x ! ax end map map character(2) h ! ah character(2) l ! al end map end union end map end union end map end union end structure record /reg/ a ! After this assignment... a.rx = 'AAAAAAAA.BBB.C.D' ! The following is true: a.rx === 'AAAAAAAA.BBB.C.D' a.rh === 'AAAAAAAA' a.rl === '.BBB.C.D' a.ex === '.BBB.C.D' a.eh === '.BBB' a.el === '.C.D' a.x === '.C.D' a.h === '.C' a.l === '.D' @end example @node Type variants for integer intrinsics @subsection Type variants for integer intrinsics @cindex intrinsics, integer Similar to the D/C prefixes to real functions to specify the input/output types, GNU Fortran offers B/I/J/K prefixes to integer functions for compatibility with DEC programs. The types implied by each are: @example @code{B} - @code{INTEGER(kind=1)} @code{I} - @code{INTEGER(kind=2)} @code{J} - @code{INTEGER(kind=4)} @code{K} - @code{INTEGER(kind=8)} @end example GNU Fortran supports these with the flag @option{-fdec-intrinsic-ints}. Intrinsics for which prefixed versions are available and in what form are noted in @ref{Intrinsic Procedures}. The complete list of supported intrinsics is here: @multitable @columnfractions .2 .2 .2 .2 .2 @headitem Intrinsic @tab B @tab I @tab J @tab K @item @code{@ref{ABS}} @tab @code{BABS} @tab @code{IIABS} @tab @code{JIABS} @tab @code{KIABS} @item @code{@ref{BTEST}} @tab @code{BBTEST} @tab @code{BITEST} @tab @code{BJTEST} @tab @code{BKTEST} @item @code{@ref{IAND}} @tab @code{BIAND} @tab @code{IIAND} @tab @code{JIAND} @tab @code{KIAND} @item @code{@ref{IBCLR}} @tab @code{BBCLR} @tab @code{IIBCLR} @tab @code{JIBCLR} @tab @code{KIBCLR} @item @code{@ref{IBITS}} @tab @code{BBITS} @tab @code{IIBITS} @tab @code{JIBITS} @tab @code{KIBITS} @item @code{@ref{IBSET}} @tab @code{BBSET} @tab @code{IIBSET} @tab @code{JIBSET} @tab @code{KIBSET} @item @code{@ref{IEOR}} @tab @code{BIEOR} @tab @code{IIEOR} @tab @code{JIEOR} @tab @code{KIEOR} @item @code{@ref{IOR}} @tab @code{BIOR} @tab @code{IIOR} @tab @code{JIOR} @tab @code{KIOR} @item @code{@ref{ISHFT}} @tab @code{BSHFT} @tab @code{IISHFT} @tab @code{JISHFT} @tab @code{KISHFT} @item @code{@ref{ISHFTC}} @tab @code{BSHFTC} @tab @code{IISHFTC} @tab @code{JISHFTC} @tab @code{KISHFTC} @item @code{@ref{MOD}} @tab @code{BMOD} @tab @code{IMOD} @tab @code{JMOD} @tab @code{KMOD} @item @code{@ref{NOT}} @tab @code{BNOT} @tab @code{INOT} @tab @code{JNOT} @tab @code{KNOT} @item @code{@ref{REAL}} @tab @code{--} @tab @code{FLOATI} @tab @code{FLOATJ} @tab @code{FLOATK} @end multitable @node AUTOMATIC and STATIC attributes @subsection @code{AUTOMATIC} and @code{STATIC} attributes @cindex variable attributes @cindex @code{AUTOMATIC} @cindex @code{STATIC} With @option{-fdec-static} GNU Fortran supports the DEC extended attributes @code{STATIC} and @code{AUTOMATIC} to provide explicit specification of entity storage. These follow the syntax of the Fortran standard @code{SAVE} attribute. @code{STATIC} is exactly equivalent to @code{SAVE}, and specifies that an entity should be allocated in static memory. As an example, @code{STATIC} local variables will retain their values across multiple calls to a function. Entities marked @code{AUTOMATIC} will be stack automatic whenever possible. @code{AUTOMATIC} is the default for local variables smaller than @option{-fmax-stack-var-size}, unless @option{-fno-automatic} is given. This attribute overrides @option{-fno-automatic}, @option{-fmax-stack-var-size}, and blanket @code{SAVE} statements. Examples: @example subroutine f integer, automatic :: i ! automatic variable integer x, y ! static variables save ... endsubroutine @end example @example subroutine f integer a, b, c, x, y, z static :: x save y automatic z, c ! a, b, c, and z are automatic ! x and y are static endsubroutine @end example @example ! Compiled with -fno-automatic subroutine f integer a, b, c, d automatic :: a ! a is automatic; b, c, and d are static endsubroutine @end example @node Extended math intrinsics @subsection Extended math intrinsics @cindex intrinsics, math @cindex intrinsics, trigonometric functions GNU Fortran supports an extended list of mathematical intrinsics with the compile flag @option{-fdec-math} for compatability with legacy code. These intrinsics are described fully in @ref{Intrinsic Procedures} where it is noted that they are extensions and should be avoided whenever possible. Specifically, @option{-fdec-math} enables the @ref{COTAN} intrinsic, and trigonometric intrinsics which accept or produce values in degrees instead of radians. Here is a summary of the new intrinsics: @multitable @columnfractions .5 .5 @headitem Radians @tab Degrees @item @code{@ref{ACOS}} @tab @code{@ref{ACOSD}}* @item @code{@ref{ASIN}} @tab @code{@ref{ASIND}}* @item @code{@ref{ATAN}} @tab @code{@ref{ATAND}}* @item @code{@ref{ATAN2}} @tab @code{@ref{ATAN2D}}* @item @code{@ref{COS}} @tab @code{@ref{COSD}}* @item @code{@ref{COTAN}}* @tab @code{@ref{COTAND}}* @item @code{@ref{SIN}} @tab @code{@ref{SIND}}* @item @code{@ref{TAN}} @tab @code{@ref{TAND}}* @end multitable * Enabled with @option{-fdec-math}. For advanced users, it may be important to know the implementation of these functions. They are simply wrappers around the standard radian functions, which have more accurate builtin versions. These functions convert their arguments (or results) to degrees (or radians) by taking the value modulus 360 (or 2*pi) and then multiplying it by a constant radian-to-degree (or degree-to-radian) factor, as appropriate. The factor is computed at compile-time as 180/pi (or pi/180). @node Form feed as whitespace @subsection Form feed as whitespace @cindex form feed whitespace Historically, legacy compilers allowed insertion of form feed characters ('\f', ASCII 0xC) at the beginning of lines for formatted output to line printers, though the Fortran standard does not mention this. GNU Fortran supports the interpretation of form feed characters in source as whitespace for compatibility. @node TYPE as an alias for PRINT @subsection TYPE as an alias for PRINT @cindex type alias print For compatibility, GNU Fortran will interpret @code{TYPE} statements as @code{PRINT} statements with the flag @option{-fdec}. With this flag asserted, the following two examples are equivalent: @smallexample TYPE *, 'hello world' @end smallexample @smallexample PRINT *, 'hello world' @end smallexample @node %LOC as an rvalue @subsection %LOC as an rvalue @cindex LOC Normally @code{%LOC} is allowed only in parameter lists. However the intrinsic function @code{LOC} does the same thing, and is usable as the right-hand-side of assignments. For compatibility, GNU Fortran supports the use of @code{%LOC} as an alias for the builtin @code{LOC} with @option{-std=legacy}. With this feature enabled the following two examples are equivalent: @smallexample integer :: i, l l = %loc(i) call sub(l) @end smallexample @smallexample integer :: i call sub(%loc(i)) @end smallexample @node .XOR. operator @subsection .XOR. operator @cindex operators, xor GNU Fortran supports @code{.XOR.} as a logical operator with @code{-std=legacy} for compatibility with legacy code. @code{.XOR.} is equivalent to @code{.NEQV.}. That is, the output is true if and only if the inputs differ. @node Bitwise logical operators @subsection Bitwise logical operators @cindex logical, bitwise With @option{-fdec}, GNU Fortran relaxes the type constraints on logical operators to allow integer operands, and performs the corresponding bitwise operation instead. This flag is for compatibility only, and should be avoided in new code. Consider: @smallexample INTEGER :: i, j i = z'33' j = z'cc' print *, i .AND. j @end smallexample In this example, compiled with @option{-fdec}, GNU Fortran will replace the @code{.AND.} operation with a call to the intrinsic @code{@ref{IAND}} function, yielding the bitwise-and of @code{i} and @code{j}. Note that this conversion will occur if at least one operand is of integral type. As a result, a logical operand will be converted to an integer when the other operand is an integer in a logical operation. In this case, @code{.TRUE.} is converted to @code{1} and @code{.FALSE.} to @code{0}. Here is the mapping of logical operator to bitwise intrinsic used with @option{-fdec}: @multitable @columnfractions .25 .25 .5 @headitem Operator @tab Intrinsic @tab Bitwise operation @item @code{.NOT.} @tab @code{@ref{NOT}} @tab complement @item @code{.AND.} @tab @code{@ref{IAND}} @tab intersection @item @code{.OR.} @tab @code{@ref{IOR}} @tab union @item @code{.NEQV.} @tab @code{@ref{IEOR}} @tab exclusive or @item @code{.EQV.} @tab @code{@ref{NOT}(@ref{IEOR})} @tab complement of exclusive or @end multitable @node Extended I/O specifiers @subsection Extended I/O specifiers @cindex @code{CARRIAGECONTROL} @cindex @code{READONLY} @cindex @code{SHARE} @cindex @code{SHARED} @cindex @code{NOSHARED} @cindex I/O specifiers GNU Fortran supports the additional legacy I/O specifiers @code{CARRIAGECONTROL}, @code{READONLY}, and @code{SHARE} with the compile flag @option{-fdec}, for compatibility. @table @code @item CARRIAGECONTROL The @code{CARRIAGECONTROL} specifier allows a user to control line termination settings between output records for an I/O unit. The specifier has no meaning for readonly files. When @code{CARRAIGECONTROL} is specified upon opening a unit for formatted writing, the exact @code{CARRIAGECONTROL} setting determines what characters to write between output records. The syntax is: @smallexample OPEN(..., CARRIAGECONTROL=cc) @end smallexample Where @emph{cc} is a character expression that evaluates to one of the following values: @multitable @columnfractions .2 .8 @item @code{'LIST'} @tab One line feed between records (default) @item @code{'FORTRAN'} @tab Legacy interpretation of the first character (see below) @item @code{'NONE'} @tab No separator between records @end multitable With @code{CARRIAGECONTROL='FORTRAN'}, when a record is written, the first character of the input record is not written, and instead determines the output record separator as follows: @multitable @columnfractions .3 .3 .4 @headitem Leading character @tab Meaning @tab Output separating character(s) @item @code{'+'} @tab Overprinting @tab Carriage return only @item @code{'-'} @tab New line @tab Line feed and carriage return @item @code{'0'} @tab Skip line @tab Two line feeds and carriage return @item @code{'1'} @tab New page @tab Form feed and carriage return @item @code{''} @tab Prompting @tab Line feed (no carriage return) @item @code{CHAR(0)} @tab Overprinting (no advance) @tab None @end multitable @item READONLY The @code{READONLY} specifier may be given upon opening a unit, and is equivalent to specifying @code{ACTION='READ'}, except that the file may not be deleted on close (i.e. @code{CLOSE} with @code{STATUS="DELETE"}). The syntax is: @smallexample @code{OPEN(..., READONLY)} @end smallexample @item SHARE The @code{SHARE} specifier allows system-level locking on a unit upon opening it for controlled access from multiple processes/threads. The @code{SHARE} specifier has several forms: @smallexample OPEN(..., SHARE=sh) OPEN(..., SHARED) OPEN(..., NOSHARED) @end smallexample Where @emph{sh} in the first form is a character expression that evaluates to a value as seen in the table below. The latter two forms are aliases for particular values of @emph{sh}: @multitable @columnfractions .3 .3 .4 @headitem Explicit form @tab Short form @tab Meaning @item @code{SHARE='DENYRW'} @tab @code{NOSHARED} @tab Exclusive (write) lock @item @code{SHARE='DENYNONE'} @tab @code{SHARED} @tab Shared (read) lock @end multitable In general only one process may hold an exclusive (write) lock for a given file at a time, whereas many processes may hold shared (read) locks for the same file. The behavior of locking may vary with your operating system. On POSIX systems, locking is implemented with @code{fcntl}. Consult your corresponding operating system's manual pages for further details. Locking via @code{SHARE=} is not supported on other systems. @end table @node Legacy PARAMETER statements @subsection Legacy PARAMETER statements @cindex PARAMETER For compatibility, GNU Fortran supports legacy PARAMETER statements without parentheses with @option{-std=legacy}. A warning is emitted if used with @option{-std=gnu}, and an error is acknowledged with a real Fortran standard flag (@option{-std=f95}, etc...). These statements take the following form: @smallexample implicit real (E) parameter e = 2.718282 real c parameter c = 3.0e8 @end smallexample @node Default exponents @subsection Default exponents @cindex exponent For compatibility, GNU Fortran supports a default exponent of zero in real constants with @option{-fdec}. For example, @code{9e} would be interpreted as @code{9e0}, rather than an error. @node Extensions not implemented in GNU Fortran @section Extensions not implemented in GNU Fortran @cindex extensions, not implemented The long history of the Fortran language, its wide use and broad userbase, the large number of different compiler vendors and the lack of some features crucial to users in the first standards have lead to the existence of a number of important extensions to the language. While some of the most useful or popular extensions are supported by the GNU Fortran compiler, not all existing extensions are supported. This section aims at listing these extensions and offering advice on how best make code that uses them running with the GNU Fortran compiler. @c More can be found here: @c -- https://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 * ENCODE and DECODE statements:: * Variable FORMAT expressions:: @c * TYPE and ACCEPT I/O Statements:: @c * DEFAULTFILE, DISPOSE and RECORDTYPE I/O specifiers:: @c * Omitted arguments in procedure call:: * Alternate complex function syntax:: * Volatile COMMON blocks:: * OPEN( ... NAME=):: * Q edit descriptor:: @end menu @node ENCODE and DECODE statements @subsection @code{ENCODE} and @code{DECODE} statements @cindex @code{ENCODE} @cindex @code{DECODE} GNU Fortran does not support the @code{ENCODE} and @code{DECODE} statements. These statements are best replaced by @code{READ} and @code{WRITE} statements involving internal files (@code{CHARACTER} variables and arrays), which have been part of the Fortran standard since Fortran 77. For example, replace a code fragment like @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. @node Volatile COMMON blocks @subsection Volatile @code{COMMON} blocks @cindex @code{VOLATILE} @cindex @code{COMMON} Some Fortran compilers, including @command{g77}, let the user declare @code{COMMON} with the @code{VOLATILE} attribute. This is invalid standard Fortran syntax and is not supported by @command{gfortran}. Note that @command{gfortran} accepts @code{VOLATILE} variables in @code{COMMON} blocks since revision 4.3. @node OPEN( ... NAME=) @subsection @code{OPEN( ... NAME=)} @cindex @code{NAME} Some Fortran compilers, including @command{g77}, let the user declare @code{OPEN( ... NAME=)}. This is invalid standard Fortran syntax and is not supported by @command{gfortran}. @code{OPEN( ... NAME=)} should be replaced with @code{OPEN( ... FILE=)}. @node Q edit descriptor @subsection @code{Q} edit descriptor @cindex @code{Q} edit descriptor Some Fortran compilers provide the @code{Q} edit descriptor, which transfers the number of characters left within an input record into an integer variable. A direct replacement of the @code{Q} edit descriptor is not available in @command{gfortran}. How to replicate its functionality using standard-conforming code depends on what the intent of the original code is. Options to replace @code{Q} may be to read the whole line into a character variable and then counting the number of non-blank characters left using @code{LEN_TRIM}. Another method may be to use formatted stream, read the data up to the position where the @code{Q} descriptor occurred, use @code{INQUIRE} to get the file position, count the characters up to the next @code{NEW_LINE} and then start reading from the position marked previously. @c --------------------------------------------------------------------- @c --------------------------------------------------------------------- @c Mixed-Language Programming @c --------------------------------------------------------------------- @node Mixed-Language Programming @chapter Mixed-Language Programming @cindex Interoperability @cindex Mixed-language programming @menu * Interoperability with C:: * GNU Fortran Compiler Directives:: * Non-Fortran Main Program:: * Naming and argument-passing conventions:: @end menu This chapter is about mixed-language interoperability, but also applies if one links Fortran code compiled by different compilers. In most cases, use of the C Binding features of the Fortran 2003 standard is sufficient, and their use is highly recommended. @node Interoperability with C @section Interoperability with C @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}. For logical types, please note that the Fortran standard only guarantees interoperability between C99's @code{_Bool} and Fortran's @code{C_Bool}-kind logicals and C99 defines that @code{true} has the value 1 and @code{false} the value 0. Using any other integer value with GNU Fortran's @code{LOGICAL} (with any kind parameter) gives an undefined result. (Passing other integer values than 0 and 1 to GCC's @code{_Bool} is also undefined, unless the integer is explicitly or implicitly casted to @code{_Bool}.) @node Derived Types and struct @subsection Derived Types and struct For compatibility of derived types with @code{struct}, one needs to use the @code{BIND(C)} attribute in the type declaration. For instance, the following type declaration @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 components are irrelevant for interoperability. As there exist no direct Fortran equivalents, neither unions nor structs with bit field or variable-length array members are interoperable. @node Interoperable Global Variables @subsection Interoperable Global Variables Variables can be made accessible from C using the C binding attribute, optionally together with specifying a binding name. Those variables have to be declared in the declaration part of a @code{MODULE}, be of interoperable type, and have neither the @code{pointer} nor the @code{allocatable} attribute. @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 in Fortran 2003 and 2008 only explicit size and assumed-size arrays are supported but not assumed-shape or deferred-shape (i.e. allocatable or pointer) arrays. However, those are allowed since the Technical Specification 29113, see @ref{Further Interoperability of Fortran with C} To pass a variable by value, use the @code{VALUE} attribute. Thus, the following C prototype @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 some applications, using an assumed type (@code{TYPE(*)}) can be an alternative to a C pointer; see @ref{Further Interoperability of Fortran with C}. For example, @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 us consider two examples of actually passing a procedure pointer from C to Fortran and vice versa. Note that these examples are also very similar to passing ordinary pointers between both languages. First, consider this code in C: @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 The Technical Specification ISO/IEC TS 29113:2012 on further interoperability of Fortran with C extends the interoperability support of Fortran 2003 and Fortran 2008. Besides removing some restrictions and constraints, it adds assumed-type (@code{TYPE(*)}) and assumed-rank (@code{dimension}) variables and allows for interoperability of assumed-shape, assumed-rank and deferred-shape arrays, including allocatables and pointers. Note: Currently, GNU Fortran does not use internally the array descriptor (dope vector) as specified in the Technical Specification, but uses an array descriptor with different fields. Assumed type and assumed rank formal arguments are converted in the library to the specified form. The ISO_Fortran_binding API functions (also Fortran 2018 18.4) are implemented in libgfortran. Alternatively, the Chasm Language Interoperability Tools, @url{http://chasm-interop.sourceforge.net/}, provide an interface to GNU Fortran's array descriptor. The Technical Specification adds the following new features, which are supported by GNU Fortran: @itemize @bullet @item The @code{ASYNCHRONOUS} attribute has been clarified and extended to allow its use with asynchronous communication in user-provided libraries such as in implementations of the Message Passing Interface specification. @item Many constraints have been relaxed, in particular for the @code{C_LOC} and @code{C_F_POINTER} intrinsics. @item The @code{OPTIONAL} attribute is now allowed for dummy arguments; an absent argument matches a @code{NULL} pointer. @item Assumed types (@code{TYPE(*)}) have been added, which may only be used for dummy arguments. They are unlimited polymorphic but contrary to @code{CLASS(*)} they do not contain any type information, similar to C's @code{void *} pointers. Expressions of any type and kind can be passed; thus, it can be used as replacement for @code{TYPE(C_PTR)}, avoiding the use of @code{C_LOC} in the caller. Note, however, that @code{TYPE(*)} only accepts scalar arguments, unless the @code{DIMENSION} is explicitly specified. As @code{DIMENSION(*)} only supports array (including array elements) but no scalars, it is not a full replacement for @code{C_LOC}. On the other hand, assumed-type assumed-rank dummy arguments (@code{TYPE(*), DIMENSION(..)}) allow for both scalars and arrays, but require special code on the callee side to handle the array descriptor. @item Assumed-rank arrays (@code{DIMENSION(..)}) as dummy argument allow that scalars and arrays of any rank can be passed as actual argument. As the Technical Specification does not provide for direct means to operate with them, they have to be used either from the C side or be converted using @code{C_LOC} and @code{C_F_POINTER} to scalars or arrays of a specific rank. The rank can be determined using the @code{RANK} intrinisic. @end itemize Currently unimplemented: @itemize @bullet @item GNU Fortran always uses an array descriptor, which does not match the one of the Technical Specification. The @code{ISO_Fortran_binding.h} header file and the C functions it specifies are not available. @item Using assumed-shape, assumed-rank and deferred-shape arrays in @code{BIND(C)} procedures is not fully supported. In particular, C interoperable strings of other length than one are not supported as this requires the new array descriptor. @end itemize @node GNU Fortran Compiler Directives @section GNU Fortran Compiler Directives @menu * ATTRIBUTES directive:: * UNROLL directive:: * BUILTIN directive:: * IVDEP directive:: * VECTOR directive:: * NOVECTOR directive:: @end menu @node ATTRIBUTES directive @subsection ATTRIBUTES directive The Fortran standard describes how a conforming program shall behave; however, the exact implementation is not standardized. In order to allow the user to choose specific implementation details, compiler directives can be used to set attributes of variables and procedures which are not part of the standard. Whether a given attribute is supported and its exact effects depend on both the operating system and on the processor; see @ref{Top,,C Extensions,gcc,Using the GNU Compiler Collection (GCC)} for details. For procedures and procedure pointers, the following attributes can be used to change the calling convention: @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 For dummy arguments, the @code{NO_ARG_CHECK} attribute can be used; in other compilers, it is also known as @code{IGNORE_TKR}. For dummy arguments with this attribute actual arguments of any type and kind (similar to @code{TYPE(*)}), scalars and arrays of any rank (no equivalent in Fortran standard) are accepted. As with @code{TYPE(*)}, the argument is unlimited polymorphic and no type information is available. Additionally, the argument may only be passed to dummy arguments with the @code{NO_ARG_CHECK} attribute and as argument to the @code{PRESENT} intrinsic function and to @code{C_LOC} of the @code{ISO_C_BINDING} module. Variables with @code{NO_ARG_CHECK} attribute shall be of assumed-type (@code{TYPE(*)}; recommended) or of type @code{INTEGER}, @code{LOGICAL}, @code{REAL} or @code{COMPLEX}. They shall not have the @code{ALLOCATE}, @code{CODIMENSION}, @code{INTENT(OUT)}, @code{POINTER} or @code{VALUE} attribute; furthermore, they shall be either scalar or of assumed-size (@code{dimension(*)}). As @code{TYPE(*)}, the @code{NO_ARG_CHECK} attribute requires an explicit interface. @itemize @item @code{NO_ARG_CHECK} -- disable the type, kind and rank checking @item @code{DEPRECATED} -- print a warning when using a such-tagged deprecated procedure, variable or parameter; the warning can be suppressed with @option{-Wno-deprecated-declarations}. @end itemize The attributes are specified using the syntax @code{!GCC$ATTRIBUTES} @var{attribute-list} @code{::} @var{variable-list} where in free-form source code only whitespace is allowed before @code{!GCC$} and in fixed-form source code @code{!GCC$}, @code{cGCC$} or @code{*GCC$} shall start in the first column. For procedures, the compiler directives shall be placed into the body of the procedure; for variables and procedure pointers, they shall be in the same declaration part as the variable or procedure pointer. @node UNROLL directive @subsection UNROLL directive The syntax of the directive is @code{!GCC$ unroll N} You can use this directive to control how many times a loop should be unrolled. It must be placed immediately before a @code{DO} loop and applies only to the loop that follows. N is an integer constant specifying the unrolling factor. The values of 0 and 1 block any unrolling of the loop. @node BUILTIN directive @subsection BUILTIN directive The syntax of the directive is @code{!GCC$BUILTIN (B) attributes simd FLAGS IF('target')} You can use this directive to define which middle-end built-ins provide vector implementations. @code{B} is name of the middle-end built-in. @code{FLAGS} are optional and must be either "(inbranch)" or "(notinbranch)". @code{IF} statement is optional and is used to filter multilib ABIs for the built-in that should be vectorized. Example usage: @smallexample !GCC$ builtin (sinf) attributes simd (notinbranch) if('x86_64') @end smallexample