The GNU Fortran Compiler

Short Table of Contents

Table of Contents

Introduction

This manual documents the use of gfortran, the GNU Fortran compiler. You can find in this manual how to invoke gfortran, as well as its features and incompatibilities.

Part I: Invoking GNU Fortran

Part II: Language Reference

1 Introduction

The GNU Fortran compiler front end was designed initially as a free replacement for, or alternative to, the unix f95 command; gfortran is the command you’ll use to invoke the compiler.

1.1 About GNU Fortran

The GNU Fortran compiler supports the Fortran 77, 90 and 95 standards completely, parts of the Fortran 2003 and Fortran 2008 standards, and several vendor extensions. The development goal is to provide the following features:

• Read a user’s program, stored in a file and containing instructions written in Fortran 77, Fortran 90, Fortran 95, Fortran 2003 or Fortran 2008. This file contains source code.
• Translate the user’s program into instructions a computer can carry out more quickly than it takes to translate the instructions in the first place. The result after compilation of a program is machine code, code designed to be efficiently translated and processed by a machine such as your computer. Humans usually aren’t as good writing machine code as they are at writing Fortran (or C++, Ada, or Java), because it is easy to make tiny mistakes writing machine code.
• 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 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 warning message.

• Provide optional information about the translation passes from the source code to machine code. This can help a user of the compiler to find the cause of certain bugs which may not be obvious in the source code, but may be more easily found at a lower level compiler output. It also helps developers to find bugs in the compiler itself.
• Provide information in the generated machine code that can make it easier to find bugs in the program (using a debugging tool, called a debugger, such as the GNU Debugger gdb).
• Locate and gather machine code already generated to perform actions requested by statements in the user’s program. This machine code is organized into modules and is located and linked to the user program.

The GNU Fortran compiler consists of several components:

• A version of the gcc command (which also might be installed as the system’s cc command) that also understands and accepts Fortran source code. The gcc command is the driver program for all the languages in the GNU Compiler Collection (GCC); With gcc, you can compile the source code of any language for which a front end is available in GCC.
• The gfortran command itself, which also might be installed as the system’s f95 command. gfortran is just another driver program, but specifically for the Fortran compiler only. The difference with gcc is that gfortran will automatically link the correct libraries to your program.
• A collection of run-time libraries. These libraries contain the machine code needed to support capabilities of the Fortran language that are not directly provided by the machine code generated by the gfortran compilation phase, such as intrinsic functions and subroutines, and routines for interaction with files and the operating system.
• The Fortran compiler itself, (f951). This is the GNU Fortran parser and code generator, linked to and interfaced with the GCC backend library. f951 “translates” the source code to assembler code. You would typically not use this program directly; instead, the gcc or gfortran driver programs will call it for you.

1.2 GNU Fortran and GCC

GNU Fortran is a part of GCC, the 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 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 (gcc) which provides the command-line interface for the compiler. It calls the relevant compiler front-end program (e.g., 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, gcc will recognize files with .f, .for, .ftn, .f90, .f95, .f03 and .f08 extensions as Fortran source code, and compile it accordingly. A gfortran driver program is also provided, which is identical to gcc except that it automatically links the Fortran runtime libraries into the compiled program.

Source files with .f, .for, .fpp, .ftn, .F, .FOR, .FPP, and .FTN extensions are treated as fixed form. Source files with .f90, .f95, .f03, .f08, .F90, .F95, .F03 and .F08 extensions are treated as free form. The capitalized versions of either form are run through preprocessing. Source files with the lower case .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 Introduction in Using the GNU Compiler Collection (GCC). The two manuals together provide a complete reference for the GNU Fortran compiler.

1.3 Preprocessing and conditional compilation

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 .F, .FOR, .FTN, .fpp, .FPP, .F90, .F95, .F03 or .F08. To manually invoke the preprocessor on any file, use -cpp, to disable preprocessing on files where the preprocessor is run automatically, use -nocpp.

If a preprocessed file includes another file with the Fortran INCLUDE statement, the included file is not preprocessed. To preprocess included files, use the equivalent preprocessor statement #include.

If GNU Fortran invokes the preprocessor, __GFORTRAN__ is defined and __GNUC__, __GNUC_MINOR__ and __GNUC_PATCHLEVEL__ can be used to determine the version of the compiler. See Overview in The C Preprocessor for details.

While CPP is the de-facto standard for preprocessing Fortran code, Part 3 of the Fortran 95 standard (ISO/IEC 1539-3:1998) defines Conditional Compilation, which is not widely used and not directly supported by the GNU Fortran compiler. You can use the program coco to preprocess such files (http://users.erols.com/dnagle/coco.html).

1.4 GNU Fortran and G77

The GNU Fortran compiler is the successor to 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 g77.

1.5 Project Status

As soon as gfortran can parse all of the statements correctly, it will be in the “larva” state. When we generate code, the “puppa” state. When gfortran is done, we’ll see if it will be a beautiful butterfly, or just a big bug....

–Andy Vaught, April 2000

The start of the GNU Fortran 95 project was announced on the GCC homepage in March 18, 2000 (even though Andy had already been working on it for a while, of course).

The GNU Fortran compiler is able to compile nearly all standard-compliant Fortran 95, Fortran 90, and Fortran 77 programs, including a number of standard and non-standard extensions, and can be used on real-world programs. In particular, the supported extensions include OpenMP, Cray-style pointers, and several Fortran 2003 and Fortran 2008 features such as enumeration, stream I/O, and some of the enhancements to allocatable array support from TR 15581. However, it is still under development and has a few remaining rough edges.

At present, the GNU Fortran compiler passes the NIST Fortran 77 Test Suite, and produces acceptable results on the LAPACK Test Suite. It also provides respectable performance on the Polyhedron Fortran compiler benchmarks and the Livermore Fortran Kernels test. It has been used to compile a number of large real-world programs, including the HIRLAM weather-forecasting code and the Tonto quantum chemistry package; see http://gcc.gnu.org/wiki/GfortranApps for an extended list.

Among other things, the GNU Fortran compiler is intended as a replacement for G77. At this point, nearly all programs that could be compiled with G77 can be compiled with GNU Fortran, although there are a few minor known regressions.

The primary work remaining to be done on GNU Fortran falls into three categories: bug fixing (primarily regarding the treatment of invalid code and providing useful error messages), improving the compiler optimizations and the performance of compiled code, and extending the compiler to support future standards—in particular, Fortran 2003 and Fortran 2008.

1.6 Standards

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, and the OpenMP Application Program Interface v2.5 specification.

In the future, the GNU Fortran compiler will also support ISO/IEC 1539-1:2004 (Fortran 2003) and future Fortran standards. Partial support of that standard is already provided; the current status of Fortran 2003 support is reported in the Fortran 2003 status section of the documentation.

The next version of the Fortran standard (Fortran 2008) is currently being developed and the GNU Fortran compiler supports some of its new features. This support is based on the latest draft of the standard (available from http://www.nag.co.uk/sc22wg5/) and no guarantee of future compatibility is made, as the final standard might differ from the draft. For more information, see the Fortran 2008 status section.

Additionally, the GNU Fortran compilers supports the OpenMP specification (version 3.0, http://openmp.org/wp/openmp-specifications/).

1.6.1 Varying Length Character Strings

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 http://www.fortran.com/iso_varying_string.f95 and at ftp://ftp.nag.co.uk/sc22wg5/ISO_VARYING_STRING/.

2 GNU Fortran Command Options

The gfortran command supports all the options supported by the gcc command. Only options specific to GNU Fortran are documented here.

See GCC Command Options in Using the GNU Compiler Collection (GCC), for information on the non-Fortran-specific aspects of the gcc command (and, therefore, the gfortran command).

All GCC and GNU Fortran options are accepted both by gfortran and by gcc (as well as any other drivers built at the same time, such as g++), since adding GNU Fortran to the GCC distribution enables acceptance of GNU Fortran options by all of the relevant drivers.

In some cases, options have positive and negative forms; the negative form of -ffoo would be -fno-foo. This manual documents only one of these two forms, whichever one is not the default.

2.1 Option summary

Here is a summary of all the options specific to GNU Fortran, grouped by type. Explanations are in the following sections.

Fortran Language Options

See Options controlling Fortran dialect.

-fall-intrinsics  -ffree-form  -fno-fixed-form 
-fdollar-ok  -fimplicit-none  -fmax-identifier-length 
-std=std -fd-lines-as-code  -fd-lines-as-comments 
-ffixed-line-length-n  -ffixed-line-length-none 
-ffree-line-length-n  -ffree-line-length-none 
-fdefault-double-8  -fdefault-integer-8  -fdefault-real-8 
-fcray-pointer  -fopenmp  -fno-range-check -fbackslash -fmodule-private

Preprocessing Options

See Enable and customize preprocessing.

-cpp -dD -dI -dM -dN -dU -fworking-directory 
-imultilib dir -iprefix file -isysroot dir 
-iquote -isystem dir -nocpp -nostdinc -undef 
-Aquestion=answer -A-question[=answer] 
-C -CC -Dmacro[=defn] -Umacro -H -P

Error and Warning Options

See Options to request or suppress errors and warnings.

-fmax-errors=n 
-fsyntax-only  -pedantic  -pedantic-errors 
-Wall  -Waliasing  -Wampersand  -Warray-bounds -Wcharacter-truncation 
-Wconversion -Wimplicit-interface  -Wimplicit-procedure  -Wline-truncation 
-Wintrinsics-std  -Wsurprising  -Wno-tabs  -Wunderflow  -Wunused-parameter 
-Wintrinsics-shadow  -Wno-align-commons

Debugging Options

See Options for debugging your program or GNU Fortran.

-fdump-parse-tree  -ffpe-trap=list 
-fdump-core -fbacktrace

Directory Options

See Options for directory search.

-Idir  -Jdir -fintrinsic-modules-path dir

Link Options

See Options for influencing the linking step.

-static-libgfortran

Runtime Options

See Options for influencing runtime behavior.

-fconvert=conversion  -fno-range-check
-frecord-marker=length   -fmax-subrecord-length=length
-fsign-zero

Code Generation Options

See Options for code generation conventions.

-fno-automatic  -ff2c  -fno-underscoring 
-fwhole-file -fsecond-underscore 
-fbounds-check -fcheck-array-temporaries  -fmax-array-constructor =n 
-fcheck=<all|array-temps|bounds|do|mem|pointer|recursion>
-fmax-stack-var-size=n 
-fpack-derived  -frepack-arrays  -fshort-enums  -fexternal-blas 
-fblas-matmul-limit=n -frecursive -finit-local-zero 
-finit-integer=n -finit-real=<zero|inf|-inf|nan|snan> 
-finit-logical=<true|false> -finit-character=n 
-fno-align-commons -fno-protect-parens

2.2 Options controlling Fortran dialect

The following options control the details of the Fortran dialect accepted by the compiler:

-ffree-form

-ffixed-form

Specify the layout used by the source file. The free form layout was introduced in Fortran 90. Fixed form was traditionally used in older Fortran programs. When neither option is specified, the source form is determined by the file extension.

-fall-intrinsics

This option causes all intrinsic procedures (including the GNU-specific extensions) to be accepted. This can be useful with -std=f95 to force standard-compliance but get access to the full range of intrinsics available with gfortran. As a consequence, -Wintrinsics-std will be ignored and no user-defined procedure with the same name as any intrinsic will be called except when it is explicitly declared EXTERNAL.

-fd-lines-as-code

-fd-lines-as-comments

Enable special treatment for lines beginning with d or D in fixed form sources. If the -fd-lines-as-code option is given they are treated as if the first column contained a blank. If the -fd-lines-as-comments option is given, they are treated as comment lines.

-fdefault-double-8

Set the DOUBLE PRECISION type to an 8 byte wide type. If -fdefault-real-8 is given, DOUBLE PRECISION would instead be promoted to 16 bytes if possible, and -fdefault-double-8 can be used to prevent this. The kind of real constants like 1.d0 will not be changed by -fdefault-real-8 though, so also -fdefault-double-8 does not affect it.

-fdefault-integer-8

Set the default integer and logical types to an 8 byte wide type. Do nothing if this is already the default. This option also affects the kind of integer constants like 42.

-fdefault-real-8

Set the default real type to an 8 byte wide type. Do nothing if this is already the default. This option also affects the kind of non-double real constants like 1.0, and does promote the default width of DOUBLE PRECISION to 16 bytes if possible, unless -fdefault-double-8 is given, too.

-fdollar-ok

Allow ‘$’ as a valid non-first character in a symbol name. Symbols that start with ‘$’ are rejected since it is unclear which rules to apply to implicit typing as different vendors implement different rules. Using ‘$’ in IMPLICIT statements is also rejected.

-fbackslash

Change the interpretation of backslashes in string literals from a single backslash character to “C-style” escape characters. The following combinations are expanded \a, \b, \f, \n, \r, \t, \v, \\, and \0 to the ASCII characters alert, backspace, form feed, newline, carriage return, horizontal tab, vertical tab, backslash, and NUL, respectively. Additionally, \xnn, \unnnn and \Unnnnnnnn (where each n is a hexadecimal digit) are translated into the Unicode characters corresponding to the specified code points. All other combinations of a character preceded by \ are unexpanded.

-fmodule-private

Set the default accessibility of module entities to PRIVATE. Use-associated entities will not be accessible unless they are explicitly declared as PUBLIC.

-ffixed-line-length-n

Set column after which characters are ignored in typical fixed-form lines in the source file, and through which spaces are assumed (as if padded to that length) after the ends of short fixed-form lines.

Popular values for n include 72 (the standard and the default), 80 (card image), and 132 (corresponding to “extended-source” options in some popular compilers). n may also be ‘none’, meaning that the entire line is meaningful and that continued character constants never have implicit spaces appended to them to fill out the line. -ffixed-line-length-0 means the same thing as -ffixed-line-length-none.

-ffree-line-length-n

Set column after which characters are ignored in typical free-form lines in the source file. The default value is 132. n may be ‘none’, meaning that the entire line is meaningful. -ffree-line-length-0 means the same thing as -ffree-line-length-none.

-fmax-identifier-length=n

Specify the maximum allowed identifier length. Typical values are 31 (Fortran 95) and 63 (Fortran 2003 and Fortran 2008).

-fimplicit-none

Specify that no implicit typing is allowed, unless overridden by explicit IMPLICIT statements. This is the equivalent of adding implicit none to the start of every procedure.

-fcray-pointer

Enable the Cray pointer extension, which provides C-like pointer functionality.

-fopenmp

Enable the OpenMP extensions. This includes OpenMP !$omp directives in free form and c$omp, *$omp and !$omp directives in fixed form, !$ conditional compilation sentinels in free form and c$, *$ and !$ sentinels in fixed form, and when linking arranges for the OpenMP runtime library to be linked in. The option -fopenmp implies -frecursive.

-fno-range-check

Disable range checking on results of simplification of constant expressions during compilation. For example, GNU Fortran will give an error at compile time when simplifying a = 1. / 0. With this option, no error will be given and a will be assigned the value +Infinity. If an expression evaluates to a value outside of the relevant range of [-HUGE():HUGE()], then the expression will be replaced by -Inf or +Inf as appropriate. Similarly, DATA i/Z'FFFFFFFF'/ will result in an integer overflow on most systems, but with -fno-range-check the value will “wrap around” and i will be initialized to -1 instead.

-std=std

Specify the standard to which the program is expected to conform, which may be one of ‘f95’, ‘f2003’, ‘f2008’, ‘gnu’, or ‘legacy’. The default value for std is ‘gnu’, which specifies a superset of the Fortran 95 standard that includes all of the extensions supported by GNU Fortran, although warnings will be given for obsolete extensions not recommended for use in new code. The ‘legacy’ value is equivalent but without the warnings for obsolete extensions, and may be useful for old non-standard programs. The ‘f95’, ‘f2003’ and ‘f2008’ values specify strict conformance to the Fortran 95, Fortran 2003 and Fortran 2008 standards, respectively; errors are given for all extensions beyond the relevant language standard, and warnings are given for the Fortran 77 features that are permitted but obsolescent in later standards.

2.3 Enable and customize preprocessing

Preprocessor related options. See section Preprocessing and conditional compilation for more detailed information on preprocessing in gfortran.

-cpp

-nocpp

Enable preprocessing. The preprocessor is automatically invoked if the file extension is .fpp, .FPP, .F, .FOR, .FTN, .F90, .F95, .F03 or .F08. Use this option to manually enable preprocessing of any kind of Fortran file.

To disable preprocessing of files with any of the above listed extensions, use the negative form: -nocpp.

The preprocessor is run in traditional mode, be aware that any restrictions of the file-format, e.g. fixed-form line width, apply for preprocessed output as well.

-dM

Instead of the normal output, generate a list of '#define' directives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no file foo.f90, the command

  touch foo.f90; gfortran -cpp -dM foo.f90

will show all the predefined macros.

-dD

Like -dM except in two respects: it does not include the predefined macros, and it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file.

-dN

Like -dD, but emit only the macro names, not their expansions.

-dU

Like dD except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and '#undef' directives are also output for macros tested but undefined at the time.

-dI

Output '#include' directives in addition to the result of preprocessing.

-fworking-directory

Enable generation of linemarkers in the preprocessor output that will let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor will emit, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC will use this directory, when it’s present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect, since no #line directives are emitted whatsoever.

-idirafter dir

Search dir for include files, but do it after all directories specified with -I and the standard system directories have been exhausted. dir is treated as a system include directory. If dir begins with =, then the = will be replaced by the sysroot prefix; see --sysroot and -isysroot.

-imultilib dir

Use dir as a subdirectory of the directory containing target-specific C++ headers.

-iprefix prefix

Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final '/'.

-isysroot dir

This option is like the --sysroot option, but applies only to header files. See the --sysroot option for more information.

-iquote dir

Search dir only for header files requested with #include "file"; they are not searched for #include <file>, before all directories specified by -I and before the standard system directories. If dir begins with =, then the = will be replaced by the sysroot prefix; see --sysroot and -isysroot.

-isystem dir

Search dir for header files, after all directories specified by -I but before the standard system directories. Mark it as a system directory, so that it gets the same special treatment as is applied to the standard system directories. If dir begins with =, then the = will be replaced by the sysroot prefix; see --sysroot and -isysroot.

-nostdinc

Do not search the standard system directories for header files. Only the directories you have specified with -I options (and the directory of the current file, if appropriate) are searched.

-undef

Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.

-Apredicate=answer

Make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A predicate(answer), which is still supported, because it does not use shell special characters.

-A-predicate=answer

Cancel an assertion with the predicate predicate and answer answer.

-C

Do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.

You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a '#'.

Warning: this currently handles C-Style comments only. The preprocessor does not yet recognize Fortran-style comments.

-CC

Do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded.

In addition to the side-effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line. The -CC option is generally used to support lint comments.

Warning: this currently handles C- and C++-Style comments only. The preprocessor does not yet recognize Fortran-style comments.

-Dname

Predefine name as a macro, with definition 1.

-Dname=definition

The contents of definition are tokenized and processed as if they appeared during translation phase three in a '#define' directive. In particular, the definition will be truncated by embedded newline characters.

If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell’s quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.

If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you will need to quote the option. With sh and csh, -D'name(args...)=definition' works.

-D and -U options are processed in the order they are given on the command line. All -imacros file and -include file options are processed after all -D and -U options.

-H

Print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the '#include' stack it is.

-P

Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers.

-Uname

Cancel any previous definition of name, either built in or provided with a -D option.

2.4 Options to request or suppress errors and warnings

Errors are diagnostic messages that report that the GNU Fortran compiler cannot compile the relevant piece of source code. The compiler will continue to process the program in an attempt to report further errors to aid in debugging, but will not produce any compiled output.

Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there is likely to be a bug in the program. Unless -Werror is specified, they do not prevent compilation of the program.

You can request many specific warnings with options beginning -W, for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two forms, whichever is not the default.

These options control the amount and kinds of errors and warnings produced by GNU Fortran:

-fmax-errors=n

Limits the maximum number of error messages to n, at which point GNU Fortran bails out rather than attempting to continue processing the source code. If n is 0, there is no limit on the number of error messages produced.

-fsyntax-only

Check the code for syntax errors, but don’t actually compile it. This will generate module files for each module present in the code, but no other output file.

-pedantic

Issue warnings for uses of extensions to Fortran 95. -pedantic also applies to C-language constructs where they occur in GNU Fortran source files, such as use of ‘\e’ in a character constant within a directive like #include.

Valid Fortran 95 programs should compile properly with or without this option. However, without this option, certain GNU extensions and traditional Fortran features are supported as well. With this option, many of them are rejected.

Some users try to use -pedantic to check programs for conformance. They soon find that it does not do quite what they want—it finds some nonstandard practices, but not all. However, improvements to GNU Fortran in this area are welcome.

This should be used in conjunction with -std=f95, -std=f2003 or -std=f2008.

-pedantic-errors

Like -pedantic, except that errors are produced rather than warnings.

-Wall

Enables commonly used warning options pertaining to usage that we recommend avoiding and that we believe are easy to avoid. This currently includes -Waliasing, -Wampersand, -Wsurprising, -Wintrinsics-std, -Wno-tabs, -Wintrinsic-shadow and -Wline-truncation.

-Waliasing

Warn about possible aliasing of dummy arguments. Specifically, it warns if the same actual argument is associated with a dummy argument with INTENT(IN) and a dummy argument with INTENT(OUT) in a call with an explicit interface.

The following example will trigger the warning.

  interface
    subroutine bar(a,b)
      integer, intent(in) :: a
      integer, intent(out) :: b
    end subroutine
  end interface
  integer :: a

  call bar(a,a)

-Wampersand

Warn about missing ampersand in continued character constants. The warning is given with -Wampersand, -pedantic, -std=f95, -std=f2003 and -std=f2008. Note: With no ampersand given in a continued character constant, GNU Fortran assumes continuation at the first non-comment, non-whitespace character after the ampersand that initiated the continuation.

-Warray-temporaries

Warn about array temporaries generated by the compiler. The information generated by this warning is sometimes useful in optimization, in order to avoid such temporaries.

-Wcharacter-truncation

Warn when a character assignment will truncate the assigned string.

-Wline-truncation

Warn when a source code line will be truncated.

-Wconversion

Warn about implicit conversions between different types.

-Wimplicit-interface

Warn if a procedure is called without an explicit interface. Note this only checks that an explicit interface is present. It does not check that the declared interfaces are consistent across program units.

-Wimplicit-procedure

Warn if a procedure is called that has neither an explicit interface nor has been declared as EXTERNAL.

-Wintrinsics-std

Warn if gfortran finds a procedure named like an intrinsic not available in the currently selected standard (with -std) and treats it as EXTERNAL procedure because of this. -fall-intrinsics can be used to never trigger this behavior and always link to the intrinsic regardless of the selected standard.

-Wsurprising

Produce a warning when “suspicious” code constructs are encountered. While technically legal these usually indicate that an error has been made.

This currently produces a warning under the following circumstances:

• An INTEGER SELECT construct has a CASE that can never be matched as its lower value is greater than its upper value.
• A LOGICAL SELECT construct has three CASE statements.
• A TRANSFER specifies a source that is shorter than the destination.
• The type of a function result is declared more than once with the same type. If -pedantic or standard-conforming mode is enabled, this is an error.
• A CHARACTER variable is declared with negative length.

-Wtabs

By default, tabs are accepted as whitespace, but tabs are not members of the Fortran Character Set. For continuation lines, a tab followed by a digit between 1 and 9 is supported. -Wno-tabs will cause a warning to be issued if a tab is encountered. Note, -Wno-tabs is active for -pedantic, -std=f95, -std=f2003, -std=f2008 and -Wall.

-Wunderflow

Produce a warning when numerical constant expressions are encountered, which yield an UNDERFLOW during compilation.

-Wintrinsic-shadow

Warn if a user-defined procedure or module procedure has the same name as an intrinsic; in this case, an explicit interface or EXTERNAL or INTRINSIC declaration might be needed to get calls later resolved to the desired intrinsic/procedure.

-Wunused-parameter

Contrary to gcc’s meaning of -Wunused-parameter, gfortran’s implementation of this option does not warn about unused dummy arguments, but about unused PARAMETER values. -Wunused-parameter is not included in -Wall but is implied by -Wall -Wextra.

-Walign-commons

By default, gfortran warns about any occasion of variables being padded for proper alignment inside a COMMON block. This warning can be turned off via -Wno-align-commons. See also -falign-commons.

-Werror

Turns all warnings into errors.

See Options to Request or Suppress Errors and Warnings in Using the GNU Compiler Collection (GCC), for information on more options offered by the GBE shared by gfortran, gcc and other GNU compilers.

Some of these have no effect when compiling programs written in Fortran.

2.5 Options for debugging your program or GNU Fortran

GNU Fortran has various special options that are used for debugging either your program or the GNU Fortran compiler.

-fdump-parse-tree

Output the internal parse tree before starting code generation. Only really useful for debugging the GNU Fortran compiler itself.

-ffpe-trap=list

Specify a list of IEEE exceptions when a Floating Point Exception (FPE) should be raised. On most systems, this will result in a SIGFPE signal being sent and the program being interrupted, producing a core file useful for debugging. list is a (possibly empty) comma-separated list of the following IEEE exceptions: ‘invalid’ (invalid floating point operation, such as SQRT(-1.0)), ‘zero’ (division by zero), ‘overflow’ (overflow in a floating point operation), ‘underflow’ (underflow in a floating point operation), ‘precision’ (loss of precision during operation) and ‘denormal’ (operation produced a denormal value).

Some of the routines in the Fortran runtime library, like ‘CPU_TIME’, are likely to trigger floating point exceptions when ffpe-trap=precision is used. For this reason, the use of ffpe-trap=precision is not recommended.

-fbacktrace

Specify that, when a runtime error is encountered or a deadly signal is emitted (segmentation fault, illegal instruction, bus error or floating-point exception), the Fortran runtime library should output a backtrace of the error. This option only has influence for compilation of the Fortran main program.

-fdump-core

Request that a core-dump file is written to disk when a runtime error is encountered on systems that support core dumps. This option is only effective for the compilation of the Fortran main program.

See Options for Debugging Your Program or GCC in Using the GNU Compiler Collection (GCC), for more information on debugging options.

2.6 Options for directory search

These options affect how GNU Fortran searches for files specified by the INCLUDE directive and where it searches for previously compiled modules.

It also affects the search paths used by cpp when used to preprocess Fortran source.

-Idir

These affect interpretation of the INCLUDE directive (as well as of the #include directive of the cpp preprocessor).

Also note that the general behavior of -I and INCLUDE is pretty much the same as of -I with #include in the cpp preprocessor, with regard to looking for header.gcc files and other such things.

This path is also used to search for .mod files when previously compiled modules are required by a USE statement.

See Options for Directory Search in Using the GNU Compiler Collection (GCC), for information on the -I option.

-Jdir

This option specifies where to put .mod files for compiled modules. It is also added to the list of directories to searched by an USE statement.

The default is the current directory.

-fintrinsic-modules-path dir

This option specifies the location of pre-compiled intrinsic modules, if they are not in the default location expected by the compiler.

2.7 Influencing the linking step

These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.

-static-libgfortran

On systems that provide libgfortran as a shared and a static library, this option forces the use of the static version. If no shared version of libgfortran was built when the compiler was configured, this option has no effect.

2.8 Influencing runtime behavior

These options affect the runtime behavior of programs compiled with GNU Fortran.

-fconvert=conversion

Specify the representation of data for unformatted files. Valid values for conversion are: ‘native’, the default; ‘swap’, swap between big- and little-endian; ‘big-endian’, use big-endian representation for unformatted files; ‘little-endian’, use little-endian representation for unformatted files.

This option has an effect only when used in the main program. The CONVERT specifier and the GFORTRAN_CONVERT_UNIT environment variable override the default specified by -fconvert.

-fno-range-check

Disable range checking of input values during integer READ operations. For example, GNU Fortran will give an error if an input value is outside of the relevant range of [-HUGE():HUGE()]. In other words, with INTEGER (kind=4) :: i , attempting to read -2147483648 will give an error unless -fno-range-check is given.

-frecord-marker=length

Specify the length of record markers for unformatted files. Valid values for length are 4 and 8. Default is 4. This is different from previous versions of gfortran, which specified a default record marker length of 8 on most systems. If you want to read or write files compatible with earlier versions of gfortran, use -frecord-marker=8.

-fmax-subrecord-length=length

Specify the maximum length for a subrecord. The maximum permitted value for length is 2147483639, which is also the default. Only really useful for use by the gfortran testsuite.

-fsign-zero

When enabled, floating point numbers of value zero with the sign bit set are written as negative number in formatted output and treated as negative in the SIGN intrinsic. fno-sign-zero does not print the negative sign of zero values and regards zero as positive number in the SIGN intrinsic for compatibility with F77. Default behavior is to show the negative sign.

2.9 Options for code generation conventions

These machine-independent options control the interface conventions used in code generation.

Most of them have both positive and negative forms; the negative form of -ffoo would be -fno-foo. In the table below, only one of the forms is listed—the one which is not the default. You can figure out the other form by either removing no- or adding it.

-fno-automatic

Treat each program unit (except those marked as RECURSIVE) as if the SAVE statement were specified for every local variable and array referenced in it. Does not affect common blocks. (Some Fortran compilers provide this option under the name -static or -save.) The default, which is -fautomatic, uses the stack for local variables smaller than the value given by -fmax-stack-var-size. Use the option -frecursive to use no static memory.

-ff2c

Generate code designed to be compatible with code generated by g77 and f2c.

The calling conventions used by g77 (originally implemented in f2c) require functions that return type default REAL to actually return the C type double, and functions that return type COMPLEX to return the values via an extra argument in the calling sequence that points to where to store the return value. Under the default GNU calling conventions, such functions simply return their results as they would in GNU C—default REAL functions return the C type float, and COMPLEX functions return the GNU C type complex. Additionally, this option implies the -fsecond-underscore option, unless -fno-second-underscore is explicitly requested.

This does not affect the generation of code that interfaces with the libgfortran library.

Caution: It is not a good idea to mix Fortran code compiled with -ff2c with code compiled with the default -fno-f2c calling conventions as, calling COMPLEX or default REAL functions between program parts which were compiled with different calling conventions will break at execution time.

Caution: This will break code which passes intrinsic functions of type default REAL or COMPLEX as actual arguments, as the library implementations use the -fno-f2c calling conventions.

-fno-underscoring

Do not transform names of entities specified in the Fortran source file by appending underscores to them.

With -funderscoring in effect, GNU Fortran appends one underscore to external names with no underscores. This is done to ensure compatibility with code produced by many UNIX Fortran compilers.

Caution: The default behavior of GNU Fortran is incompatible with f2c and g77, please use the -ff2c option if you want object files compiled with GNU Fortran to be compatible with object code created with these tools.

Use of -fno-underscoring is not recommended unless you are experimenting with issues such as integration of GNU Fortran into existing system environments (vis-à-vis existing libraries, tools, and so on).

For example, with -funderscoring, and assuming other defaults like -fcase-lower and that j() and max_count() are external functions while my_var and lvar are local variables, a statement like

I = J() + MAX_COUNT (MY_VAR, LVAR)

is implemented as something akin to:

i = j_() + max_count__(&my_var__, &lvar);

With -fno-underscoring, the same statement is implemented as:

i = j() + max_count(&my_var, &lvar);

Use of -fno-underscoring allows direct specification of user-defined names while debugging and when interfacing GNU Fortran code with other languages.

Note that just because the names match does not mean that the interface implemented by GNU Fortran for an external name matches the interface implemented by some other language for that same name. That is, getting code produced by GNU Fortran to link to code produced by some other compiler using this or any other method can be only a small part of the overall solution—getting the code generated by both compilers to agree on issues other than naming can require significant effort, and, unlike naming disagreements, linkers normally cannot detect disagreements in these other areas.

Also, note that with -fno-underscoring, the lack of appended underscores introduces the very real possibility that a user-defined external name will conflict with a name in a system library, which could make finding unresolved-reference bugs quite difficult in some cases—they might occur at program run time, and show up only as buggy behavior at run time.

In future versions of GNU Fortran we hope to improve naming and linking issues so that debugging always involves using the names as they appear in the source, even if the names as seen by the linker are mangled to prevent accidental linking between procedures with incompatible interfaces.

-fwhole-file

By default, GNU Fortran parses, resolves and translates each procedure in a file separately. Using this option modifies this such that the whole file is parsed and placed in a single front-end tree. During resolution, in addition to all the usual checks and fixups, references to external procedures that are in the same file effect resolution of that procedure, if not already done, and a check of the interfaces. The dependences are resolved by changing the order in which the file is translated into the backend tree. Thus, a procedure that is referenced is translated before the reference and the duplication of backend tree declarations eliminated.

-fsecond-underscore

By default, GNU Fortran appends an underscore to external names. If this option is used GNU Fortran appends two underscores to names with underscores and one underscore to external names with no underscores. GNU Fortran also appends two underscores to internal names with underscores to avoid naming collisions with external names.

This option has no effect if -fno-underscoring is in effect. It is implied by the -ff2c option.

Otherwise, with this option, an external name such as MAX_COUNT is implemented as a reference to the link-time external symbol max_count__, instead of max_count_. This is required for compatibility with g77 and f2c, and is implied by use of the -ff2c option.

-fcheck=<keyword>

Enable the generation of run-time checks; the argument shall be a comma-delimited list of the following keywords.

‘all’

Enable all run-time test of -fcheck.

‘array-temps’

Warns at run time when for passing an actual argument a temporary array had to be generated. The information generated by this warning is sometimes useful in optimization, in order to avoid such temporaries.

Note: The warning is only printed once per location.

‘bounds’

Enable generation of run-time checks for array subscripts and against the declared minimum and maximum values. It also checks array indices for assumed and deferred shape arrays against the actual allocated bounds and ensures that all string lengths are equal for character array constructors without an explicit typespec.

Some checks require that -fcheck=bounds is set for the compilation of the main program.

Note: In the future this may also include other forms of checking, e.g., checking substring references.

‘do’

Enable generation of run-time checks for invalid modification of loop iteration variables.

‘mem’

Enable generation of run-time checks for memory allocation. Note: This option does not affect explicit allocations using the ALLOCATE statement, which will be always checked.

‘pointer’

Enable generation of run-time checks for pointers and allocatables.

‘recursion’

Enable generation of run-time checks for recursively called subroutines and functions which are not marked as recursive. See also -frecursive. Note: This check does not work for OpenMP programs and is disabled if used together with -frecursive and -fopenmp.

-fbounds-check

Deprecated alias for -fcheck=bounds.

-fcheck-array-temporaries

Deprecated alias for -fcheck=array-temps.

-fmax-array-constructor=n

This option can be used to increase the upper limit permitted in array constructors. The code below requires this option to expand the array at compile time.

program test
implicit none
integer j
integer, parameter :: n = 100000
integer, parameter :: i(n) = (/ (2*j, j = 1, n) /)
print '(10(I0,1X))', i
end program test

Caution: This option can lead to long compile times and excessively large object files.

The default value for n is 65535.

-fmax-stack-var-size=n

This option specifies the size in bytes of the largest array that will be put on the stack; if the size is exceeded static memory is used (except in procedures marked as RECURSIVE). Use the option -frecursive to allow for recursive procedures which do not have a RECURSIVE attribute or for parallel programs. Use -fno-automatic to never use the stack.

This option currently only affects local arrays declared with constant bounds, and may not apply to all character variables. Future versions of GNU Fortran may improve this behavior.

The default value for n is 32768.

-fpack-derived

This option tells GNU Fortran to pack derived type members as closely as possible. Code compiled with this option is likely to be incompatible with code compiled without this option, and may execute slower.

-frepack-arrays

In some circumstances GNU Fortran may pass assumed shape array sections via a descriptor describing a noncontiguous area of memory. This option adds code to the function prologue to repack the data into a contiguous block at runtime.

This should result in faster accesses to the array. However it can introduce significant overhead to the function call, especially when the passed data is noncontiguous.

-fshort-enums

This option is provided for interoperability with C code that was compiled with the -fshort-enums option. It will make GNU Fortran choose the smallest INTEGER kind a given enumerator set will fit in, and give all its enumerators this kind.

-fexternal-blas

This option will make gfortran generate calls to BLAS functions for some matrix operations like MATMUL, instead of using our own algorithms, if the size of the matrices involved is larger than a given limit (see -fblas-matmul-limit). This may be profitable if an optimized vendor BLAS library is available. The BLAS library will have to be specified at link time.

-fblas-matmul-limit=n

Only significant when -fexternal-blas is in effect. Matrix multiplication of matrices with size larger than (or equal to) n will be performed by calls to BLAS functions, while others will be handled by gfortran internal algorithms. If the matrices involved are not square, the size comparison is performed using the geometric mean of the dimensions of the argument and result matrices.

The default value for n is 30.

-frecursive

Allow indirect recursion by forcing all local arrays to be allocated on the stack. This flag cannot be used together with -fmax-stack-var-size= or -fno-automatic.

-finit-local-zero

-finit-integer=n

-finit-real=<zero|inf|-inf|nan|snan>

-finit-logical=<true|false>

-finit-character=n

The -finit-local-zero option instructs the compiler to initialize local INTEGER, REAL, and COMPLEX variables to zero, LOGICAL variables to false, and CHARACTER variables to a string of null bytes. Finer-grained initialization options are provided by the -finit-integer=n, -finit-real=<zero|inf|-inf|nan|snan> (which also initializes the real and imaginary parts of local COMPLEX variables), -finit-logical=<true|false>, and -finit-character=n (where n is an ASCII character value) options. These options do not initialize components of derived type variables, nor do they initialize variables that appear in an EQUIVALENCE statement. (This limitation may be removed in future releases).

Note that the -finit-real=nan option initializes REAL and COMPLEX variables with a quiet NaN. For a signalling NaN use -finit-real=snan; note, however, that compile-time optimizations may convert them into quiet NaN and that trapping needs to be enabled (e.g. via -ffpe-trap).

-falign-commons

By default, gfortran enforces proper alignment of all variables in a COMMON block by padding them as needed. On certain platforms this is mandatory, on others it increases performance. If a COMMON block is not declared with consistent data types everywhere, this padding can cause trouble, and -fno-align-commons can be used to disable automatic alignment. The same form of this option should be used for all files that share a COMMON block. To avoid potential alignment issues in COMMON blocks, it is recommended to order objects from largests to smallest.

-fno-protect-parens

By default the parentheses in expression are honored for all optimization levels such that the compiler does not do any re-association. Using -fno-protect-parens allows the compiler to reorder REAL and COMPLEX expressions to produce faster code. Note that for the re-association optimization -fno-signed-zeros and -fno-trapping-math need to be in effect.

See Options for Code Generation Conventions in Using the GNU Compiler Collection (GCC), for information on more options offered by the GBE shared by gfortran, gcc, and other GNU compilers.

2.10 Environment variables affecting gfortran

The gfortran compiler currently does not make use of any environment variables to control its operation above and beyond those that affect the operation of gcc.

See Environment Variables Affecting GCC in Using the GNU Compiler Collection (GCC), for information on environment variables.

See Runtime, for environment variables that affect the run-time behavior of programs compiled with GNU Fortran.

3 Runtime: Influencing runtime behavior with environment variables

The behavior of the gfortran can be influenced by environment variables.

Malformed environment variables are silently ignored.

3.1 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.

3.2 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.

3.3 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.

3.4 GFORTRAN_USE_STDERR—Send library output to standard error

This environment variable controls where library output is sent. If the first letter is ‘y’, ‘Y’ or ‘1’, standard error is used. If the first letter is ‘n’, ‘N’ or ‘0’, standard output is used.

3.5 GFORTRAN_TMPDIR—Directory for scratch files

This environment variable controls where scratch files are created. If this environment variable is missing, GNU Fortran searches for the environment variable TMP. If this is also missing, the default is /tmp.

3.6 GFORTRAN_UNBUFFERED_ALL—Don’t buffer I/O on all units

This environment variable controls whether all I/O is unbuffered. If the first letter is ‘y’, ‘Y’ or ‘1’, all I/O is unbuffered. This will slow down small sequential reads and writes. If the first letter is ‘n’, ‘N’ or ‘0’, I/O is buffered. This is the default.

3.7 GFORTRAN_UNBUFFERED_PRECONNECTED—Don’t buffer I/O on preconnected units

The environment variable named GFORTRAN_UNBUFFERED_PRECONNECTED controls whether I/O on a preconnected unit (i.e. STDOUT or STDERR) is unbuffered. If the first letter is ‘y’, ‘Y’ or ‘1’, I/O is unbuffered. This will slow down small sequential reads and writes. If the first letter is ‘n’, ‘N’ or ‘0’, I/O is buffered. This is the default.

3.8 GFORTRAN_SHOW_LOCUS—Show location for runtime errors

If the first letter is ‘y’, ‘Y’ or ‘1’, filename and line numbers for runtime errors are printed. If the first letter is ‘n’, ‘N’ or ‘0’, don’t print filename and line numbers for runtime errors. The default is to print the location.

3.9 GFORTRAN_OPTIONAL_PLUS—Print leading + where permitted

If the first letter is ‘y’, ‘Y’ or ‘1’, a plus sign is printed where permitted by the Fortran standard. If the first letter is ‘n’, ‘N’ or ‘0’, a plus sign is not printed in most cases. Default is not to print plus signs.

3.10 GFORTRAN_DEFAULT_RECL—Default record length for new files

This environment variable specifies the default record length, in bytes, for files which are opened without a RECL tag in the OPEN statement. This must be a positive integer. The default value is 1073741824 bytes (1 GB).

3.11 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

$ GFORTRAN_LIST_SEPARATOR='  ,  ' ./a.out

when a.out is the compiled Fortran program that you want to run. Default is a single space.

3.12 GFORTRAN_CONVERT_UNIT—Set endianness for unformatted I/O

By setting the GFORTRAN_CONVERT_UNIT variable, it is possible to change the representation of data for unformatted files. The syntax for the GFORTRAN_CONVERT_UNIT variable is:

GFORTRAN_CONVERT_UNIT: mode | mode ';' exception | exception ;
mode: 'native' | 'swap' | 'big_endian' | 'little_endian' ;
exception: mode ':' unit_list | unit_list ;
unit_list: unit_spec | unit_list unit_spec ;
unit_spec: INTEGER | INTEGER '-' INTEGER ;

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 CONVERT specifier:

• NATIVE Use the native format. This is the default.
• SWAP Swap between little- and big-endian.
• LITTLE_ENDIAN Use the little-endian format for unformatted files.
• BIG_ENDIAN Use the big-endian format for unformatted files.

A missing mode for an exception is taken to mean BIG_ENDIAN. Examples of values for GFORTRAN_CONVERT_UNIT are:

• 'big_endian' Do all unformatted I/O in big_endian mode.
• '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.
• '10-20' Units 10 to 20 are big-endian, the rest is native.

Setting the environment variables should be done on the command line or via the export command for sh-compatible shells and via setenv for csh-compatible shells.

Example for sh:

$ gfortran foo.f90
$ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out

Example code for csh:

% gfortran foo.f90
% setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
% ./a.out

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.

See CONVERT specifier, for an alternative way to specify the data representation for unformatted files. See Runtime Options, for setting a default data representation for the whole program. The CONVERT specifier overrides the -fconvert compile options.

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.

3.13 GFORTRAN_ERROR_DUMPCORE—Dump core on run-time errors

If the GFORTRAN_ERROR_DUMPCORE variable is set to ‘y’, ‘Y’ or ‘1’ (only the first letter is relevant) then library run-time errors cause core dumps. To disable the core dumps, set the variable to ‘n’, ‘N’, ‘0’. Default is not to core dump unless the -fdump-core compile option was used.

3.14 GFORTRAN_ERROR_BACKTRACE—Show backtrace on run-time errors

If the GFORTRAN_ERROR_BACKTRACE variable is set to ‘y’, ‘Y’ or ‘1’ (only the first letter is relevant) then a backtrace is printed when a run-time error occurs. To disable the backtracing, set the variable to ‘n’, ‘N’, ‘0’. Default is not to print a backtrace unless the -fbacktrace compile option was used.

4 Fortran 2003 and 2008 Status

4.1 Fortran 2003 status

GNU Fortran supports several Fortran 2003 features; an incomplete list can be found below. See also the wiki page about Fortran 2003.

• Intrinsics command_argument_count, get_command, get_command_argument, get_environment_variable, and move_alloc.
• Array constructors using square brackets. That is, [...] rather than (/.../). Type-specification for array constructors like (/ some-type :: ... /).
• FLUSH statement.
• IOMSG= specifier for I/O statements.
• Support for the declaration of enumeration constants via the ENUM and ENUMERATOR statements. Interoperability with gcc is guaranteed also for the case where the -fshort-enums command line option is given.
• TR 15581:
  • ALLOCATABLE dummy arguments.
  • ALLOCATABLE function results
  • ALLOCATABLE components of derived types
• The ERRMSG= tag is now supported in ALLOCATE and DEALLOCATE statements. The SOURCE= tag is supported in an ALLOCATE statement. An intrinsic-type-spec can be used as the type-spec in an ALLOCATE statement; while the use of a derived-type-name is currently unsupported.
• The OPEN statement supports the ACCESS='STREAM' specifier, allowing I/O without any record structure.
• Namelist input/output for internal files.
• The PROTECTED statement and attribute.
• The VALUE statement and attribute.
• The VOLATILE statement and attribute.
• The IMPORT statement, allowing to import host-associated derived types.
• USE statement with INTRINSIC and NON_INTRINSIC attribute; supported intrinsic modules: ISO_FORTRAN_ENV, OMP_LIB and OMP_LIB_KINDS.
• Renaming of operators in the USE statement.
• Interoperability with C (ISO C Bindings)
• BOZ as argument of INT, REAL, DBLE and CMPLX.
• Type-bound procedures with PROCEDURE or GENERIC, and operators bound to a derived-type.
• Extension of derived-types (the EXTENDS(...) syntax).
• ABSTRACT derived-types and declaring procedure bindings DEFERRED.

4.2 Fortran 2008 status

The next version of the Fortran standard after Fortran 2003 is currently being worked on by the Working Group 5 of Sub-Committee 22 of the Joint Technical Committee 1 of the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). This group is known as WG5. The next revision of the Fortran standard is informally referred to as Fortran 2008, reflecting its planned release year. The GNU Fortran compiler has support for some of the new features in Fortran 2008. This support is based on the latest draft, available from http://www.nag.co.uk/sc22wg5/. However, as the final standard may differ from the drafts, no guarantee of backward compatibility can be made and you should only use it for experimental purposes.

The wiki has some information about the current Fortran 2008 implementation status.

5 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.

5.1 KIND Type Parameters

The KIND type parameters supported by GNU Fortran for the primitive data types are:

INTEGER

1, 2, 4, 8*, 16*, default: 4 (1)

LOGICAL

1, 2, 4, 8*, 16*, default: 4 (1)

REAL

4, 8, 10**, 16**, default: 4 (2)

COMPLEX

4, 8, 10**, 16**, default: 4 (2)

CHARACTER

1, 4, default: 1

* = not available on all systems
** = not available on all systems; additionally 10 and 16 are never available at the same time
(1) Unless -fdefault-integer-8 is used
(2) Unless -fdefault-real-8 is used

The KIND value matches the storage size in bytes, except for 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 SELECT_*_KIND intrinsics instead of the concrete values.

5.2 Internal representation of LOGICAL variables

The Fortran standard does not specify how variables of LOGICAL type are represented, beyond requiring that LOGICAL variables of default kind have the same storage size as default INTEGER and REAL variables. The GNU Fortran internal representation is as follows.

A LOGICAL(KIND=N) variable is represented as an INTEGER(KIND=N) variable, however, with only two permissible values: 1 for .TRUE. and 0 for .FALSE.. Any other integer value results in undefined behavior.

Note that for mixed-language programming using the ISO_C_BINDING feature, there is a C_BOOL kind that can be used to create LOGICAL(KIND=C_BOOL) variables which are interoperable with the C99 _Bool type. The C99 _Bool type has an internal representation described in the C99 standard, which is identical to the above description, i.e. with 1 for true and 0 for false being the only permissible values. Thus the internal representation of LOGICAL variables in GNU Fortran is identical to C99 _Bool, except for a possible difference in storage size depending on the kind.

6 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.

6.1 Extensions implemented in GNU Fortran

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, -std=gnu allows the compiler to accept both types of extensions, but to warn about the use of the latter. Specifying either -std=f95, -std=f2003 or -std=f2008 disables both types of extensions, and -std=legacy allows both without warning.

6.1.1 Old-style kind specifications

GNU Fortran allows old-style kind specifications in declarations. These look like:

      TYPESPEC*size x,y,z

where TYPESPEC is a basic type (INTEGER, REAL, etc.), and where size is a byte count corresponding to the storage size of a valid kind for that type. (For COMPLEX variables, size is the total size of the real and imaginary parts.) The statement then declares x, y and z to be of type TYPESPEC with the appropriate kind. This is equivalent to the standard-conforming declaration

      TYPESPEC(k) x,y,z

where k is the kind parameter suitable for the intended precision. As kind parameters are implementation-dependent, use the KIND, SELECTED_INT_KIND and SELECTED_REAL_KIND intrinsics to retrieve the correct value, for instance REAL*8 x can be replaced by:

INTEGER, PARAMETER :: dbl = KIND(1.0d0)
REAL(KIND=dbl) :: x

6.1.2 Old-style variable initialization

GNU Fortran allows old-style initialization of variables of the form:

      INTEGER i/1/,j/2/
      REAL x(2,2) /3*0.,1./

The syntax for the initializers is as for the DATA statement, but unlike in a DATA statement, an initializer only applies to the variable immediately preceding the initialization. In other words, something like INTEGER I,J/2,3/ is not valid. This style of initialization is only allowed in declarations without double colons (::); 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:

! 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./

Note that variables which are explicitly initialized in declarations or in DATA statements automatically acquire the SAVE attribute.

6.1.3 Extensions to 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 ‘$’ instead of ‘&’

$MYNML
 X(:)%Y(2) = 1.0 2.0 3.0
 CH(1:4) = "abcd"
$END

It should be noted that the default terminator is ‘/’ rather than ‘&END’.

Querying of the namelist when inputting from stdin. After at least one space, entering ‘?’ sends to stdout the namelist name and the names of the variables in the namelist:

 ?

&mynml
 x
 x%y
 ch
&end

Entering ‘=?’ outputs the namelist to stdout, as if WRITE(*,NML = mynml) had been called:

=?

&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,  /

To aid this dialog, when input is from stdin, errors send their messages to stderr and execution continues, even if IOSTAT is set.

PRINT namelist is permitted. This causes an error if -std=f95 is used.

PROGRAM test_print
  REAL, dimension (4)  ::  x = (/1.0, 2.0, 3.0, 4.0/)
  NAMELIST /mynml/ x
  PRINT mynml
END PROGRAM test_print

Expanded namelist reads are permitted. This causes an error if -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.

&MYNML
  X(1,1) = 0.00 , 1.00 , 2.00
/

6.1.4 X format descriptor without count field

To support legacy codes, GNU Fortran permits the count field of the X edit descriptor in FORMAT statements to be omitted. When omitted, the count is implicitly assumed to be one.

       PRINT 10, 2, 3
10     FORMAT (I1, X, I1)

6.1.5 Commas in FORMAT specifications

To support legacy codes, GNU Fortran allows the comma separator to be omitted immediately before and after character string edit descriptors in FORMAT statements.

       PRINT 10, 2, 3
10     FORMAT ('FOO='I1' BAR='I2)

6.1.6 Missing period in FORMAT specifications

To support legacy codes, GNU Fortran allows missing periods in format specifications if and only if -std=legacy is given on the command line. This is considered non-conforming code and is discouraged.

       REAL :: value
       READ(*,10) value
10     FORMAT ('F4')

6.1.7 I/O item lists

To support legacy codes, GNU Fortran allows the input item list of the READ statement, and the output item lists of the WRITE and PRINT statements, to start with a comma.

6.1.8 BOZ literal constants

Besides decimal constants, Fortran also supports binary (b), octal (o) and hexadecimal (z) integer constants. The syntax is: ‘prefix quote digits quote’, were the prefix is either b, o or z, quote is either ' or " and the digits are for binary 0 or 1, for octal between 0 and 7, and for hexadecimal between 0 and F. (Example: b'01011101'.)

Up to Fortran 95, BOZ literals were only allowed to initialize integer variables in DATA statements. Since Fortran 2003 BOZ literals are also allowed as argument of REAL, DBLE, INT and CMPLX; the result is the same as if the integer BOZ literal had been converted by TRANSFER to, respectively, real, double precision, integer or complex. As GNU Fortran extension the intrinsic procedures FLOAT, DFLOAT, COMPLEX and DCMPLX are treated alike.

As an extension, GNU Fortran allows hexadecimal BOZ literal constants to be specified using the X prefix, in addition to the standard Z prefix. The BOZ literal can also be specified by adding a suffix to the string, for example, Z'ABC' and 'ABC'Z are equivalent.

Furthermore, GNU Fortran allows using BOZ literal constants outside DATA statements and the four intrinsic functions allowed by Fortran 2003. In DATA statements, in direct assignments, where the right-hand side only contains a BOZ literal constant, and for old-style initializers of the form integer i /o'0173'/, the constant is transferred as if TRANSFER had been used; for COMPLEX numbers, only the real part is initialized unless CMPLX is used. In all other cases, the BOZ literal constant is converted to an INTEGER value with the largest decimal representation. This value is then converted numerically to the type and kind of the variable in question. (For instance, real :: r = b'0000001' + 1 initializes r with 2.0.) As different compilers implement the extension differently, one should be careful when doing bitwise initialization of non-integer variables.

Note that initializing an INTEGER variable with a statement such as DATA i/Z'FFFFFFFF'/ will give an integer overflow error rather than the desired result of -1 when i is a 32-bit integer on a system that supports 64-bit integers. The ‘-fno-range-check’ option can be used as a workaround for legacy code that initializes integers in this manner.

6.1.9 Real array indices

As an extension, GNU Fortran allows the use of REAL expressions or variables as array indices.

6.1.10 Unary operators

As an extension, GNU Fortran allows unary plus and unary minus operators to appear as the second operand of binary arithmetic operators without the need for parenthesis.

       X = Y * -Z

6.1.11 Implicitly convert LOGICAL and INTEGER values

As an extension for backwards compatibility with other compilers, GNU Fortran allows the implicit conversion of LOGICAL values to INTEGER values and vice versa. When converting from a LOGICAL to an INTEGER, .FALSE. is interpreted as zero, and .TRUE. is interpreted as one. When converting from INTEGER to LOGICAL, the value zero is interpreted as .FALSE. and any nonzero value is interpreted as .TRUE..

        LOGICAL :: l
        l = 1

        INTEGER :: i
        i = .TRUE.

However, there is no implicit conversion of INTEGER values in if-statements, nor of LOGICAL or INTEGER values in I/O operations.

6.1.12 Hollerith constants support

GNU Fortran supports Hollerith constants in assignments, function arguments, and DATA and ASSIGN statements. A Hollerith constant is written as a string of characters preceded by an integer constant indicating the character count, and the letter H or h, and stored in bytewise fashion in a numeric (INTEGER, REAL, or complex) or LOGICAL variable. The constant will be padded or truncated to fit the size of the variable in which it is stored.

Examples of valid uses of Hollerith constants:

      complex*16 x(2)
      data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/
      x(1) = 16HABCDEFGHIJKLMNOP
      call foo (4h abc)

Invalid Hollerith constants examples:

      integer*4 a
      a = 8H12345678 ! Valid, but the Hollerith constant will be truncated.
      a = 0H         ! At least one character is needed.

In general, Hollerith constants were used to provide a rudimentary facility for handling character strings in early Fortran compilers, prior to the introduction of 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 TRANSFER statement, as in this example.

      INTEGER(KIND=4) :: a
      a = TRANSFER ("abcd", a)     ! equivalent to: a = 4Habcd

6.1.13 Cray pointers

Cray pointers are part of a non-standard extension that provides a C-like pointer in Fortran. This is accomplished through a pair of variables: an integer "pointer" that holds a memory address, and a "pointee" that is used to dereference the pointer.

Pointer/pointee pairs are declared in statements of the form:

        pointer ( <pointer> , <pointee> )

or,

        pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ...

The pointer is an integer that is intended to hold a memory address. The pointee may be an array or scalar. A pointee can be an assumed size array—that is, the last dimension may be left unspecified by using a * in place of a value—but a pointee cannot be an assumed shape array. No space is allocated for the pointee.

The pointee may have its type declared before or after the pointer statement, and its array specification (if any) may be declared before, during, or after the pointer statement. The pointer may be declared as an integer prior to the pointer statement. However, some machines have default integer sizes that are different than the size of a pointer, and so the following code is not portable:

        integer ipt
        pointer (ipt, iarr)

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:

        real target(10)
        real pointee(10)
        pointer (ipt, pointee)
        ipt = loc (target)
        ipt = ipt + 1       

The last statement does not set ipt to the address of target(1), as it would in C pointer arithmetic. Adding 1 to ipt just adds one byte to the address stored in 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 LOC(). The LOC() function is equivalent to the & operator in C, except the address is cast to an integer type:

        real ar(10)
        pointer(ipt, arpte(10))
        real arpte
        ipt = loc(ar)  ! Makes arpte is an alias for ar
        arpte(1) = 1.0 ! Sets ar(1) to 1.0

The pointer can also be set by a call to the MALLOC intrinsic (see MALLOC).

Cray pointees often are used to alias an existing variable. For example:

        integer target(10)
        integer iarr(10)
        pointer (ipt, iarr)
        ipt = loc(target)

As long as ipt remains unchanged, iarr is now an alias for target. The optimizer, however, will not detect this aliasing, so it is unsafe to use iarr and 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 ALLOCATABLE, INTENT, OPTIONAL, DUMMY, TARGET, INTRINSIC, or POINTER attributes. Pointers may not have the DIMENSION, POINTER, TARGET, ALLOCATABLE, EXTERNAL, or INTRINSIC attributes. Pointees may not occur in more than one pointer statement. A pointee cannot be a pointer. Pointees cannot occur in equivalence, common, or data statements.

A Cray pointer may also point to a function or a subroutine. For example, the following excerpt is valid:

  implicit none
  external sub
  pointer (subptr,subpte)
  external subpte
  subptr = loc(sub)
  call subpte()
  [...]
  subroutine sub
  [...]
  end subroutine sub

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.

6.1.14 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 CONVERT specifier on the OPEN statement. See GFORTRAN_CONVERT_UNIT, for an alternative way of specifying the data format via an environment variable.

Valid values for CONVERT are:

• CONVERT='NATIVE' Use the native format. This is the default.
• CONVERT='SWAP' Swap between little- and big-endian.
• CONVERT='LITTLE_ENDIAN' Use the little-endian representation for unformatted files.
• CONVERT='BIG_ENDIAN' Use the big-endian representation for unformatted files.

Using the option could look like this:

  open(file='big.dat',form='unformatted',access='sequential', &
       convert='big_endian')

The value of the conversion can be queried by using INQUIRE(CONVERT=ch). The values returned are 'BIG_ENDIAN' and 'LITTLE_ENDIAN'.

CONVERT works between big- and little-endian for INTEGER values of all supported kinds and for 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 REAL(KIND=10) and REAL(KIND=16), will probably not work.

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.

6.1.15 OpenMP

OpenMP (Open Multi-Processing) is an application programming interface (API) that supports multi-platform shared memory multiprocessing programming in C/C++ and Fortran on many architectures, including Unix and Microsoft Windows platforms. It consists of a set of compiler directives, library routines, and environment variables that influence run-time behavior.

GNU Fortran strives to be compatible to the OpenMP Application Program Interface v3.0.

To enable the processing of the OpenMP directive !$omp in free-form source code; the c$omp, *$omp and !$omp directives in fixed form; the !$ conditional compilation sentinels in free form; and the c$, *$ and !$ sentinels in fixed form, gfortran needs to be invoked with the -fopenmp. This also arranges for automatic linking of the GNU OpenMP runtime library libgomp in GNU OpenMP runtime library.

The OpenMP Fortran runtime library routines are provided both in a form of a Fortran 90 module named omp_lib and in a form of a Fortran include file named omp_lib.h.

An example of a parallelized loop taken from Appendix A.1 of the OpenMP Application Program Interface v2.5:

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

Please note:

• -fopenmp implies -frecursive, i.e., all local arrays will be allocated on the stack. When porting existing code to OpenMP, this may lead to surprising results, especially to segmentation faults if the stacksize is limited.
• On glibc-based systems, OpenMP enabled applications cannot be statically linked due to limitations of the underlying pthreads-implementation. It might be possible to get a working solution if -Wl,--whole-archive -lpthread -Wl,--no-whole-archive is added to the command line. However, this is not supported by gcc and thus not recommended.

6.1.16 Argument list functions %VAL, %REF and %LOC

GNU Fortran supports argument list functions %VAL, %REF and %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.

%VAL passes a scalar argument by value, %REF passes it by reference and %LOC passes its memory location. Since gfortran already passes scalar arguments by reference, %REF is in effect a do-nothing. %LOC has the same effect as a Fortran pointer.

An example of passing an argument by value to a C subroutine foo.:

C
C prototype      void foo_ (float x);
C
      external foo
      real*4 x
      x = 3.14159
      call foo (%VAL (x))
      end

For details refer to the g77 manual http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/index.html#Top.

Also, c_by_val.f and its partner c_by_val.c of the GNU Fortran testsuite are worth a look.

6.2 Extensions not implemented in GNU Fortran

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.

6.2.1 STRUCTURE and RECORD

Structures are user-defined aggregate data types; this functionality was standardized in Fortran 90 with an different syntax, under the name of “derived types”. Here is an example of code using the non portable structure syntax:

! 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)

This code can easily be rewritten in the Fortran 90 syntax as following:

! ``STRUCTURE /name/ ... END STRUCTURE'' becomes
! ``TYPE name ... END TYPE''
TYPE item
  INTEGER id
  CHARACTER(LEN=200) description
  REAL price
END TYPE

! ``RECORD /name/ variable'' becomes ``TYPE(name) variable''
TYPE(item) pear, store_catalog(100)

! Instead of using a dot (.) to access fields of a record, the
! standard syntax uses a percent sign (%)
pear%id = 92316
pear%description = "juicy D'Anjou pear"
pear%price = 0.15
store_catalog(7)%id = 7831
store_catalog(7)%description = "milk bottle"
store_catalog(7)%price = 1.2

! Assignments of a whole variable don't change
store_catalog(12) = pear
print *, store_catalog(12)

6.2.2 ENCODE and DECODE statements

GNU Fortran doesn’t support the ENCODE and DECODE statements. These statements are best replaced by READ and WRITE statements involving internal files (CHARACTER variables and arrays), which have been part of the Fortran standard since Fortran 77. For example, replace a code fragment like

      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))

with the following:

      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))

Similarly, replace a code fragment like

      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))

with the following:

      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))

6.2.3 Variable FORMAT expressions

A variable FORMAT expression is format statement which includes angle brackets enclosing a Fortran expression: FORMAT(I<N>). GNU Fortran does not support this legacy extension. The effect of variable format expressions can be reproduced by using the more powerful (and standard) combination of internal output and string formats. For example, replace a code fragment like this:

      WRITE(6,20) INT1
 20   FORMAT(I<N+1>)

with the following:

c     Variable declaration
      CHARACTER(LEN=20) F
c     
c     Other code here...
c
      WRITE(FMT,'("(I", I0, ")")') N+1
      WRITE(6,FM) INT1

or with:

c     Variable declaration
      CHARACTER(LEN=20) FMT
c     
c     Other code here...
c
      WRITE(FMT,*) N+1
      WRITE(6,"(I" // ADJUSTL(FMT) // ")") INT1

7 Mixed-Language Programming

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.

7.1 Interoperability with C

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 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 A(n,m) in Fortran matches A[m][n] in C and accessing the element A(i,j) matches A[j-1][i-1]. The element following A(i,j) (C: A[j-1][i-1]; assuming i < n) in memory is A(i+1,j) (C: A[j-1][i]).

7.1.1 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 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 ISO_C_BINDING.

7.1.2 Derived Types and struct

For compatibility of derived types with struct, one needs to use the BIND(C) attribute in the type declaration. For instance, the following type declaration

 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

matches the following struct declaration in C

 struct {
   int i1, i2;
   /* Note: "char" might be signed or unsigned.  */
   signed char i3;
   double d1;
   float _Complex c1;
   char str[5];
 } myType;

Derived types with the C binding attribute shall not have the sequence attribute, type parameters, the extends attribute, nor type-bound procedures. Every component must be of interoperable type and kind and may not have the pointer or allocatable attribute. The names of the variables are irrelevant for interoperability.

As there exist no direct Fortran equivalents, neither unions nor structs with bit field or variable-length array members are interoperable.

7.1.3 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 MODULE, be of interoperable type, and have neither the pointer nor the allocatable attribute.

  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

Here, _MyProject_flags is the case-sensitive name of the variable as seen from C programs while global_flag is the case-insensitive name as seen from Fortran. If no binding name is specified, as for 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 errno of the C library as the C standard allows it to be a macro. Use the IERRNO intrinsic (GNU extension) instead.

7.1.4 Interoperable Subroutines and Functions

Subroutines and functions have to have the BIND(C) attribute to be compatible with C. The dummy argument declaration is relatively straightforward. However, one needs to be careful because C uses call-by-value by default while Fortran behaves usually similar to call-by-reference. Furthermore, strings and pointers are handled differently. Note that only explicit size and assumed-size arrays are supported but not assumed-shape or allocatable arrays.

To pass a variable by value, use the VALUE attribute. Thus the following C prototype

int func(int i, int *j)

matches the Fortran declaration

  integer(c_int) function func(i,j)
    use iso_c_binding, only: c_int
    integer(c_int), VALUE :: i
    integer(c_int) :: j

Note that pointer arguments also frequently need the VALUE attribute.

Strings are handled quite differently in C and Fortran. In C a string is a NUL-terminated array of characters while in Fortran each string has a length associated with it and is thus not terminated (by e.g. NUL). For example, if one wants to use the following C function,

  #include <stdio.h>
  void print_C(char *string) /* equivalent: char string[]  */
  {
     printf("%s\n", string);
  }

to print “Hello World” from Fortran, one can call it using

  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)

As the example shows, one needs to ensure that the string is NUL terminated. Additionally, the dummy argument string of print_C is a length-one assumed-size array; using character(len=*) is not allowed. The example above uses c_char_"Hello World" to ensure the string literal has the right type; typically the default character kind and c_char are the same and thus "Hello World" is equivalent. However, the standard does not guarantee this.

The use of pointers is now illustrated using the C library function strncpy, whose prototype is

  char *strncpy(char *restrict s1, const char *restrict s2, size_t n);

The function strncpy copies at most n characters from string s2 to s1 and returns s1. In the following example, we ignore the return value:

  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

C pointers are represented in Fortran via the special derived type type(c_ptr), with private components. Thus one needs to use intrinsic conversion procedures to convert from or to C pointers. For example,

  use iso_c_binding
  type(c_ptr) :: cptr1, cptr2
  integer, target :: array(7), scalar
  integer, pointer :: pa(:), ps
  cptr1 = c_loc(array(1)) ! The programmer needs to ensure that the
                          ! array is contiguous if required by the C
                          ! procedure
  cptr2 = c_loc(scalar)
  call c_f_pointer(cptr2, ps)
  call c_f_pointer(cptr2, pa, shape=[7])

When converting C to Fortran arrays, the one-dimensional SHAPE argument has to be passed. Note: A pointer argument void * matches TYPE(C_PTR), VALUE while TYPE(C_PTR) matches void **.

Procedure pointers are handled analogously to pointers; the C type is TYPE(C_FUNPTR) and the intrinsic conversion procedures are C_F_PROC_POINTER and C_FUNLOC.

The intrinsic procedures are described in Intrinsic Procedures.

7.1.5 Further Interoperability of Fortran with C

Assumed-shape and allocatable arrays are passed using an array descriptor (dope vector). The internal structure of the array descriptor used by GNU Fortran is not yet documented and will change. There will also be a Technical Report (TR 29113) which standardizes an interoperable array descriptor. Until then, you can use the Chasm Language Interoperability Tools, http://chasm-interop.sourceforge.net/, which provide an interface to GNU Fortran’s array descriptor.

The technical report 29113 will presumably also include support for C-interoperable OPTIONAL and for assumed-rank and assumed-type dummy arguments. However, the TR has neither been approved nor implemented in GNU Fortran; therefore, these features are not yet available.

7.2 GNU Fortran Compiler Directives

The Fortran standard 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 C Extensions in Using the GNU Compiler Collection (GCC) for details.

For procedures and procedure pointers, the following attributes can be used to change the calling convention:

• CDECL – standard C calling convention
• STDCALL – convention where the called procedure pops the stack
• FASTCALL – part of the arguments are passed via registers instead using the stack

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 COMMON blocks) need special handling to be accessible when they are in a shared library. The following attributes are available:

• DLLEXPORT – provide a global pointer to a pointer in the DLL
• DLLIMPORT – reference the function or variable using a global pointer

The attributes are specified using the syntax

!GCC$ ATTRIBUTES attribute-list :: variable-list

where in free-form source code only whitespace is allowed before !GCC$ and in fixed-form source code !GCC$, cGCC$ or *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.

7.3 Non-Fortran Main Program

Even if you are doing mixed-language programming, it is very likely that you do not need to know or use the information in this section. Since it is about the internal structure of GNU Fortran, it may also change in GCC minor releases.

When you compile a PROGRAM with GNU Fortran, a function with the name main (in the symbol table of the object file) is generated, which initializes the libgfortran library and then calls the actual program which uses the name MAIN__, for historic reasons. If you link GNU Fortran compiled procedures to, e.g., a C or C++ program or to a Fortran program compiled by a different compiler, the libgfortran library is not initialized and thus a few intrinsic procedures do not work properly, e.g. those for obtaining the command-line arguments.

Therefore, if your PROGRAM is not compiled with GNU Fortran and the GNU Fortran compiled procedures require intrinsics relying on the library initialization, you need to initialize the library yourself. Using the default options, gfortran calls _gfortran_set_args and _gfortran_set_options. The initialization of the former is needed if the called procedures access the command line (and for backtracing); the latter sets some flags based on the standard chosen or to enable backtracing. In typical programs, it is not necessary to call any initialization function.

If your PROGRAM is compiled with GNU Fortran, you shall not call any of the following functions. The libgfortran initialization functions are shown in C syntax but using C bindings they are also accessible from Fortran.

7.3.1 _gfortran_set_args — Save command-line arguments

Description:

_gfortran_set_args saves the command-line arguments; this initialization is required if any of the command-line intrinsics is called. Additionally, it shall be called if backtracing is enabled (see _gfortran_set_options).

Syntax:

void _gfortran_set_args (int argc, char *argv[])

Arguments:

argc	number of command line argument strings
argv	the command-line argument strings; argv[0] is the pathname of the executable itself.

Example:

int main (int argc, char *argv[])
{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  return 0;
}

7.3.2 _gfortran_set_options — Set library option flags

Description:

_gfortran_set_options sets several flags related to the Fortran standard to be used, whether backtracing or core dumps should be enabled and whether range checks should be performed. The syntax allows for upward compatibility since the number of passed flags is specified; for non-passed flags, the default value is used. See also see Code Gen Options. Please note that not all flags are actually used.

Syntax:

void _gfortran_set_options (int num, int options[])

Arguments:

num	number of options passed
argv	The list of flag values

option flag list:

option[0]	Allowed standard; can give run-time errors if e.g. an input-output edit descriptor is invalid in a given standard. Possible values are (bitwise or-ed) GFC_STD_F77 (1), GFC_STD_F95_OBS (2), GFC_STD_F95_DEL (4), GFC_STD_F95 (8), GFC_STD_F2003 (16), GFC_STD_GNU (32), GFC_STD_LEGACY (64), and GFC_STD_F2008 (128). Default: GFC_STD_F95_OBS | GFC_STD_F95_DEL | GFC_STD_F2003 | GFC_STD_F2008 | GFC_STD_F95 | GFC_STD_F77 | GFC_STD_GNU | GFC_STD_LEGACY.
option[1]	Standard-warning flag; prints a warning to standard error. Default: GFC_STD_F95_DEL | GFC_STD_LEGACY.
option[2]	If non zero, enable pedantic checking. Default: off.
option[3]	If non zero, enable core dumps on run-time errors. Default: off.
option[4]	If non zero, enable backtracing on run-time errors. Default: off. Note: Installs a signal handler and requires command-line initialization using _gfortran_set_args.
option[5]	If non zero, supports signed zeros. Default: enabled.
option[6]	Enables run-time checking. Possible values are (bitwise or-ed): GFC_RTCHECK_BOUNDS (1), GFC_RTCHECK_ARRAY_TEMPS (2), GFC_RTCHECK_RECURSION (4), GFC_RTCHECK_DO (16), GFC_RTCHECK_POINTER (32). Default: disabled.
option[7]	If non zero, range checking is enabled. Default: enabled. See -frange-check (see Code Gen Options).

Example:

  /* Use gfortran 4.5 default options.  */
  static int options[] = {68, 255, 0, 0, 0, 1, 0, 1};
  _gfortran_set_options (8, &options);

7.3.3 _gfortran_set_convert — Set endian conversion

Description:

_gfortran_set_convert set the representation of data for unformatted files.

Syntax:

void _gfortran_set_convert (int conv)

Arguments:

conv	Endian conversion, possible values: GFC_CONVERT_NATIVE (0, default), GFC_CONVERT_SWAP (1), GFC_CONVERT_BIG (2), GFC_CONVERT_LITTLE (3).

Example:

int main (int argc, char *argv[])
{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  _gfortran_set_convert (1);
  return 0;
}

7.3.4 _gfortran_set_record_marker — Set length of record markers

Description:

_gfortran_set_record_marker sets the length of record markers for unformatted files.

Syntax:

void _gfortran_set_record_marker (int val)

Arguments:

val	Length of the record marker; valid values are 4 and 8. Default is 4.

Example:

int main (int argc, char *argv[])
{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  _gfortran_set_record_marker (8);
  return 0;
}

7.3.5 _gfortran_set_fpe — Set when a Floating Point Exception should be raised

Description:

_gfortran_set_fpe sets the IEEE exceptions for which a Floating Point Exception (FPE) should be raised. On most systems, this will result in a SIGFPE signal being sent and the program being interrupted.

Syntax:

void _gfortran_set_fpe (int val)

Arguments:

option[0]

IEEE exceptions. Possible values are (bitwise or-ed) zero (0, default) no trapping, GFC_FPE_INVALID (1), GFC_FPE_DENORMAL (2), GFC_FPE_ZERO (4), GFC_FPE_OVERFLOW (8), GFC_FPE_UNDERFLOW (16), and GFC_FPE_PRECISION (32).

Example:

int main (int argc, char *argv[])
{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  /* FPE for invalid operations such as SQRT(-1.0).  */
  _gfortran_set_fpe (1);
  return 0;
}

7.3.6 _gfortran_set_max_subrecord_length — Set subrecord length

Description:

_gfortran_set_max_subrecord_length set the maximum length for a subrecord. This option only makes sense for testing and debugging of unformatted I/O.

Syntax:

void _gfortran_set_max_subrecord_length (int val)

Arguments:

val	the maximum length for a subrecord; the maximum permitted value is 2147483639, which is also the default.

Example:

int main (int argc, char *argv[])
{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  _gfortran_set_max_subrecord_length (8);
  return 0;
}

8 Intrinsic Procedures

8.1 Introduction to intrinsic procedures

The intrinsic procedures provided by GNU Fortran include all of the intrinsic procedures required by the Fortran 95 standard, a set of intrinsic procedures for backwards compatibility with G77, and a selection of intrinsic procedures from the Fortran 2003 and Fortran 2008 standards. Any conflict between a description here and a description in either the Fortran 95 standard, the Fortran 2003 standard or the Fortran 2008 standard is unintentional, and the standard(s) should be considered authoritative.

The enumeration of the KIND type parameter is processor defined in the Fortran 95 standard. GNU Fortran defines the default integer type and default real type by INTEGER(KIND=4) and REAL(KIND=4), respectively. The standard mandates that both data types shall have another kind, which have more precision. On typical target architectures supported by gfortran, this kind type parameter is KIND=8. Hence, REAL(KIND=8) and DOUBLE PRECISION are equivalent. In the description of generic intrinsic procedures, the kind type parameter will be specified by KIND=*, and in the description of specific names for an intrinsic procedure the kind type parameter will be explicitly given (e.g., REAL(KIND=4) or REAL(KIND=8)). Finally, for brevity the optional KIND= syntax will be omitted.

Many of the intrinsic procedures take one or more optional arguments. This document follows the convention used in the Fortran 95 standard, and denotes such arguments by square brackets.

GNU Fortran offers the -std=f95 and -std=gnu options, which can be used to restrict the set of intrinsic procedures to a given standard. By default, gfortran sets the -std=gnu option, and so all intrinsic procedures described here are accepted. There is one caveat. For a select group of intrinsic procedures, g77 implemented both a function and a subroutine. Both classes have been implemented in gfortran for backwards compatibility with g77. It is noted here that these functions and subroutines cannot be intermixed in a given subprogram. In the descriptions that follow, the applicable standard for each intrinsic procedure is noted.

8.2 ABORT — Abort the program

Description:

ABORT causes immediate termination of the program. On operating systems that support a core dump, ABORT will produce a core dump even if the option -fno-dump-core is in effect, which is suitable for debugging purposes.

Standard:

GNU extension

Class:

Subroutine

Syntax:

CALL ABORT

Return value:

Does not return.

Example:

program test_abort
  integer :: i = 1, j = 2
  if (i /= j) call abort
end program test_abort

See also:

EXIT, KILL

8.3 ABS — Absolute value

Description:

ABS(A) computes the absolute value of A.

Standard:

Fortran 77 and later, has overloads that are GNU extensions

Class:

Elemental function

Syntax:

RESULT = ABS(A)

Arguments:

A	The type of the argument shall be an INTEGER, REAL, or COMPLEX.

Return value:

The return value is of the same type and kind as the argument except the return value is REAL for a COMPLEX argument.

Example:

program test_abs
  integer :: i = -1
  real :: x = -1.e0
  complex :: z = (-1.e0,0.e0)
  i = abs(i)
  x = abs(x)
  x = abs(z)
end program test_abs

Specific names:

Name	Argument	Return type	Standard
CABS(A)	COMPLEX(4) Z	REAL(4)	Fortran 77 and later
DABS(A)	REAL(8) X	REAL(8)	Fortran 77 and later
IABS(A)	INTEGER(4) I	INTEGER(4)	Fortran 77 and later
ZABS(A)	COMPLEX(8) Z	COMPLEX(8)	GNU extension
CDABS(A)	COMPLEX(8) Z	COMPLEX(8)	GNU extension

8.4 ACCESS — Checks file access modes

Description:

ACCESS(NAME, MODE) checks whether the file NAME exists, is readable, writable or executable. Except for the executable check, ACCESS can be replaced by Fortran 95’s INQUIRE.

Standard:

GNU extension

Class:

Inquiry function

Syntax:

RESULT = ACCESS(NAME, MODE)

Arguments:

NAME	Scalar CHARACTER of default kind with the file name. Tailing blank are ignored unless the character achar(0) is present, then all characters up to and excluding achar(0) are used as file name.
MODE	Scalar CHARACTER of default kind with the file access mode, may be any concatenation of "r" (readable), "w" (writable) and "x" (executable), or " " to check for existence.

Return value:

Returns a scalar INTEGER, which is 0 if the file is accessible in the given mode; otherwise or if an invalid argument has been given for MODE the value 1 is returned.

Example:

program access_test
  implicit none
  character(len=*), parameter :: file  = 'test.dat'
  character(len=*), parameter :: file2 = 'test.dat  '//achar(0)
  if(access(file,' ') == 0) print *, trim(file),' is exists'
  if(access(file,'r') == 0) print *, trim(file),' is readable'
  if(access(file,'w') == 0) print *, trim(file),' is writable'
  if(access(file,'x') == 0) print *, trim(file),' is executable'
  if(access(file2,'rwx') == 0) &
    print *, trim(file2),' is readable, writable and executable'
end program access_test

Specific names:

See also:

8.5 ACHAR — Character in ASCII collating sequence

Description:

ACHAR(I) returns the character located at position I in the ASCII collating sequence.

Standard:

Fortran 77 and later, with KIND argument Fortran 2003 and later

Class:

Elemental function

Syntax:

RESULT = ACHAR(I [, KIND])

Arguments:

I	The type shall be INTEGER.
KIND	(Optional) An INTEGER initialization expression indicating the kind parameter of the result.

Return value:

The return value is of type CHARACTER with a length of one. If the KIND argument is present, the return value is of the specified kind and of the default kind otherwise.

Example:

program test_achar
  character c
  c = achar(32)
end program test_achar

Note:

See ICHAR for a discussion of converting between numerical values and formatted string representations.

See also:

CHAR, IACHAR, ICHAR

8.6 ACOS — Arccosine function

Description:

ACOS(X) computes the arccosine of X (inverse of COS(X)).

Standard:

Fortran 77 and later, for a complex argument Fortran 2008 or later

Class:

Elemental function

Syntax:

RESULT = ACOS(X)

Arguments:

X	The type shall either be REAL with a magnitude that is less than or equal to one - or the type shall be COMPLEX.

Return value:

The return value is of the same type and kind as X. The real part of the result is in radians and lies in the range 0 \leq \Re \acos(x) \leq \pi.

Example:

program test_acos
  real(8) :: x = 0.866_8
  x = acos(x)
end program test_acos

Specific names:

Name	Argument	Return type	Standard
DACOS(X)	REAL(8) X	REAL(8)	Fortran 77 and later

See also:

Inverse function: COS

8.7 ACOSH — Inverse hyperbolic cosine function

Description:

ACOSH(X) computes the inverse hyperbolic cosine of X.

Standard:

Fortran 2008 and later

Class:

Elemental function

Syntax:

RESULT = ACOSH(X)

Arguments:

X	The type shall be REAL or COMPLEX.

Return value:

The return value has the same type and kind as X. If X is complex, the imaginary part of the result is in radians and lies between 0 \leq \Im \acosh(x) \leq \pi.

Example:

PROGRAM test_acosh
  REAL(8), DIMENSION(3) :: x = (/ 1.0, 2.0, 3.0 /)
  WRITE (*,*) ACOSH(x)
END PROGRAM

Specific names:

Name	Argument	Return type	Standard
DACOSH(X)	REAL(8) X	REAL(8)	GNU extension

See also:

Inverse function: COSH

8.8 ADJUSTL — Left adjust a string

Description:

ADJUSTL(STRING) will left adjust a string by removing leading spaces. Spaces are inserted at the end of the string as needed.

Standard:

Fortran 90 and later

Class:

Elemental function

Syntax:

RESULT = ADJUSTL(STRING)

Arguments:

STRING

The type shall be CHARACTER.

Return value:

The return value is of type CHARACTER and of the same kind as STRING where leading spaces are removed and the same number of spaces are inserted on the end of STRING.

Example:

program test_adjustl
  character(len=20) :: str = '   gfortran'
  str = adjustl(str)
  print *, str
end program test_adjustl

See also:

ADJUSTR, TRIM

8.9 ADJUSTR — Right adjust a string

Description:

ADJUSTR(STRING) will right adjust a string by removing trailing spaces. Spaces are inserted at the start of the string as needed.

Standard:

Fortran 95 and later

Class:

Elemental function

Syntax:

RESULT = ADJUSTR(STRING)

Arguments:

STR	The type shall be CHARACTER.

Return value:

The return value is of type CHARACTER and of the same kind as STRING where trailing spaces are removed and the same number of spaces are inserted at the start of STRING.

Example:

program test_adjustr
  character(len=20) :: str = 'gfortran'
  str = adjustr(str)
  print *, str
end program test_adjustr

See also:

ADJUSTL, TRIM

8.10 AIMAG — Imaginary part of complex number

Description:

AIMAG(Z) yields the imaginary part of complex argument Z. The IMAG(Z) and IMAGPART(Z) intrinsic functions are provided for compatibility with g77, and their use in new code is strongly discouraged.

Standard:

Fortran 77 and later, has overloads that are GNU extensions

Class:

Elemental function

Syntax:

RESULT = AIMAG(Z)

Arguments:

Z	The type of the argument shall be COMPLEX.

Return value:

The return value is of type REAL with the kind type parameter of the argument.

Example:

program test_aimag
  complex(4) z4
  complex(8) z8
  z4 = cmplx(1.e0_4, 0.e0_4)
  z8 = cmplx(0.e0_8, 1.e0_8)
  print *, aimag(z4), dimag(z8)
end program test_aimag

Specific names:

Name	Argument	Return type	Standard
DIMAG(Z)	COMPLEX(8) Z	REAL(8)	GNU extension
IMAG(Z)	COMPLEX Z	REAL	GNU extension
IMAGPART(Z)	COMPLEX Z	REAL	GNU extension

8.11 AINT — Truncate to a whole number

Description:

AINT(A [, KIND]) truncates its argument to a whole number.

Standard:

Fortran 77 and later

Class:

Elemental function

Syntax:

RESULT = AINT(A [, KIND])

Arguments:

A	The type of the argument shall be REAL.
KIND	(Optional) An INTEGER initialization expression indicating the kind parameter of the result.

Return value:

The return value is of type REAL with the kind type parameter of the argument if the optional KIND is absent; otherwise, the kind type parameter will be given by KIND. If the magnitude of X is less than one, AINT(X) returns zero. If the magnitude is equal to or greater than one then it returns the largest whole number that does not exceed its magnitude. The sign is the same as the sign of X.

Example:

program test_aint
  real(4) x4
  real(8) x8
  x4 = 1.234E0_4
  x8 = 4.321_8
  print *, aint(x4), dint(x8)
  x8 = aint(x4,8)
end program test_aint

Specific names:

Name	Argument	Return type	Standard
DINT(X)	REAL(8) X	REAL(8)	Fortran 77 and later

8.12 ALARM — Execute a routine after a given delay

Description:

ALARM(SECONDS, HANDLER [, STATUS]) causes external subroutine HANDLER to be executed after a delay of SECONDS by using alarm(2) to set up a signal and signal(2) to catch it. If STATUS is supplied, it will be returned with the number of seconds remaining until any previously scheduled alarm was due to be delivered, or zero if there was no previously scheduled alarm.

Standard:

GNU extension

Class:

Subroutine

Syntax:

CALL ALARM(SECONDS, HANDLER [, STATUS])

Arguments:

SECONDS	The type of the argument shall be a scalar INTEGER. It is INTENT(IN).
HANDLER	Signal handler (INTEGER FUNCTION or SUBROUTINE) or dummy/global INTEGER scalar. The scalar values may be either SIG_IGN=1 to ignore the alarm generated or SIG_DFL=0 to set the default action. It is INTENT(IN).
STATUS	(Optional) STATUS shall be a scalar variable of the default INTEGER kind. It is INTENT(OUT).

Example:

program test_alarm
  external handler_print
  integer i
  call alarm (3, handler_print, i)
  print *, i
  call sleep(10)
end program test_alarm

This will cause the external routine handler_print to be called after 3 seconds.

8.13 ALL — All values in MASK along DIM are true

Description:

ALL(MASK [, DIM]) determines if all the values are true in MASK in the array along dimension DIM.

Standard:

Fortran 95 and later

Class:

Transformational function

Syntax:

RESULT = ALL(MASK [, DIM])

Arguments:

MASK	The type of the argument shall be LOGICAL and it shall not be scalar.
DIM	(Optional) DIM shall be a scalar integer with a value that lies between one and the rank of MASK.

Return value:

ALL(MASK) returns a scalar value of type LOGICAL where the kind type parameter is the same as the kind type parameter of MASK. If DIM is present, then ALL(MASK, DIM) returns an array with the rank of MASK minus 1. The shape is determined from the shape of MASK where the DIM dimension is elided.

(A)

ALL(MASK) is true if all elements of MASK are true. It also is true if MASK has zero size; otherwise, it is false.

(B)

If the rank of MASK is one, then ALL(MASK,DIM) is equivalent to ALL(MASK). If the rank is greater than one, then ALL(MASK,DIM) is determined by applying ALL to the array sections.

Example:

program test_all
  logical l
  l = all((/.true., .true., .true./))
  print *, l
  call section
  contains
    subroutine section
      integer a(2,3), b(2,3)
      a = 1
      b = 1
      b(2,2) = 2
      print *, all(a .eq. b, 1)
      print *, all(a .eq. b, 2)
    end subroutine section
end program test_all

8.14 ALLOCATED — Status of an allocatable entity

Description:

ALLOCATED(ARRAY) checks the status of whether X is allocated.

Standard:

Fortran 95 and later

Class:

Inquiry function

Syntax:

RESULT = ALLOCATED(ARRAY)

Arguments:

ARRAY

The argument shall be an ALLOCATABLE array.

Return value:

The return value is a scalar LOGICAL with the default logical kind type parameter. If ARRAY is allocated, ALLOCATED(ARRAY) is .TRUE.; otherwise, it returns .FALSE.

Example:

program test_allocated
  integer :: i = 4
  real(4), allocatable :: x(:)
  if (.not. allocated(x)) allocate(x(i))
end program test_allocated

8.15 AND — Bitwise logical AND

Description:

Bitwise logical AND.

This intrinsic routine is provided for backwards compatibility with GNU Fortran 77. For integer arguments, programmers should consider the use of the IAND intrinsic defined by the Fortran standard.

Standard:

GNU extension

Class:

Function

Syntax:

RESULT = AND(I, J)

Arguments:

I	The type shall be either a scalar INTEGER type or a scalar LOGICAL type.
J	The type shall be the same as the type of I.

Return value:

The return type is either a scalar INTEGER or a scalar LOGICAL. If the kind type parameters differ, then the smaller kind type is implicitly converted to larger kind, and the return has the larger kind.

Example:

PROGRAM test_and
  LOGICAL :: T = .TRUE., F = .FALSE.
  INTEGER :: a, b
  DATA a / Z'F' /, b / Z'3' /

  WRITE (*,*) AND(T, T), AND(T, F), AND(F, T), AND(F, F)
  WRITE (*,*) AND(a, b)
END PROGRAM

See also:

Fortran 95 elemental function: IAND

8.16 ANINT — Nearest whole number

Description:

ANINT(A [, KIND]) rounds its argument to the nearest whole number.

Standard:

Fortran 77 and later

Class:

Elemental function

Syntax:

RESULT = ANINT(A [, KIND])

Arguments:

A	The type of the argument shall be REAL.
KIND	(Optional) An INTEGER initialization expression indicating the kind parameter of the result.

Return value:

The return value is of type real with the kind type parameter of the argument if the optional KIND is absent; otherwise, the kind type parameter will be given by KIND. If A is greater than zero, ANINT(A) returns AINT(X+0.5). If A is less than or equal to zero then it returns AINT(X-0.5).

Example:

program test_anint
  real(4) x4
  real(8) x8
  x4 = 1.234E0_4
  x8 = 4.321_8
  print *, anint(x4), dnint(x8)
  x8 = anint(x4,8)
end program test_anint

Specific names:

Name	Argument	Return type	Standard
DNINT(A)	REAL(8) A	REAL(8)	Fortran 77 and later

8.17 ANY — Any value in MASK along DIM is true

Description:

ANY(MASK [, DIM]) determines if any of the values in the logical array MASK along dimension DIM are .TRUE..

Standard:

Fortran 95 and later

Class:

Transformational function

Syntax:

RESULT = ANY(MASK [, DIM])

Arguments:

MASK	The type of the argument shall be LOGICAL and it shall not be scalar.
DIM	(Optional) DIM shall be a scalar integer with a value that lies between one and the rank of MASK.

Return value:

ANY(MASK) returns a scalar value of type LOGICAL where the kind type parameter is the same as the kind type parameter of MASK. If DIM is present, then ANY(MASK, DIM) returns an array with the rank of MASK minus 1. The shape is determined from the shape of MASK where the DIM dimension is elided.

(A)

ANY(MASK) is true if any element of MASK is true; otherwise, it is false. It also is false if MASK has zero size.

(B)

If the rank of MASK is one, then ANY(MASK,DIM) is equivalent to ANY(MASK). If the rank is greater than one, then ANY(MASK,DIM) is determined by applying ANY to the array sections.

Example:

program test_any
  logical l
  l = any((/.true., .true., .true./))
  print *, l
  call section
  contains
    subroutine section
      integer a(2,3), b(2,3)
      a = 1
      b = 1
      b(2,2) = 2
      print *, any(a .eq. b, 1)
      print *, any(a .eq. b, 2)
    end subroutine section
end program test_any

8.18 ASIN — Arcsine function

Description:

ASIN(X) computes the arcsine of its X (inverse of SIN(X)).

Standard:

Fortran 77 and later, for a complex argument Fortran 2008 or later

Class:

Elemental function

Syntax:

RESULT = ASIN(X)

Arguments:

X	The type shall be either REAL and a magnitude that is less than or equal to one - or be COMPLEX.

Return value:

The return value is of the same type and kind as X. The real part of the result is in radians and lies in the range -\pi/2 \leq \Re \asin(x) \leq \pi/2.

Example:

program test_asin
  real(8) :: x = 0.866_8
  x = asin(x)
end program test_asin

Specific names:

Name	Argument	Return type	Standard
DASIN(X)	REAL(8) X	REAL(8)	Fortran 77 and later

See also:

Inverse function: SIN

8.19 ASINH — Inverse hyperbolic sine function

Description:

ASINH(X) computes the inverse hyperbolic sine of X.

Standard:

Fortran 2008 and later

Class:

Elemental function

Syntax:

RESULT = ASINH(X)

Arguments:

X	The type shall be REAL or COMPLEX.

Return value:

The return value is of the same type and kind as X. If X is complex, the imaginary part of the result is in radians and lies between -\pi/2 \leq \Im \asinh(x) \leq \pi/2.

Example:

PROGRAM test_asinh
  REAL(8), DIMENSION(3) :: x = (/ -1.0, 0.0, 1.0 /)
  WRITE (*,*) ASINH(x)
END PROGRAM

Specific names:

Name	Argument	Return type	Standard
DASINH(X)	REAL(8) X	REAL(8)	GNU extension.

See also:

Inverse function: SINH

8.20 ASSOCIATED — Status of a pointer or pointer/target pair

Description:

ASSOCIATED(POINTER [, TARGET]) determines the status of the pointer POINTER or if POINTER is associated with the target TARGET.

Standard:

Fortran 95 and later

Class:

Inquiry function

Syntax:

RESULT = ASSOCIATED(POINTER [, TARGET])

Arguments:

POINTER	POINTER shall have the POINTER attribute and it can be of any type.
TARGET	(Optional) TARGET shall be a pointer or a target. It must have the same type, kind type parameter, and array rank as POINTER.

The association status of neither POINTER nor TARGET shall be undefined.

Return value:

ASSOCIATED(POINTER) returns a scalar value of type LOGICAL(4). There are several cases:

(A) When the optional TARGET is not present then

ASSOCIATED(POINTER) is true if POINTER is associated with a target; otherwise, it returns false.

(B) If TARGET is present and a scalar target, the result is true if

TARGET is not a zero-sized storage sequence and the target associated with POINTER occupies the same storage units. If POINTER is disassociated, the result is false.

(C) If TARGET is present and an array target, the result is true if

TARGET and POINTER have the same shape, are not zero-sized arrays, are arrays whose elements are not zero-sized storage sequences, and TARGET and POINTER occupy the same storage units in array element order. As in case(B), the result is false, if POINTER is disassociated.

(D) If TARGET is present and an scalar pointer, the result is true

if TARGET is associated with POINTER, the target associated with TARGET are not zero-sized storage sequences and occupy the same storage units. The result is false, if either TARGET or POINTER is disassociated.

(E) If TARGET is present and an array pointer, the result is true if

target associated with POINTER and the target associated with TARGET have the same shape, are not zero-sized arrays, are arrays whose elements are not zero-sized storage sequences, and TARGET and POINTER occupy the same storage units in array element order. The result is false, if either TARGET or POINTER is disassociated.

Example:

program test_associated
   implicit none
   real, target  :: tgt(2) = (/1., 2./)
   real, pointer :: ptr(:)
   ptr => tgt
   if (associated(ptr)     .eqv. .false.) call abort
   if (associated(ptr,tgt) .eqv. .false.) call abort
end program test_associated

See also:

NULL

8.21 ATAN — Arctangent function

Description:

ATAN(X) computes the arctangent of X.

Standard:

Fortran 77 and later, for a complex argument and for two arguments Fortran 2008 or later

Class:

Elemental function

Syntax:

RESULT = ATAN(X) RESULT = ATAN(Y, X)

Arguments:

X	The type shall be REAL or COMPLEX; if Y is present, X shall be REAL.
Y shall be of the same type and kind as X.

Return value:

The return value is of the same type and kind as X. If Y is present, the result is identical to ATAN2(Y,X). Otherwise, it the arcus tangent of X, where the real part of the result is in radians and lies in the range -\pi/2 \leq \Re \atan(x) \leq \pi/2.

Example:

program test_atan
  real(8) :: x = 2.866_8
  x = atan(x)
end program test_atan

Specific names:

Name	Argument	Return type	Standard
DATAN(X)	REAL(8) X	REAL(8)	Fortran 77 and later

See also:

Inverse function: TAN

8.22 ATAN2 — Arctangent function

Description:

ATAN2(Y, X) computes the principal value of the argument function of the complex number X + i Y. This function can be used to transform from carthesian into polar coordinates and allows to determine the angle in the correct quadrant.

Standard:

Fortran 77 and later

Class:

Elemental function

Syntax:

RESULT = ATAN2(Y, X)

Arguments:

Y	The type shall be REAL.
X	The type and kind type parameter shall be the same as Y. If Y is zero, then X must be nonzero.

Return value:

The return value has the same type and kind type parameter as Y. It is the principal value of the complex number X + i Y. If X is nonzero, then it lies in the range -\pi \le \atan (x) \leq \pi. The sign is positive if Y is positive. If Y is zero, then the return value is zero if X is positive and \pi if X is negative. Finally, if X is zero, then the magnitude of the result is \pi/2.

Example:

program test_atan2
  real(4) :: x = 1.e0_4, y = 0.5e0_4
  x = atan2(y,x)
end program test_atan2

Specific names:

Name	Argument	Return type	Standard
DATAN2(X, Y)	REAL(8) X, REAL(8) Y	REAL(8)	Fortran 77 and later

8.23 ATANH — Inverse hyperbolic tangent function

Description:

ATANH(X) computes the inverse hyperbolic tangent of X.

Standard:

Fortran 2008 and later

Class:

Elemental function

Syntax:

RESULT = ATANH(X)

Arguments:

X	The type shall be REAL or COMPLEX.

Return value:

The return value has same type and kind as X. If X is complex, the imaginary part of the result is in radians and lies between -\pi/2 \leq \Im \atanh(x) \leq \pi/2.

Example:

PROGRAM test_atanh
  REAL, DIMENSION(3) :: x = (/ -1.0, 0.0, 1.0 /)
  WRITE (*,*) ATANH(x)
END PROGRAM

Specific names:

Name	Argument	Return type	Standard
DATANH(X)	REAL(8) X	REAL(8)	GNU extension

See also:

Inverse function: TANH