gfortranTMPDIR—Directory for scratch filesGFORTRAN_STDIN_UNIT—Unit number for standard inputGFORTRAN_STDOUT_UNIT—Unit number for standard outputGFORTRAN_STDERR_UNIT—Unit number for standard errorGFORTRAN_UNBUFFERED_ALL—Do not buffer I/O on all unitsGFORTRAN_UNBUFFERED_PRECONNECTED—Do not buffer I/O on preconnected unitsGFORTRAN_SHOW_LOCUS—Show location for runtime errorsGFORTRAN_OPTIONAL_PLUS—Print leading + where permittedGFORTRAN_DEFAULT_RECL—Default record length for new filesGFORTRAN_LIST_SEPARATOR—Separator for list outputGFORTRAN_CONVERT_UNIT—Set endianness for unformatted I/OGFORTRAN_ERROR_BACKTRACE—Show backtrace on run-time errorsX format descriptor without count fieldFORMAT specificationsFORMAT specificationsQ exponent-letterLOGICAL and INTEGER valuesCONVERT specifier%VAL, %REF and %LOC_gfortran_set_args — Save command-line arguments_gfortran_set_options — Set library option flags_gfortran_set_convert — Set endian conversion_gfortran_set_record_marker — Set length of record markers_gfortran_set_fpe — Enable floating point exception traps_gfortran_set_max_subrecord_length — Set subrecord length_gfortran_caf_init — Initialiation function_gfortran_caf_finish — Finalization function_gfortran_caf_this_image — Querying the image number_gfortran_caf_num_images — Querying the maximal number of images_gfortran_caf_register — Registering coarrays_gfortran_caf_deregister — Deregistering coarrays_gfortran_caf_send — Sending data from a local image to a remote image_gfortran_caf_get — Getting data from a remote image_gfortran_caf_sendget — Sending data between remote images_gfortran_caf_lock — Locking a lock variable_gfortran_caf_lock — Unlocking a lock variable_gfortran_caf_event_post — Post an event_gfortran_caf_event_wait — Wait that an event occurred_gfortran_caf_event_query — Query event count_gfortran_caf_sync_all — All-image barrier_gfortran_caf_sync_images — Barrier for selected images_gfortran_caf_sync_memory — Wait for completion of segment-memory operations_gfortran_caf_error_stop — Error termination with exit code_gfortran_caf_error_stop_str — Error termination with string_gfortran_caf_atomic_define — Atomic variable assignment_gfortran_caf_atomic_ref — Atomic variable reference_gfortran_caf_atomic_cas — Atomic compare and swap_gfortran_caf_atomic_op — Atomic operation_gfortran_caf_co_broadcast — Sending data to all images_gfortran_caf_co_max — Collective maximum reduction_gfortran_caf_co_min — Collective minimum reduction_gfortran_caf_co_sum — Collective summing reduction_gfortran_caf_co_reduce — Generic collective reductionABORT — Abort the programABS — Absolute valueACCESS — Checks file access modesACHAR — Character in ASCII collating sequenceACOS — Arccosine functionACOSH — Inverse hyperbolic cosine functionADJUSTL — Left adjust a stringADJUSTR — Right adjust a stringAIMAG — Imaginary part of complex numberAINT — Truncate to a whole numberALARM — Execute a routine after a given delayALL — All values in MASK along DIM are trueALLOCATED — Status of an allocatable entityAND — Bitwise logical ANDANINT — Nearest whole numberANY — Any value in MASK along DIM is trueASIN — Arcsine functionASINH — Inverse hyperbolic sine functionASSOCIATED — Status of a pointer or pointer/target pairATAN — Arctangent functionATAN2 — Arctangent functionATANH — Inverse hyperbolic tangent functionATOMIC_ADD — Atomic ADD operationATOMIC_AND — Atomic bitwise AND operationATOMIC_CAS — Atomic compare and swapATOMIC_DEFINE — Setting a variable atomicallyATOMIC_FETCH_ADD — Atomic ADD operation with prior fetchATOMIC_FETCH_AND — Atomic bitwise AND operation with prior fetchATOMIC_FETCH_OR — Atomic bitwise OR operation with prior fetchATOMIC_FETCH_XOR — Atomic bitwise XOR operation with prior fetchATOMIC_OR — Atomic bitwise OR operationATOMIC_REF — Obtaining the value of a variable atomicallyATOMIC_XOR — Atomic bitwise OR operationBACKTRACE — Show a backtraceBESSEL_J0 — Bessel function of the first kind of order 0BESSEL_J1 — Bessel function of the first kind of order 1BESSEL_JN — Bessel function of the first kindBESSEL_Y0 — Bessel function of the second kind of order 0BESSEL_Y1 — Bessel function of the second kind of order 1BESSEL_YN — Bessel function of the second kindBGE — Bitwise greater than or equal toBGT — Bitwise greater thanBIT_SIZE — Bit size inquiry functionBLE — Bitwise less than or equal toBLT — Bitwise less thanBTEST — Bit test functionC_ASSOCIATED — Status of a C pointerC_F_POINTER — Convert C into Fortran pointerC_F_PROCPOINTER — Convert C into Fortran procedure pointerC_FUNLOC — Obtain the C address of a procedureC_LOC — Obtain the C address of an objectC_SIZEOF — Size in bytes of an expressionCEILING — Integer ceiling functionCHAR — Character conversion functionCHDIR — Change working directoryCHMOD — Change access permissions of filesCMPLX — Complex conversion functionCO_BROADCAST — Copy a value to all images the current set of imagesCO_MAX — Maximal value on the current set of imagesCO_MIN — Minimal value on the current set of imagesCO_REDUCE — Reduction of values on the current set of imagesCO_SUM — Sum of values on the current set of imagesCOMMAND_ARGUMENT_COUNT — Get number of command line argumentsCOMPILER_OPTIONS — Options passed to the compilerCOMPILER_VERSION — Compiler version stringCOMPLEX — Complex conversion functionCONJG — Complex conjugate functionCOS — Cosine functionCOSH — Hyperbolic cosine functionCOUNT — Count functionCPU_TIME — CPU elapsed time in secondsCSHIFT — Circular shift elements of an arrayCTIME — Convert a time into a stringDATE_AND_TIME — Date and time subroutineDBLE — Double conversion functionDCMPLX — Double complex conversion functionDIGITS — Significant binary digits functionDIM — Positive differenceDOT_PRODUCT — Dot product functionDPROD — Double product functionDREAL — Double real part functionDSHIFTL — Combined left shiftDSHIFTR — Combined right shiftDTIME — Execution time subroutine (or function)EOSHIFT — End-off shift elements of an arrayEPSILON — Epsilon functionERF — Error functionERFC — Error functionERFC_SCALED — Error functionETIME — Execution time subroutine (or function)EXECUTE_COMMAND_LINE — Execute a shell commandEXIT — Exit the program with status.EXP — Exponential functionEXPONENT — Exponent functionEXTENDS_TYPE_OF — Query dynamic type for extensionFDATE — Get the current time as a stringFGET — Read a single character in stream mode from stdinFGETC — Read a single character in stream modeFLOOR — Integer floor functionFLUSH — Flush I/O unit(s)FNUM — File number functionFPUT — Write a single character in stream mode to stdoutFPUTC — Write a single character in stream modeFRACTION — Fractional part of the model representationFREE — Frees memoryFSEEK — Low level file positioning subroutineFSTAT — Get file statusFTELL — Current stream positionGAMMA — Gamma functionGERROR — Get last system error messageGETARG — Get command line argumentsGET_COMMAND — Get the entire command lineGET_COMMAND_ARGUMENT — Get command line argumentsGETCWD — Get current working directoryGETENV — Get an environmental variableGET_ENVIRONMENT_VARIABLE — Get an environmental variableGETGID — Group ID functionGETLOG — Get login nameGETPID — Process ID functionGETUID — User ID functionGMTIME — Convert time to GMT infoHOSTNM — Get system host nameHUGE — Largest number of a kindHYPOT — Euclidean distance functionIACHAR — Code in ASCII collating sequenceIALL — Bitwise AND of array elementsIAND — Bitwise logical andIANY — Bitwise OR of array elementsIARGC — Get the number of command line argumentsIBCLR — Clear bitIBITS — Bit extractionIBSET — Set bitICHAR — Character-to-integer conversion functionIDATE — Get current local time subroutine (day/month/year)IEOR — Bitwise logical exclusive orIERRNO — Get the last system error numberIMAGE_INDEX — Function that converts a cosubscript to an image indexINDEX — Position of a substring within a stringINT — Convert to integer typeINT2 — Convert to 16-bit integer typeINT8 — Convert to 64-bit integer typeIOR — Bitwise logical orIPARITY — Bitwise XOR of array elementsIRAND — Integer pseudo-random numberIS_IOSTAT_END — Test for end-of-file valueIS_IOSTAT_EOR — Test for end-of-record valueISATTY — Whether a unit is a terminal device.ISHFT — Shift bitsISHFTC — Shift bits circularlyISNAN — Test for a NaNITIME — Get current local time subroutine (hour/minutes/seconds)KILL — Send a signal to a processKIND — Kind of an entityLBOUND — Lower dimension bounds of an arrayLCOBOUND — Lower codimension bounds of an arrayLEADZ — Number of leading zero bits of an integerLEN — Length of a character entityLEN_TRIM — Length of a character entity without trailing blank charactersLGE — Lexical greater than or equalLGT — Lexical greater thanLINK — Create a hard linkLLE — Lexical less than or equalLLT — Lexical less thanLNBLNK — Index of the last non-blank character in a stringLOC — Returns the address of a variableLOG — Natural logarithm functionLOG10 — Base 10 logarithm functionLOG_GAMMA — Logarithm of the Gamma functionLOGICAL — Convert to logical typeLONG — Convert to integer typeLSHIFT — Left shift bitsLSTAT — Get file statusLTIME — Convert time to local time infoMALLOC — Allocate dynamic memoryMASKL — Left justified maskMASKR — Right justified maskMATMUL — matrix multiplicationMAX — Maximum value of an argument listMAXEXPONENT — Maximum exponent of a real kindMAXLOC — Location of the maximum value within an arrayMAXVAL — Maximum value of an arrayMCLOCK — Time functionMCLOCK8 — Time function (64-bit)MERGE — Merge variablesMERGE_BITS — Merge of bits under maskMIN — Minimum value of an argument listMINEXPONENT — Minimum exponent of a real kindMINLOC — Location of the minimum value within an arrayMINVAL — Minimum value of an arrayMOD — Remainder functionMODULO — Modulo functionMOVE_ALLOC — Move allocation from one object to anotherMVBITS — Move bits from one integer to anotherNEAREST — Nearest representable numberNEW_LINE — New line characterNINT — Nearest whole numberNORM2 — Euclidean vector normsNOT — Logical negationNULL — Function that returns an disassociated pointerNUM_IMAGES — Function that returns the number of imagesOR — Bitwise logical ORPACK — Pack an array into an array of rank onePARITY — Reduction with exclusive ORPERROR — Print system error messagePOPCNT — Number of bits setPOPPAR — Parity of the number of bits setPRECISION — Decimal precision of a real kindPRESENT — Determine whether an optional dummy argument is specifiedPRODUCT — Product of array elementsRADIX — Base of a model numberRAN — Real pseudo-random numberRAND — Real pseudo-random numberRANDOM_NUMBER — Pseudo-random numberRANDOM_SEED — Initialize a pseudo-random number sequenceRANGE — Decimal exponent rangeRANK — Rank of a data objectREAL — Convert to real typeRENAME — Rename a fileREPEAT — Repeated string concatenationRESHAPE — Function to reshape an arrayRRSPACING — Reciprocal of the relative spacingRSHIFT — Right shift bitsSAME_TYPE_AS — Query dynamic types for equalitySCALE — Scale a real valueSCAN — Scan a string for the presence of a set of charactersSECNDS — Time functionSECOND — CPU time functionSELECTED_CHAR_KIND — Choose character kindSELECTED_INT_KIND — Choose integer kindSELECTED_REAL_KIND — Choose real kindSET_EXPONENT — Set the exponent of the modelSHAPE — Determine the shape of an arraySHIFTA — Right shift with fillSHIFTL — Left shiftSHIFTR — Right shiftSIGN — Sign copying functionSIGNAL — Signal handling subroutine (or function)SIN — Sine functionSINH — Hyperbolic sine functionSIZE — Determine the size of an arraySIZEOF — Size in bytes of an expressionSLEEP — Sleep for the specified number of secondsSPACING — Smallest distance between two numbers of a given typeSPREAD — Add a dimension to an arraySQRT — Square-root functionSRAND — Reinitialize the random number generatorSTAT — Get file statusSTORAGE_SIZE — Storage size in bitsSUM — Sum of array elementsSYMLNK — Create a symbolic linkSYSTEM — Execute a shell commandSYSTEM_CLOCK — Time functionTAN — Tangent functionTANH — Hyperbolic tangent functionTHIS_IMAGE — Function that returns the cosubscript index of this imageTIME — Time functionTIME8 — Time function (64-bit)TINY — Smallest positive number of a real kindTRAILZ — Number of trailing zero bits of an integerTRANSFER — Transfer bit patternsTRANSPOSE — Transpose an array of rank twoTRIM — Remove trailing blank characters of a stringTTYNAM — Get the name of a terminal device.UBOUND — Upper dimension bounds of an arrayUCOBOUND — Upper codimension bounds of an arrayUMASK — Set the file creation maskUNLINK — Remove a file from the file systemUNPACK — Unpack an array of rank one into an arrayVERIFY — Scan a string for characters not a given setXOR — Bitwise logical exclusive ORNext: Introduction, Up: (dir) [Contents][Index]
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.
| • Introduction: | ||
Part I: Invoking GNU Fortran | ||
|---|---|---|
| • Invoking GNU Fortran: | Command options supported by gfortran.
| |
| • Runtime: | Influencing runtime behavior with environment variables. | |
Part II: Language Reference | ||
| • Fortran 2003 and 2008 status: | Fortran 2003 and 2008 features supported by GNU Fortran. | |
| • Compiler Characteristics: | User-visible implementation details. | |
| • Extensions: | Language extensions implemented by GNU Fortran. | |
| • Mixed-Language Programming: | Interoperability with C | |
| • Coarray Programming: | ||
| • Intrinsic Procedures: | Intrinsic procedures supported by GNU Fortran. | |
| • Intrinsic Modules: | Intrinsic modules supported by GNU Fortran. | |
| • Contributing: | How you can help. | |
| • Copying: | GNU General Public License says how you can copy and share GNU Fortran. | |
| • GNU Free Documentation License: | How you can copy and share this manual. | |
| • Funding: | How to help assure continued work for free software. | |
| • Option Index: | Index of command line options | |
| • Keyword Index: | Index of concepts | |
Next: Invoking GNU Fortran, Previous: Top, Up: Top [Contents][Index]
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 will use to invoke the compiler.
| • About GNU Fortran: | What you should know about the GNU Fortran compiler. | |
| • GNU Fortran and GCC: | You can compile Fortran, C, or other programs. | |
| • Preprocessing and conditional compilation: | The Fortran preprocessor | |
| • GNU Fortran and G77: | Why we chose to start from scratch. | |
| • Project Status: | Status of GNU Fortran, roadmap, proposed extensions. | |
| • Standards: | Standards supported by GNU Fortran. |
Next: GNU Fortran and GCC, Up: Introduction [Contents][Index]
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:
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.
gdb).
The GNU Fortran compiler consists of several components:
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.
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.
gfortran compilation phase,
such as intrinsic functions and subroutines,
and routines for interaction with files and the operating system.
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.
Next: Preprocessing and conditional compilation, Previous: About GNU Fortran, Up: Introduction [Contents][Index]
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.
Next: GNU Fortran and G77, Previous: GNU Fortran and GCC, Up: Introduction [Contents][Index]
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://www.daniellnagle.com/coco.html).
Next: Project Status, Previous: Preprocessing and conditional compilation, Up: Introduction [Contents][Index]
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.
Next: Standards, Previous: GNU Fortran and G77, Up: Introduction [Contents][Index]
As soon as
gfortrancan parse all of the statements correctly, it will be in the “larva” state. When we generate code, the “puppa” state. Whengfortranis 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, including TR 15581. However, it is still under development and has a few remaining rough edges. There also is initial support for OpenACC. Note that this is an experimental feature, incomplete, and subject to change in future versions of GCC. See https://gcc.gnu.org/wiki/OpenACC for more information.
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 HARMONIE and HIRLAM weather forecasting code and the Tonto quantum chemistry package; see https://gcc.gnu.org/wiki/GfortranApps for an extended list.
Among other things, the GNU Fortran compiler is intended as a replacement for G77. At this point, nearly all programs that could be compiled with G77 can be compiled with GNU Fortran, although there are a few minor known regressions.
The primary work remaining to be done on GNU Fortran falls into three categories: bug fixing (primarily regarding the treatment of invalid code and providing useful error messages), improving the compiler optimizations and the performance of compiled code, and extending the compiler to support future standards—in particular, Fortran 2003 and Fortran 2008.
Previous: Project Status, Up: Introduction [Contents][Index]
| • Varying Length Character Strings: |
The GNU Fortran compiler implements ISO/IEC 1539:1997 (Fortran 95). As such, it can also compile essentially all standard-compliant Fortran 90 and Fortran 77 programs. It also supports the ISO/IEC TR-15581 enhancements to allocatable arrays.
GNU Fortran also have a partial support for ISO/IEC 1539-1:2004 (Fortran
2003), ISO/IEC 1539-1:2010 (Fortran 2008), the Technical Specification
Further Interoperability of Fortran with C (ISO/IEC TS 29113:2012).
Full support of those standards and future Fortran standards is planned.
The current status of the support is can be found in the
Fortran 2003 status, Fortran 2008 status and
TS 29113 status sections of the documentation.
Additionally, the GNU Fortran compilers supports the OpenMP specification (version 4.0, http://openmp.org/wp/openmp-specifications/). There also is initial support for the OpenACC specification (targeting version 2.0, http://www.openacc.org/). Note that this is an experimental feature, incomplete, and subject to change in future versions of GCC. See https://gcc.gnu.org/wiki/OpenACC for more information.
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/.
Deferred-length character strings of Fortran 2003 supports part of
the features of ISO_VARYING_STRING and should be considered as
replacement. (Namely, allocatable or pointers of the type
character(len=:).)
Next: Runtime, Previous: Introduction, Up: Top [Contents][Index]
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.
| • Option Summary: | Brief list of all gfortran options,
without explanations.
| |
| • Fortran Dialect Options: | Controlling the variant of Fortran language compiled. | |
| • Preprocessing Options: | Enable and customize preprocessing. | |
| • Error and Warning Options: | How picky should the compiler be? | |
| • Debugging Options: | Symbol tables, measurements, and debugging dumps. | |
| • Directory Options: | Where to find module files | |
| • Link Options : | Influencing the linking step | |
| • Runtime Options: | Influencing runtime behavior | |
| • Code Gen Options: | Specifying conventions for function calls, data layout and register usage. | |
| • Environment Variables: | Environment variables that affect gfortran.
|
Next: Fortran Dialect Options, Up: Invoking GNU Fortran [Contents][Index]
Here is a summary of all the options specific to GNU Fortran, grouped by type. Explanations are in the following sections.
See Options controlling Fortran dialect.
-fall-intrinsics -fbackslash -fcray-pointer -fd-lines-as-code -fd-lines-as-comments -fdefault-double-8 -fdefault-integer-8 -fdefault-real-8 -fdollar-ok -ffixed-line-length-n -ffixed-line-length-none -ffree-form -ffree-line-length-n -ffree-line-length-none -fimplicit-none -finteger-4-integer-8 -fmax-identifier-length -fmodule-private -ffixed-form -fno-range-check -fopenacc -fopenmp -freal-4-real-10 -freal-4-real-16 -freal-4-real-8 -freal-8-real-10 -freal-8-real-16 -freal-8-real-4 -std=std
See Enable and customize preprocessing.
-A-question[=answer] -Aquestion=answer -C -CC -Dmacro[=defn] -H -P -Umacro -cpp -dD -dI -dM -dN -dU -fworking-directory -imultilib dir -iprefix file -iquote -isysroot dir -isystem dir -nocpp -nostdinc -undef
See Options to request or suppress errors and warnings.
-Waliasing -Wall -Wampersand -Warray-bounds -Wc-binding-type -Wcharacter-truncation -Wconversion -Wfunction-elimination -Wimplicit-interface -Wimplicit-procedure -Wintrinsic-shadow -Wuse-without-only -Wintrinsics-std -Wline-truncation -Wno-align-commons -Wno-tabs -Wreal-q-constant -Wsurprising -Wunderflow -Wunused-parameter -Wrealloc-lhs -Wrealloc-lhs-all -Wtarget-lifetime -fmax-errors=n -fsyntax-only -pedantic -pedantic-errors
See Options for debugging your program or GNU Fortran.
-fbacktrace -fdump-fortran-optimized -fdump-fortran-original -fdump-parse-tree -ffpe-trap=list -ffpe-summary=list
See Options for directory search.
-Idir -Jdir -fintrinsic-modules-path dir
See Options for influencing the linking step.
-static-libgfortran
See Options for influencing runtime behavior.
-fconvert=conversion -fmax-subrecord-length=length -frecord-marker=length -fsign-zero
See Options for code generation conventions.
-faggressive-function-elimination -fblas-matmul-limit=n -fbounds-check -fcheck-array-temporaries -fcheck=<all|array-temps|bounds|do|mem|pointer|recursion> -fcoarray=<none|single|lib> -fexternal-blas -ff2c -ffrontend-optimize -finit-character=n -finit-integer=n -finit-local-zero -finit-logical=<true|false> -finit-real=<zero|inf|-inf|nan|snan> -fmax-array-constructor=n -fmax-stack-var-size=n -fno-align-commons -fno-automatic -fno-protect-parens -fno-underscoring -fsecond-underscore -fpack-derived -frealloc-lhs -frecursive -frepack-arrays -fshort-enums -fstack-arrays
Next: Preprocessing Options, Previous: Option Summary, Up: Invoking GNU Fortran [Contents][Index]
The following options control the details of the Fortran dialect accepted by the compiler:
-ffree-form-ffixed-formSpecify 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-intrinsicsThis 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-commentsEnable 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.
-fdollar-okAllow ‘$’ as a valid non-first character in a symbol name. Symbols
that start with ‘$’ 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.
-fbackslashChange 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-privateSet 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-nSet 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-nSet 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=nSpecify the maximum allowed identifier length. Typical values are 31 (Fortran 95) and 63 (Fortran 2003 and Fortran 2008).
-fimplicit-noneSpecify 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-pointerEnable the Cray pointer extension, which provides C-like pointer functionality.
-fopenaccEnable the OpenACC extensions. This includes OpenACC !$acc
directives in free form and c$acc, *$acc and
!$acc directives in fixed form, !$ conditional
compilation sentinels in free form and c$, *$ and
!$ sentinels in fixed form, and when linking arranges for the
OpenACC runtime library to be linked in.
Note that this is an experimental feature, incomplete, and subject to change in future versions of GCC. See https://gcc.gnu.org/wiki/OpenACC for more information.
-fopenmpEnable 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-checkDisable 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.
-fdefault-integer-8Set the default integer and logical types to an 8 byte wide type. This option
also affects the kind of integer constants like 42. Unlike
-finteger-4-integer-8, it does not promote variables with explicit
kind declaration.
-fdefault-real-8Set the default real type to an 8 byte wide type. 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. Unlike -freal-4-real-8,
it does not promote variables with explicit kind declaration.
-fdefault-double-8Set the DOUBLE PRECISION type to an 8 byte wide type. Do nothing if this
is already the default. 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.
-finteger-4-integer-8Promote all INTEGER(KIND=4) entities to an INTEGER(KIND=8)
entities. If KIND=8 is unavailable, then an error will be issued.
This option should be used with care and may not be suitable for your codes.
Areas of possible concern include calls to external procedures,
alignment in EQUIVALENCE and/or COMMON, generic interfaces,
BOZ literal constant conversion, and I/O. Inspection of the intermediate
representation of the translated Fortran code, produced by
-fdump-tree-original, is suggested.
-freal-4-real-8-freal-4-real-10-freal-4-real-16-freal-8-real-4-freal-8-real-10-freal-8-real-16Promote all REAL(KIND=M) entities to REAL(KIND=N) entities.
If REAL(KIND=N) is unavailable, then an error will be issued.
All other real kind types are unaffected by this option.
These options should be used with care and may not be suitable for your
codes. Areas of possible concern include calls to external procedures,
alignment in EQUIVALENCE and/or COMMON, generic interfaces,
BOZ literal constant conversion, and I/O. Inspection of the intermediate
representation of the translated Fortran code, produced by
-fdump-tree-original, is suggested.
-std=stdSpecify 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. ‘-std=f2008ts’ allows the Fortran 2008 standard including the additions of the Technical Specification (TS) 29113 on Further Interoperability of Fortran with C and TS 18508 on Additional Parallel Features in Fortran.
Next: Error and Warning Options, Previous: Fortran Dialect Options, Up: Invoking GNU Fortran [Contents][Index]
Preprocessor related options. See section
Preprocessing and conditional compilation for more detailed
information on preprocessing in gfortran.
-cpp-nocppEnable 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. Any restrictions of the file-format, especially the limits on line length, apply for preprocessed output as well, so it might be advisable to use the -ffree-line-length-none or -ffixed-line-length-none options.
-dMInstead 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 -E -dM foo.f90
will show all the predefined macros.
-dDLike -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.
-dNLike -dD, but emit only the macro names, not their expansions.
-dULike 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.
-dIOutput '#include' directives in addition to the result
of preprocessing.
-fworking-directoryEnable 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 is 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 dirSearch 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 dirUse dir as a subdirectory of the directory containing target-specific C++ headers.
-iprefix prefixSpecify prefix as the prefix for subsequent -iwithprefix
options. If the prefix represents a directory, you should include
the final '/'.
-isysroot dirThis option is like the --sysroot option, but applies only to header files. See the --sysroot option for more information.
-iquote dirSearch 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 dirSearch 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.
-nostdincDo 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.
-undefDo not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.
-Apredicate=answerMake 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=answerCancel an assertion with the predicate predicate and answer answer.
-CDo 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.
-CCDo 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.
-DnamePredefine name as a macro, with definition 1.
-Dname=definitionThe 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.
-HPrint 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.
-PInhibit 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.
-UnameCancel any previous definition of name, either built in or provided with a -D option.
Next: Debugging Options, Previous: Preprocessing Options, Up: Invoking GNU Fortran [Contents][Index]
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=nLimits 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-onlyCheck the code for syntax errors, but do not actually compile it. This will generate module files for each module present in the code, but no other output file.
-pedanticIssue 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-errorsLike -pedantic, except that errors are produced rather than warnings.
-WallEnables commonly used warning options pertaining to usage that we recommend avoiding and that we believe are easy to avoid. This currently includes -Waliasing, -Wampersand, -Wconversion, -Wsurprising, -Wc-binding-type, -Wintrinsics-std, -Wtabs, -Wintrinsic-shadow, -Wline-truncation, -Wtarget-lifetime, -Wreal-q-constant and -Wunused.
-WaliasingWarn 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)
-WampersandWarn 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-temporariesWarn about array temporaries generated by the compiler. The information generated by this warning is sometimes useful in optimization, in order to avoid such temporaries.
-Wc-binding-typeWarn if the a variable might not be C interoperable. In particular, warn if
the variable has been declared using an intrinsic type with default kind
instead of using a kind parameter defined for C interoperability in the
intrinsic ISO_C_Binding module. This option is implied by
-Wall.
-Wcharacter-truncationWarn when a character assignment will truncate the assigned string.
-Wline-truncationWarn when a source code line will be truncated. This option is implied by -Wall. For free-form source code, the default is -Werror=line-truncation such that truncations are reported as error.
-WconversionWarn about implicit conversions that are likely to change the value of the expression after conversion. Implied by -Wall.
-Wconversion-extraWarn about implicit conversions between different types and kinds. This option does not imply -Wconversion.
-WextraEnables some warning options for usages of language features which may be problematic. This currently includes -Wcompare-reals and -Wunused-parameter.
-Wimplicit-interfaceWarn 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-procedureWarn if a procedure is called that has neither an explicit interface
nor has been declared as EXTERNAL.
-Wintrinsics-stdWarn 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.
-Wreal-q-constantProduce a warning if a real-literal-constant contains a q
exponent-letter.
-WsurprisingProduce 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:
CHARACTER variable is declared with negative length.
-WtabsBy 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. -Wtabs will cause a warning to be issued if a tab is encountered. Note, -Wtabs is active for -pedantic, -std=f95, -std=f2003, -std=f2008, -std=f2008ts and -Wall.
-WunderflowProduce a warning when numerical constant expressions are encountered, which yield an UNDERFLOW during compilation. Enabled by default.
-Wintrinsic-shadowWarn 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. This option is implied by -Wall.
-Wuse-without-onlyWarn if a USE statement has no ONLY qualifier and
thus implicitly imports all public entities of the used module.
-Wunused-dummy-argumentWarn about unused dummy arguments. This option is implied by -Wall.
-Wunused-parameterContrary to gcc’s meaning of -Wunused-parameter,
gfortran’s implementation of this option does not warn
about unused dummy arguments (see -Wunused-dummy-argument),
but about unused PARAMETER values. -Wunused-parameter
is implied by -Wextra if also -Wunused or
-Wall is used.
-Walign-commonsBy 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.
-Wfunction-eliminationWarn if any calls to functions are eliminated by the optimizations enabled by the -ffrontend-optimize option.
-Wrealloc-lhsWarn when the compiler might insert code to for allocation or reallocation of
an allocatable array variable of intrinsic type in intrinsic assignments. In
hot loops, the Fortran 2003 reallocation feature may reduce the performance.
If the array is already allocated with the correct shape, consider using a
whole-array array-spec (e.g. (:,:,:)) for the variable on the left-hand
side to prevent the reallocation check. Note that in some cases the warning
is shown, even if the compiler will optimize reallocation checks away. For
instance, when the right-hand side contains the same variable multiplied by
a scalar. See also -frealloc-lhs.
-Wrealloc-lhs-allWarn when the compiler inserts code to for allocation or reallocation of an allocatable variable; this includes scalars and derived types.
-Wcompare-realsWarn when comparing real or complex types for equality or inequality. This option is implied by -Wextra.
-Wtarget-lifetimeWarn if the pointer in a pointer assignment might be longer than the its target. This option is implied by -Wall.
-WzerotripWarn if a DO loop is known to execute zero times at compile
time. This option is implied by -Wall.
-WerrorTurns 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.
Next: Directory Options, Previous: Error and Warning Options, Up: Invoking GNU Fortran [Contents][Index]
GNU Fortran has various special options that are used for debugging either your program or the GNU Fortran compiler.
-fdump-fortran-originalOutput the internal parse tree after translating the source program into internal representation. Only really useful for debugging the GNU Fortran compiler itself.
-fdump-fortran-optimizedOutput the parse tree after front-end optimization. Only really useful for debugging the GNU Fortran compiler itself.
-fdump-parse-treeOutput the internal parse tree after translating the source program
into internal representation. Only really useful for debugging the
GNU Fortran compiler itself. This option is deprecated; use
-fdump-fortran-original instead.
-ffpe-trap=listSpecify a list of floating point exception traps to enable. On most
systems, if a floating point exception occurs and the trap for that
exception is enabled, a SIGFPE signal will be sent and the program
being aborted, producing a core file useful for debugging. list
is a (possibly empty) comma-separated list of the following
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), ‘inexact’ (loss of precision
during operation), and ‘denormal’ (operation performed on a
denormal value). The first five exceptions correspond to the five
IEEE 754 exceptions, whereas the last one (‘denormal’) is not
part of the IEEE 754 standard but is available on some common
architectures such as x86.
The first three exceptions (‘invalid’, ‘zero’, and ‘overflow’) often indicate serious errors, and unless the program has provisions for dealing with these exceptions, enabling traps for these three exceptions is probably a good idea.
Many, if not most, floating point operations incur loss of precision
due to rounding, and hence the ffpe-trap=inexact is likely to
be uninteresting in practice.
By default no exception traps are enabled.
-ffpe-summary=listSpecify a list of floating-point exceptions, whose flag status is printed
to ERROR_UNIT when invoking STOP and ERROR STOP.
list can be either ‘none’, ‘all’ or a comma-separated list
of the following exceptions: ‘invalid’, ‘zero’, ‘overflow’,
‘underflow’, ‘inexact’ and ‘denormal’. (See
-ffpe-trap for a description of the exceptions.)
By default, a summary for all exceptions but ‘inexact’ is shown.
-fno-backtraceWhen a serious runtime error is encountered or a deadly signal is
emitted (segmentation fault, illegal instruction, bus error,
floating-point exception, and the other POSIX signals that have the
action ‘core’), the Fortran runtime library tries to output a
backtrace of the error. -fno-backtrace disables the backtrace
generation. This option only has influence for 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.
Next: Link Options, Previous: Debugging Options, Up: Invoking GNU Fortran [Contents][Index]
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.
-IdirThese 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.
-JdirThis 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 dirThis option specifies the location of pre-compiled intrinsic modules, if they are not in the default location expected by the compiler.
Next: Runtime Options, Previous: Directory Options, Up: Invoking GNU Fortran [Contents][Index]
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-libgfortranOn 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.
Next: Code Gen Options, Previous: Link Options, Up: Invoking GNU Fortran [Contents][Index]
These options affect the runtime behavior of programs compiled with GNU Fortran.
-fconvert=conversionSpecify 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.
-frecord-marker=lengthSpecify 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=lengthSpecify 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-zeroWhen 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 (or values rounded to zero for I/O)
and regards zero as positive number in the SIGN intrinsic for
compatibility with Fortran 77. The default is -fsign-zero.
Next: Environment Variables, Previous: Runtime Options, Up: Invoking GNU Fortran [Contents][Index]
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-automaticTreat 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.
-ff2cGenerate 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-underscoringDo 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 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.
-fsecond-underscoreBy 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.
-fcoarray=<keyword>Disable coarray support; using coarray declarations and image-control statements will produce a compile-time error. (Default)
Single-image mode, i.e. num_images() is always one.
Library-based coarray parallelization; a suitable GNU Fortran coarray library needs to be linked.
-fcheck=<keyword>Enable the generation of run-time checks; the argument shall be a comma-delimited list of the following keywords.
Enable all run-time test of -fcheck.
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.
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.
Enable generation of run-time checks for invalid modification of loop iteration variables.
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.
Enable generation of run-time checks for pointers and allocatables.
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-checkDeprecated alias for -fcheck=bounds.
-fcheck-array-temporariesDeprecated alias for -fcheck=array-temps.
-fmax-array-constructor=nThis 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=nThis 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.
-fstack-arraysAdding this option will make the Fortran compiler put all local arrays, even those of unknown size onto stack memory. If your program uses very large local arrays it is possible that you will have to extend your runtime limits for stack memory on some operating systems. This flag is enabled by default at optimization level -Ofast.
-fpack-derivedThis 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-arraysIn 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-enumsThis 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-blasThis 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=nOnly 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.
-frecursiveAllow 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=nThe -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
EQUIVALENCE statement.
(These limitations 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).
Finally, note that enabling any of the -finit-* options will silence warnings that would have been emitted by -Wuninitialized for the affected local variables.
-falign-commonsBy 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 largest to smallest.
-fno-protect-parensBy 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. The parentheses protection is enabled by default, unless
-Ofast is given.
-frealloc-lhsAn allocatable left-hand side of an intrinsic assignment is automatically (re)allocated if it is either unallocated or has a different shape. The option is enabled by default except when -std=f95 is given. See also -Wrealloc-lhs.
-faggressive-function-eliminationFunctions with identical argument lists are eliminated within
statements, regardless of whether these functions are marked
PURE or not. For example, in
a = f(b,c) + f(b,c)
there will only be a single call to f. This option only works
if -ffrontend-optimize is in effect.
-ffrontend-optimizeThis option performs front-end optimization, based on manipulating
parts the Fortran parse tree. Enabled by default by any -O
option. Optimizations enabled by this option include elimination of
identical function calls within expressions, removing unnecessary
calls to TRIM in comparisons and assignments and replacing
TRIM(a) with a(1:LEN_TRIM(a)).
It can be deselected by specifying -fno-frontend-optimize.
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.
Previous: Code Gen Options, Up: Invoking GNU Fortran [Contents][Index]
gfortranThe 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.
Next: Fortran 2003 and 2008 status, Previous: Invoking GNU Fortran, Up: Top [Contents][Index]
The behavior of the gfortran can be influenced by
environment variables.
Malformed environment variables are silently ignored.
| • TMPDIR: | Directory for scratch files | |
| • GFORTRAN_STDIN_UNIT: | Unit number for standard input | |
| • GFORTRAN_STDOUT_UNIT: | Unit number for standard output | |
| • GFORTRAN_STDERR_UNIT: | Unit number for standard error | |
| • GFORTRAN_UNBUFFERED_ALL: | Do not buffer I/O for all units. | |
| • GFORTRAN_UNBUFFERED_PRECONNECTED: | Do not buffer I/O for preconnected units. | |
| • GFORTRAN_SHOW_LOCUS: | Show location for runtime errors | |
| • GFORTRAN_OPTIONAL_PLUS: | Print leading + where permitted | |
| • GFORTRAN_DEFAULT_RECL: | Default record length for new files | |
| • GFORTRAN_LIST_SEPARATOR: | Separator for list output | |
| • GFORTRAN_CONVERT_UNIT: | Set endianness for unformatted I/O | |
| • GFORTRAN_ERROR_BACKTRACE: | Show backtrace on run-time errors |
Next: GFORTRAN_STDIN_UNIT, Up: Runtime [Contents][Index]
TMPDIR—Directory for scratch filesWhen opening a file with STATUS='SCRATCH', GNU Fortran tries to
create the file in one of the potential directories by testing each
directory in the order below.
TMPDIR, if it exists.
GetTempPath
function. Alternatively, on the Cygwin target, the TMP and
TEMP environment variables, if they exist, in that order.
P_tmpdir macro if it is defined, otherwise the directory
/tmp.
Next: GFORTRAN_STDOUT_UNIT, Previous: TMPDIR, Up: Runtime [Contents][Index]
GFORTRAN_STDIN_UNIT—Unit number for standard inputThis 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.
Next: GFORTRAN_STDERR_UNIT, Previous: GFORTRAN_STDIN_UNIT, Up: Runtime [Contents][Index]
GFORTRAN_STDOUT_UNIT—Unit number for standard outputThis 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.
Next: GFORTRAN_UNBUFFERED_ALL, Previous: GFORTRAN_STDOUT_UNIT, Up: Runtime [Contents][Index]
GFORTRAN_STDERR_UNIT—Unit number for standard errorThis 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.
Next: GFORTRAN_UNBUFFERED_PRECONNECTED, Previous: GFORTRAN_STDERR_UNIT, Up: Runtime [Contents][Index]
GFORTRAN_UNBUFFERED_ALL—Do not buffer I/O on all unitsThis 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.
Next: GFORTRAN_SHOW_LOCUS, Previous: GFORTRAN_UNBUFFERED_ALL, Up: Runtime [Contents][Index]
GFORTRAN_UNBUFFERED_PRECONNECTED—Do not buffer I/O on preconnected unitsThe 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.
Next: GFORTRAN_OPTIONAL_PLUS, Previous: GFORTRAN_UNBUFFERED_PRECONNECTED, Up: Runtime [Contents][Index]
GFORTRAN_SHOW_LOCUS—Show location for runtime errorsIf 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’, do not print filename and line numbers for runtime errors. The default is to print the location.
Next: GFORTRAN_DEFAULT_RECL, Previous: GFORTRAN_SHOW_LOCUS, Up: Runtime [Contents][Index]
GFORTRAN_OPTIONAL_PLUS—Print leading + where permittedIf 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.
Next: GFORTRAN_LIST_SEPARATOR, Previous: GFORTRAN_OPTIONAL_PLUS, Up: Runtime [Contents][Index]
GFORTRAN_DEFAULT_RECL—Default record length for new filesThis 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).
Next: GFORTRAN_CONVERT_UNIT, Previous: GFORTRAN_DEFAULT_RECL, Up: Runtime [Contents][Index]
GFORTRAN_LIST_SEPARATOR—Separator for list outputThis 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.
Next: GFORTRAN_ERROR_BACKTRACE, Previous: GFORTRAN_LIST_SEPARATOR, Up: Runtime [Contents][Index]
GFORTRAN_CONVERT_UNIT—Set endianness for unformatted I/OBy 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.
Previous: GFORTRAN_CONVERT_UNIT, Up: Runtime [Contents][Index]
GFORTRAN_ERROR_BACKTRACE—Show backtrace on run-time errorsIf 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 serious run-time error occurs. To disable
the backtracing, set the variable to ‘n’, ‘N’, ‘0’.
Default is to print a backtrace unless the -fno-backtrace
compile option was used.
Next: Compiler Characteristics, Previous: Runtime, Up: Top [Contents][Index]
| • Fortran 2003 status: | ||
| • Fortran 2008 status: | ||
| • TS 29113 status: |
Next: Fortran 2008 status, Up: Fortran 2003 and 2008 status [Contents][Index]
GNU Fortran supports several Fortran 2003 features; an incomplete list can be found below. See also the wiki page about Fortran 2003.
PASS attribute.
PASS, PROCEDURE and GENERIC, and
operators bound to a type.
CLASS”) for derived types and unlimited
polymorphism (“CLASS(*)”) – including SAME_TYPE_AS,
EXTENDS_TYPE_OF and SELECT TYPE for scalars and arrays and
finalization.
ASSOCIATE construct.
ALLOCATE statement, allowing for a
type-specification with type parameter and for allocation and initialization
from a SOURCE= expression; ALLOCATE and DEALLOCATE
optionally return an error message string via ERRMSG=.
MOVE_ALLOC.
PRIVATE and PUBLIC attributes may be given individually
to derived-type components.
INTENT may be specified which affect the
association status not the value of the pointer target.
command_argument_count, get_command,
get_command_argument, and get_environment_variable.
SELECTED_CHAR_KIND and NEW_LINE intrinsic functions.
INT, REAL, CMPLX and DBLE.
[...] rather
than (/.../). Type-specification for array constructors like
(/ some-type :: ... /).
FLUSH statement.
IOMSG= specifier for I/O statements.
ENUM and ENUMERATOR statements. Interoperability with
gcc is guaranteed also for the case where the
-fshort-enums command line option is given.
OPEN statement supports the ACCESS='STREAM' specifier,
allowing I/O without any record structure.
strtod honours
the rounding mode, the rounding mode is also supported for input.
PROTECTED statement and attribute.
VALUE statement and attribute.
VOLATILE statement and attribute.
IMPORT statement, allowing to import
host-associated derived types.
ISO_FORTRAN_ENVIRONMENT is supported,
which contains parameters of the I/O units, storage sizes. Additionally,
procedures for C interoperability are available in the ISO_C_BINDING
module.
USE statement with INTRINSIC and NON_INTRINSIC
attribute; supported intrinsic modules: ISO_FORTRAN_ENV,
ISO_C_BINDING, OMP_LIB and OMP_LIB_KINDS,
and OPENACC.
USE statement.
Next: TS 29113 status, Previous: Fortran 2003 status, Up: Fortran 2003 and 2008 status [Contents][Index]
The latest version of the Fortran standard is ISO/IEC 1539-1:2010, informally known as Fortran 2008. The official version is available from International Organization for Standardization (ISO) or its national member organizations. The the final draft (FDIS) can be downloaded free of charge from http://www.nag.co.uk/sc22wg5/links.html. Fortran is developed by the Working Group 5 of Sub-Committee 22 of the Joint Technical Committee 1 of the International Organization for Standardization and the International Electrotechnical Commission (IEC). This group is known as WG5.
The GNU Fortran compiler supports several of the new features of Fortran 2008; the wiki has some information about the current Fortran 2008 implementation status. In particular, the following is implemented.
OPEN statement now supports the NEWUNIT= option,
which returns a unique file unit, thus preventing inadvertent use of the
same unit in different parts of the program.
g0 format descriptor and unlimited format items.
ASINH, ACOSH, ATANH,
ERF, ERFC, GAMMA, LOG_GAMMA, BESSEL_J0,
BESSEL_J1, BESSEL_JN, BESSEL_Y0, BESSEL_Y1,
BESSEL_YN, HYPOT, NORM2, and ERFC_SCALED.
TAN, SINH, COSH,
TANH, ASIN, ACOS, and ATAN is now possible;
ATAN(Y,X) is now an alias for ATAN2(Y,X).
PARITY intrinsic functions.
LEADZ and TRAILZ for
counting the number of leading and trailing zero bits, POPCNT and
POPPAR for counting the number of one bits and returning the parity;
BGE, BGT, BLE, and BLT for bitwise comparisons;
DSHIFTL and DSHIFTR for combined left and right shifts,
MASKL and MASKR for simple left and right justified masks,
MERGE_BITS for a bitwise merge using a mask, SHIFTA,
SHIFTL and SHIFTR for shift operations, and the
transformational bit intrinsics IALL, IANY and IPARITY.
EXECUTE_COMMAND_LINE intrinsic subroutine.
STORAGE_SIZE intrinsic inquiry function.
INT{8,16,32} and REAL{32,64,128} kind type
parameters and the array-valued named constants INTEGER_KINDS,
LOGICAL_KINDS, REAL_KINDS and CHARACTER_KINDS of
the intrinsic module ISO_FORTRAN_ENV.
C_SIZEOF of the intrinsic module
ISO_C_BINDINGS and COMPILER_VERSION and COMPILER_OPTIONS
of ISO_FORTRAN_ENV.
DO CONCURRENT construct is supported.
BLOCK construct is supported.
STOP and the new ERROR STOP statements now
support all constant expressions. Both show the signals which were signaling
at termination.
CONTIGUOUS attribute.
ALLOCATE with MOLD.
IMPURE attribute for procedures, which
allows for ELEMENTAL procedures without the restrictions of
PURE.
NULL()) and not-allocated variables
can be used as actual argument to optional non-pointer, non-allocatable
dummy arguments, denoting an absent argument.
TARGET attribute can be used as
actual argument to POINTER dummies with INTENT(IN).
NULL.
EXIT statement (with construct-name) can be now be
used to leave not only the DO but also the ASSOCIATE,
BLOCK, IF, SELECT CASE and SELECT TYPE
constructs.
ENTRY with
-std=f2008; a line may start with a semicolon; for internal
and module procedures END can be used instead of
END SUBROUTINE and END FUNCTION; SELECTED_REAL_KIND
now also takes a RADIX argument; intrinsic types are supported
for TYPE(intrinsic-type-spec); multiple type-bound procedures
can be declared in a single PROCEDURE statement; implied-shape
arrays are supported for named constants (PARAMETER).
Previous: Fortran 2008 status, Up: Fortran 2003 and 2008 status [Contents][Index]
GNU Fortran supports some of the new features of the Technical Specification (TS) 29113 on Further Interoperability of Fortran with C. The wiki has some information about the current TS 29113 implementation status. In particular, the following is implemented.
See also Further Interoperability of Fortran with C.
OPTIONAL attribute is allowed for dummy arguments
of BIND(C) procedures.
RANK intrinsic is supported.
ASYNCHRONOUS
attribute is compatible with TS 29113.
TYPE(*).
DIMENSION(..)). However, the array descriptor
of the TS is not yet supported.
Next: Extensions, Previous: Fortran 2003 and 2008 status, Up: Top [Contents][Index]
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.
The KIND type parameters supported by GNU Fortran for the primitive
data types are:
INTEGER1, 2, 4, 8*, 16*, default: 4**
LOGICAL1, 2, 4, 8*, 16*, default: 4**
REAL4, 8, 10*, 16*, default: 4***
COMPLEX4, 8, 10*, 16*, default: 4***
DOUBLE PRECISION4, 8, 10*, 16*, default: 8***
CHARACTER1, 4, default: 1
* not available on all systems
** unless -fdefault-integer-8 is used
*** unless -fdefault-real-8 is used (see Fortran Dialect Options)
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 SELECTED_CHAR_KIND, SELECTED_INT_KIND and
SELECTED_REAL_KIND intrinsics or the INT8, INT16,
INT32, INT64, REAL32, REAL64, and REAL128
parameters of the ISO_FORTRAN_ENV module instead of the concrete values.
The available kind parameters can be found in the constant arrays
CHARACTER_KINDS, INTEGER_KINDS, LOGICAL_KINDS and
REAL_KINDS in the ISO_FORTRAN_ENV module. For C interoperability,
the kind parameters of the ISO_C_BINDING module should be used.
Next: Thread-safety of the runtime library, Previous: KIND Type Parameters, Up: Compiler Characteristics [Contents][Index]
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.
See also Argument passing conventions and Interoperability with C.
Next: Data consistency and durability, Previous: Internal representation of LOGICAL variables, Up: Compiler Characteristics [Contents][Index]
GNU Fortran can be used in programs with multiple threads, e.g. by
using OpenMP, by calling OS thread handling functions via the
ISO_C_BINDING facility, or by GNU Fortran compiled library code
being called from a multi-threaded program.
The GNU Fortran runtime library, (libgfortran), supports being
called concurrently from multiple threads with the following
exceptions.
During library initialization, the C getenv function is used,
which need not be thread-safe. Similarly, the getenv
function is used to implement the GET_ENVIRONMENT_VARIABLE and
GETENV intrinsics. It is the responsibility of the user to
ensure that the environment is not being updated concurrently when any
of these actions are taking place.
The EXECUTE_COMMAND_LINE and SYSTEM intrinsics are
implemented with the system function, which need not be
thread-safe. It is the responsibility of the user to ensure that
system is not called concurrently.
For platforms not supporting thread-safe POSIX functions, further functionality might not be thread-safe. For details, please consult the documentation for your operating system.
The GNU Fortran runtime library uses various C library functions that
depend on the locale, such as strtod and snprintf. In
order to work correctly in locale-aware programs that set the locale
using setlocale, the locale is reset to the default “C”
locale while executing a formatted READ or WRITE
statement. On targets supporting the POSIX 2008 per-thread locale
functions (e.g. newlocale, uselocale,
freelocale), these are used and thus the global locale set
using setlocale or the per-thread locales in other threads are
not affected. However, on targets lacking this functionality, the
global LC_NUMERIC locale is set to “C” during the formatted I/O.
Thus, on such targets it’s not safe to call setlocale
concurrently from another thread while a Fortran formatted I/O
operation is in progress. Also, other threads doing something
dependent on the LC_NUMERIC locale might not work correctly if a
formatted I/O operation is in progress in another thread.
Next: Files opened without an explicit ACTION= specifier, Previous: Thread-safety of the runtime library, Up: Compiler Characteristics [Contents][Index]
This section contains a brief overview of data and metadata consistency and durability issues when doing I/O.
With respect to durability, GNU Fortran makes no effort to ensure that
data is committed to stable storage. If this is required, the GNU
Fortran programmer can use the intrinsic FNUM to retrieve the
low level file descriptor corresponding to an open Fortran unit. Then,
using e.g. the ISO_C_BINDING feature, one can call the
underlying system call to flush dirty data to stable storage, such as
fsync on POSIX, _commit on MingW, or fcntl(fd,
F_FULLSYNC, 0) on Mac OS X. The following example shows how to call
fsync:
! Declare the interface for POSIX fsync function
interface
function fsync (fd) bind(c,name="fsync")
use iso_c_binding, only: c_int
integer(c_int), value :: fd
integer(c_int) :: fsync
end function fsync
end interface
! Variable declaration
integer :: ret
! Opening unit 10
open (10,file="foo")
! ...
! Perform I/O on unit 10
! ...
! Flush and sync
flush(10)
ret = fsync(fnum(10))
! Handle possible error
if (ret /= 0) stop "Error calling FSYNC"
With respect to consistency, for regular files GNU Fortran uses
buffered I/O in order to improve performance. This buffer is flushed
automatically when full and in some other situations, e.g. when
closing a unit. It can also be explicitly flushed with the
FLUSH statement. Also, the buffering can be turned off with the
GFORTRAN_UNBUFFERED_ALL and
GFORTRAN_UNBUFFERED_PRECONNECTED environment variables. Special
files, such as terminals and pipes, are always unbuffered. Sometimes,
however, further things may need to be done in order to allow other
processes to see data that GNU Fortran has written, as follows.
The Windows platform supports a relaxed metadata consistency model,
where file metadata is written to the directory lazily. This means
that, for instance, the dir command can show a stale size for a
file. One can force a directory metadata update by closing the unit,
or by calling _commit on the file descriptor. Note, though,
that _commit will force all dirty data to stable storage, which
is often a very slow operation.
The Network File System (NFS) implements a relaxed consistency model
called open-to-close consistency. Closing a file forces dirty data and
metadata to be flushed to the server, and opening a file forces the
client to contact the server in order to revalidate cached
data. fsync will also force a flush of dirty data and metadata
to the server. Similar to open and close, acquiring and
releasing fcntl file locks, if the server supports them, will
also force cache validation and flushing dirty data and metadata.
Previous: Data consistency and durability, Up: Compiler Characteristics [Contents][Index]
The Fortran standard says that if an OPEN statement is executed
without an explicit ACTION= specifier, the default value is
processor dependent. GNU Fortran behaves as follows:
ACTION='READWRITE'
ACTION='READ'
ACTION='WRITE'
Next: Mixed-Language Programming, Previous: Compiler Characteristics, Up: Top [Contents][Index]
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.
| • Extensions implemented in GNU Fortran: | ||
| • Extensions not implemented in GNU Fortran: |
Next: Extensions not implemented in GNU Fortran, Up: Extensions [Contents][Index]
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.
Next: Old-style variable initialization, Up: Extensions implemented in GNU Fortran [Contents][Index]
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
Next: Extensions to namelist, Previous: Old-style kind specifications, Up: Extensions implemented in GNU Fortran [Contents][Index]
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.
Next: X format descriptor without count field, Previous: Old-style variable initialization, Up: Extensions implemented in GNU Fortran [Contents][Index]
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 /
When writing a namelist, if no DELIM= is specified, by default a
double quote is used to delimit character strings. If -std=F95, F2003,
or F2008, etc, the delim status is set to ’none’. Defaulting to
quotes ensures that namelists with character strings can be subsequently
read back in accurately.
Next: Commas in FORMAT specifications, Previous: Extensions to namelist, Up: Extensions implemented in GNU Fortran [Contents][Index]
X format descriptor without count fieldTo 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)
Next: Missing period in FORMAT specifications, Previous: X format descriptor without count field, Up: Extensions implemented in GNU Fortran [Contents][Index]
FORMAT specificationsTo 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)
Next: I/O item lists, Previous: Commas in FORMAT specifications, Up: Extensions implemented in GNU Fortran [Contents][Index]
FORMAT specificationsTo 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')
Next: Q exponent-letter, Previous: Missing period in FORMAT specifications, Up: Extensions implemented in GNU Fortran [Contents][Index]
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.
Next: BOZ literal constants, Previous: I/O item lists, Up: Extensions implemented in GNU Fortran [Contents][Index]
Q exponent-letterGNU Fortran accepts real literal constants with an exponent-letter
of Q, for example, 1.23Q45. The constant is interpreted
as a REAL(16) entity on targets that support this type. If
the target does not support REAL(16) but has a REAL(10)
type, then the real-literal-constant will be interpreted as a
REAL(10) entity. In the absence of REAL(16) and
REAL(10), an error will occur.
Next: Real array indices, Previous: Q exponent-letter, Up: Extensions implemented in GNU Fortran [Contents][Index]
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.
Next: Unary operators, Previous: BOZ literal constants, Up: Extensions implemented in GNU Fortran [Contents][Index]
As an extension, GNU Fortran allows the use of REAL expressions
or variables as array indices.
Next: Implicitly convert LOGICAL and INTEGER values, Previous: Real array indices, Up: Extensions implemented in GNU Fortran [Contents][Index]
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
Next: Hollerith constants support, Previous: Unary operators, Up: Extensions implemented in GNU Fortran [Contents][Index]
LOGICAL and INTEGER valuesAs 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.
Next: Cray pointers, Previous: Implicitly convert LOGICAL and INTEGER values, Up: Extensions implemented in GNU Fortran [Contents][Index]
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
Next: CONVERT specifier, Previous: Hollerith constants support, Up: Extensions implemented in GNU Fortran [Contents][Index]
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, nor
may they be function results. Pointees may not occur in more than one
pointer statement. A pointee cannot be a pointer. Pointees cannot occur
in equivalence, common, or data statements.
A Cray pointer may also point to a function or a subroutine. For example, the following excerpt is valid:
implicit none external sub pointer (subptr,subpte) external subpte subptr = loc(sub) call subpte() [...] subroutine sub [...] end subroutine sub
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.
Next: OpenMP, Previous: Cray pointers, Up: Extensions implemented in GNU Fortran [Contents][Index]
CONVERT specifierGNU 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.
Next: OpenACC, Previous: CONVERT specifier, Up: Extensions implemented in GNU Fortran [Contents][Index]
OpenMP (Open Multi-Processing) is an application programming interface (API) that supports multi-platform shared memory multiprocessing programming in C/C++ and Fortran on many architectures, including Unix and Microsoft Windows platforms. It consists of a set of compiler directives, library routines, and environment variables that influence run-time behavior.
GNU Fortran strives to be compatible to the OpenMP Application Program Interface v4.0.
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 Offloading and Multi Processing Runtime Library
libgomp in GNU Offloading and Multi Processing Runtime
Library.
The OpenMP Fortran runtime library routines are provided both in a
form of a Fortran 90 module named 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:
-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.
Next: Argument list functions, Previous: OpenMP, Up: Extensions implemented in GNU Fortran [Contents][Index]
OpenACC is an application programming interface (API) that supports offloading of code to accelerator devices. It consists of a set of compiler directives, library routines, and environment variables that influence run-time behavior.
GNU Fortran strives to be compatible to the OpenACC Application Programming Interface v2.0.
To enable the processing of the OpenACC directive !$acc in
free-form source code; the c$acc, *$acc and !$acc
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 -fopenacc. This also arranges for automatic linking of
the GNU Offloading and Multi Processing Runtime Library
libgomp in GNU Offloading and Multi Processing Runtime
Library.
The OpenACC Fortran runtime library routines are provided both in a
form of a Fortran 90 module named openacc and in a form of a
Fortran include file named openacc_lib.h.
Note that this is an experimental feature, incomplete, and subject to change in future versions of GCC. See https://gcc.gnu.org/wiki/OpenACC for more information.
Next: Read/Write after EOF marker, Previous: OpenACC, Up: Extensions implemented in GNU Fortran [Contents][Index]
%VAL, %REF and %LOCGNU 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 https://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.
Previous: Argument list functions, Up: Extensions implemented in GNU Fortran [Contents][Index]
Some legacy codes rely on allowing READ or WRITE after the
EOF file marker in order to find the end of a file. GNU Fortran normally
rejects these codes with a run-time error message and suggests the user
consider BACKSPACE or REWIND to properly position
the file before the EOF marker. As an extension, the run-time error may
be disabled using -std=legacy.
Previous: Extensions implemented in GNU Fortran, Up: Extensions [Contents][Index]
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.
| • STRUCTURE and RECORD: | ||
| • ENCODE and DECODE statements: | ||
| • Variable FORMAT expressions: | ||
| • Alternate complex function syntax: | ||
| • Volatile COMMON blocks: |
STRUCTURE and RECORDRecord structures are a pre-Fortran-90 vendor extension to create user-defined aggregate data types. GNU Fortran does not support record structures, only Fortran 90’s “derived types”, which have a different syntax.
In many cases, record structures can easily be converted to derived types.
To convert, replace STRUCTURE /structure-name/
by TYPE type-name. Additionally, replace
RECORD /structure-name/ by
TYPE(type-name). Finally, in the component access,
replace the period (.) by the percent sign (%).
Here is an example of code using the non portable record structure syntax:
! Declaring a structure named ``item'' and containing three fields: ! an integer ID, an description string and a floating-point price. 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 do not change store_catalog(12) = pear print *, store_catalog(12)
Next: Variable FORMAT expressions, Previous: STRUCTURE and RECORD, Up: Extensions not implemented in GNU Fortran [Contents][Index]
ENCODE and DECODE statementsGNU Fortran does not 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))
Next: Alternate complex function syntax, Previous: ENCODE and DECODE statements, Up: Extensions not implemented in GNU Fortran [Contents][Index]
FORMAT expressionsA 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) FMT
c
c Other code here...
c
WRITE(FMT,'("(I", I0, ")")') N+1
WRITE(6,FMT) 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
Next: Volatile COMMON blocks, Previous: Variable FORMAT expressions, Up: Extensions not implemented in GNU Fortran [Contents][Index]
Some Fortran compilers, including g77, let the user declare
complex functions with the syntax COMPLEX FUNCTION name*16(), as
well as COMPLEX*16 FUNCTION name(). Both are non-standard, legacy
extensions. gfortran accepts the latter form, which is more
common, but not the former.
Previous: Alternate complex function syntax, Up: Extensions not implemented in GNU Fortran [Contents][Index]
COMMON blocksSome Fortran compilers, including g77, let the user declare
COMMON with the VOLATILE attribute. This is
invalid standard Fortran syntax and is not supported by
gfortran. Note that gfortran accepts
VOLATILE variables in COMMON blocks since revision 4.3.
Next: Coarray Programming, Previous: Extensions, Up: Top [Contents][Index]
| • Interoperability with C: | ||
| • GNU Fortran Compiler Directives: | ||
| • Non-Fortran Main Program: | ||
| • Naming and argument-passing conventions: |
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.
| • Intrinsic Types: | ||
| • Derived Types and struct: | ||
| • Interoperable Global Variables: | ||
| • Interoperable Subroutines and Functions: | ||
| • Working with Pointers: | ||
| • Further Interoperability of Fortran 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]).
Next: Derived Types and struct, Up: Interoperability with C [Contents][Index]
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.
For logical types, please note that the Fortran standard only guarantees
interoperability between C99’s _Bool and Fortran’s C_Bool-kind
logicals and C99 defines that true has the value 1 and false
the value 0. Using any other integer value with GNU Fortran’s LOGICAL
(with any kind parameter) gives an undefined result. (Passing other integer
values than 0 and 1 to GCC’s _Bool is also undefined, unless the
integer is explicitly or implicitly casted to _Bool.)
Next: Interoperable Global Variables, Previous: Intrinsic Types, Up: Interoperability with C [Contents][Index]
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
components are irrelevant for interoperability.
As there exist no direct Fortran equivalents, neither unions nor structs with bit field or variable-length array members are interoperable.
Next: Interoperable Subroutines and Functions, Previous: Derived Types and struct, Up: Interoperability with C [Contents][Index]
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.
Next: Working with Pointers, Previous: Interoperable Global Variables, Up: Interoperability with C [Contents][Index]
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 in Fortran 2003 and 2008 only explicit size
and assumed-size arrays are supported but not assumed-shape or
deferred-shape (i.e. allocatable or pointer) arrays. However, those
are allowed since the Technical Specification 29113, see
Further Interoperability of Fortran with C
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,
see Working with Pointers.
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 strings is now further 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
The intrinsic procedures are described in Intrinsic Procedures.
Next: Further Interoperability of Fortran with C, Previous: Interoperable Subroutines and Functions, Up: Interoperability with C [Contents][Index]
C pointers are represented in Fortran via the special opaque derived type
type(c_ptr) (with private components). Thus one needs to
use intrinsic conversion procedures to convert from or to C pointers.
For some applications, using an assumed type (TYPE(*)) can be an
alternative to a C pointer; see
Further Interoperability of Fortran with C.
For example,
use iso_c_binding
type(c_ptr) :: cptr1, cptr2
integer, target :: array(7), scalar
integer, pointer :: pa(:), ps
cptr1 = c_loc(array(1)) ! The programmer needs to ensure that the
! array is contiguous if required by the C
! procedure
cptr2 = c_loc(scalar)
call c_f_pointer(cptr2, ps)
call c_f_pointer(cptr2, pa, shape=[7])
When converting C to Fortran arrays, the one-dimensional SHAPE argument
has to be passed.
If a pointer is a dummy-argument of an interoperable procedure, it usually
has to be declared using the VALUE attribute. void*
matches TYPE(C_PTR), VALUE, while TYPE(C_PTR) alone
matches void**.
Procedure pointers are handled analogously to pointers; the C type is
TYPE(C_FUNPTR) and the intrinsic conversion procedures are
C_F_PROCPOINTER and C_FUNLOC.
Let us consider two examples of actually passing a procedure pointer from C to Fortran and vice versa. Note that these examples are also very similar to passing ordinary pointers between both languages. First, consider this code in C:
/* Procedure implemented in Fortran. */
void get_values (void (*)(double));
/* Call-back routine we want called from Fortran. */
void
print_it (double x)
{
printf ("Number is %f.\n", x);
}
/* Call Fortran routine and pass call-back to it. */
void
foobar ()
{
get_values (&print_it);
}
A matching implementation for get_values in Fortran, that correctly
receives the procedure pointer from C and is able to call it, is given
in the following MODULE:
MODULE m
IMPLICIT NONE
! Define interface of call-back routine.
ABSTRACT INTERFACE
SUBROUTINE callback (x)
USE, INTRINSIC :: ISO_C_BINDING
REAL(KIND=C_DOUBLE), INTENT(IN), VALUE :: x
END SUBROUTINE callback
END INTERFACE
CONTAINS
! Define C-bound procedure.
SUBROUTINE get_values (cproc) BIND(C)
USE, INTRINSIC :: ISO_C_BINDING
TYPE(C_FUNPTR), INTENT(IN), VALUE :: cproc
PROCEDURE(callback), POINTER :: proc
! Convert C to Fortran procedure pointer.
CALL C_F_PROCPOINTER (cproc, proc)
! Call it.
CALL proc (1.0_C_DOUBLE)
CALL proc (-42.0_C_DOUBLE)
CALL proc (18.12_C_DOUBLE)
END SUBROUTINE get_values
END MODULE m
Next, we want to call a C routine that expects a procedure pointer argument and pass it a Fortran procedure (which clearly must be interoperable!). Again, the C function may be:
int
call_it (int (*func)(int), int arg)
{
return func (arg);
}
It can be used as in the following Fortran code:
MODULE m
USE, INTRINSIC :: ISO_C_BINDING
IMPLICIT NONE
! Define interface of C function.
INTERFACE
INTEGER(KIND=C_INT) FUNCTION call_it (func, arg) BIND(C)
USE, INTRINSIC :: ISO_C_BINDING
TYPE(C_FUNPTR), INTENT(IN), VALUE :: func
INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
END FUNCTION call_it
END INTERFACE
CONTAINS
! Define procedure passed to C function.
! It must be interoperable!
INTEGER(KIND=C_INT) FUNCTION double_it (arg) BIND(C)
INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
double_it = arg + arg
END FUNCTION double_it
! Call C function.
SUBROUTINE foobar ()
TYPE(C_FUNPTR) :: cproc
INTEGER(KIND=C_INT) :: i
! Get C procedure pointer.
cproc = C_FUNLOC (double_it)
! Use it.
DO i = 1_C_INT, 10_C_INT
PRINT *, call_it (cproc, i)
END DO
END SUBROUTINE foobar
END MODULE m
Previous: Working with Pointers, Up: Interoperability with C [Contents][Index]
The Technical Specification ISO/IEC TS 29113:2012 on further
interoperability of Fortran with C extends the interoperability support
of Fortran 2003 and Fortran 2008. Besides removing some restrictions
and constraints, it adds assumed-type (TYPE(*)) and assumed-rank
(dimension) variables and allows for interoperability of
assumed-shape, assumed-rank and deferred-shape arrays, including
allocatables and pointers.
Note: Currently, GNU Fortran does not support the array descriptor (dope vector) as specified in the Technical Specification, but uses an array descriptor with different fields. The Chasm Language Interoperability Tools, http://chasm-interop.sourceforge.net/, provide an interface to GNU Fortran’s array descriptor.
The Technical Specification adds the following new features, which are supported by GNU Fortran:
ASYNCHRONOUS attribute has been clarified and
extended to allow its use with asynchronous communication in
user-provided libraries such as in implementations of the
Message Passing Interface specification.
C_LOC and C_F_POINTER intrinsics.
OPTIONAL attribute is now allowed for dummy
arguments; an absent argument matches a NULL pointer.
TYPE(*)) have been added, which may
only be used for dummy arguments. They are unlimited polymorphic
but contrary to CLASS(*) they do not contain any type
information, similar to C’s void * pointers. Expressions
of any type and kind can be passed; thus, it can be used as
replacement for TYPE(C_PTR), avoiding the use of
C_LOC in the caller.
Note, however, that TYPE(*) only accepts scalar arguments,
unless the DIMENSION is explicitly specified. As
DIMENSION(*) only supports array (including array elements) but
no scalars, it is not a full replacement for C_LOC. On the
other hand, assumed-type assumed-rank dummy arguments
(TYPE(*), DIMENSION(..)) allow for both scalars and arrays, but
require special code on the callee side to handle the array descriptor.
DIMENSION(..)) as dummy argument
allow that scalars and arrays of any rank can be passed as actual
argument. As the Technical Specification does not provide for direct
means to operate with them, they have to be used either from the C side
or be converted using C_LOC and C_F_POINTER to scalars
or arrays of a specific rank. The rank can be determined using the
RANK intrinisic.
Currently unimplemented:
ISO_Fortran_binding.h header file and the C functions it
specifies are not available.
BIND(C) procedures is not fully supported. In particular,
C interoperable strings of other length than one are not supported
as this requires the new array descriptor.
Next: Non-Fortran Main Program, Previous: Interoperability with C, Up: Mixed-Language Programming [Contents][Index]
The Fortran standard describes how a conforming program shall behave; however, the exact implementation is not standardized. In order to allow the user to choose specific implementation details, compiler directives can be used to set attributes of variables and procedures which are not part of the standard. Whether a given attribute is supported and its exact effects depend on both the operating system and on the processor; see 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
For dummy arguments, the NO_ARG_CHECK attribute can be used; in
other compilers, it is also known as IGNORE_TKR. For dummy arguments
with this attribute actual arguments of any type and kind (similar to
TYPE(*)), scalars and arrays of any rank (no equivalent
in Fortran standard) are accepted. As with TYPE(*), the argument
is unlimited polymorphic and no type information is available.
Additionally, the argument may only be passed to dummy arguments
with the NO_ARG_CHECK attribute and as argument to the
PRESENT intrinsic function and to C_LOC of the
ISO_C_BINDING module.
Variables with NO_ARG_CHECK attribute shall be of assumed-type
(TYPE(*); recommended) or of type INTEGER, LOGICAL,
REAL or COMPLEX. They shall not have the ALLOCATE,
CODIMENSION, INTENT(OUT), POINTER or VALUE
attribute; furthermore, they shall be either scalar or of assumed-size
(dimension(*)). As TYPE(*), the NO_ARG_CHECK attribute
requires an explicit interface.
NO_ARG_CHECK – disable the type, kind and rank checking
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.
Next: Naming and argument-passing conventions, Previous: GNU Fortran Compiler Directives, Up: Mixed-Language Programming [Contents][Index]
| • _gfortran_set_args: | Save command-line arguments | |
| • _gfortran_set_options: | Set library option flags | |
| • _gfortran_set_convert: | Set endian conversion | |
| • _gfortran_set_record_marker: | Set length of record markers | |
| • _gfortran_set_fpe: | Set when a Floating Point Exception should be raised | |
| • _gfortran_set_max_subrecord_length: | Set subrecord length |
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.
Next: _gfortran_set_options, Up: Non-Fortran Main Program [Contents][Index]
_gfortran_set_args — Save command-line arguments_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).
void _gfortran_set_args (int argc, char *argv[])
| argc | number of command line argument strings |
| argv | the command-line argument strings; argv[0] is the pathname of the executable itself. |
int main (int argc, char *argv[])
{
/* Initialize libgfortran. */
_gfortran_set_args (argc, argv);
return 0;
}