GFORTRAN(1)

📦 version 16.2.0, in arch rolling

💖 GFORTRAN(1) 💖

okay bestie, welcome to the GNU manual page for gfortran — the GNU Fortran compiler. grab a strawberry 🍓 and let’s get into it.

🌸 NAME 🌸

gfortran — GNU Fortran compiler

✨ SYNOPSIS ✨

gfortran [-c|-S|-E]
         [-g] [-pg] [-Olevel]
         [-Wwarn...] [-pedantic]
         [-Idir...] [-Ldir...]
         [-Dmacro[=defn]...] [-Umacro]
         [-foption...]
         [-mmachine-option...]
         [-o outfile] infile...

Only the most useful options are listed here; see below for the remainder. Translation: this is the starter pack, not the whole closet 💅

💅 DESCRIPTION 💅

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

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.

👑 OPTIONS 👑

Here is a summary of all the options specific to GNU Fortran, grouped by type. Explanations are in the following sections. no cap, every single one is below 🫶

🌈 Fortran Language Options 🌈

-fall-intrinsics -fallow-argument-mismatch -fallow-invalid-boz
-fbackslash -fcray-pointer -fd-lines-as-code -fd-lines-as-comments
-fdec -fdec-char-conversions -fdec-structure -fdec-intrinsic-ints
-fdec-static -fdec-math -fdec-include -fdec-format-defaults
-fdec-blank-format-item -fdefault-double-8 -fdefault-integer-8
-fdefault-real-8 -fdefault-real-10 -fdefault-real-16 -fdollar-ok
-ffixed-line-length-n -ffixed-line-length-none -fpad-source
-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
-fopenmp-allocators -fopenmp-simd -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 -ftest-forall-temp -funsigned

🎀 Preprocessing Options 🎀

-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

⚠️ Error and Warning Options ⚠️

-Waliasing -Wall -Wampersand -Warray-bounds -Wc-binding-type
-Wcharacter-truncation -Wconversion -Wno-deprecated-openmp
-Wdo-subscript -Wfunction-elimination -Wimplicit-interface
-Wimplicit-procedure -Wintrinsic-shadow -Wuse-without-only
-Wintrinsics-std -Wline-truncation -Wno-align-commons
-Wno-overwrite-recursive -Wno-tabs -Wreal-q-constant -Wsurprising
-Wunderflow -Wunused-parameter -Wrealloc-lhs -Wrealloc-lhs-all
-Wfrontend-loop-interchange -Wtarget-lifetime -fmax-errors=n
-fsyntax-only -pedantic -pedantic-errors

🕵️ Debugging Options 🕵️

-fbacktrace -fdebug-aux-vars -ffpe-trap=list -ffpe-summary=list

📁 Directory Options 📁

-Idir  -Jdir -fintrinsic-modules-path dir
-static-libgfortran  -static-libquadmath

🏃 Runtime Options 🏃

-fconvert=conversion -fmax-subrecord-length=length
-frecord-marker=length -fsign-zero

🤝 Interoperability Options 🤝

-fc-prototypes -fc-prototypes-external

⚙️ Code Generation Options ⚙️

-faggressive-function-elimination -fblas-matmul-limit=n
-fbounds-check -ftail-call-workaround -ftail-call-workaround=n
-fcheck-array-temporaries
-fcheck=<all|array-temps|bits|bounds|do|mem|pointer|recursion>
-fcoarray=<none|single|shared|lib> -fexternal-blas -fexternal-blas64
-ff2c -ffrontend-loop-interchange -ffrontend-optimize
-finit-character=n -finit-integer=n -finit-local-zero -finit-derived
-finit-logical=<true|false> -finit-real=<zero|inf|-inf|nan|snan>
-finline-intrinsics[=<minloc,maxloc>] -finline-matmul-limit=n
-finline-arg-packing -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

🛠️ Developer Options 🛠️

-fdump-fortran-global -fdump-fortran-optimized
-fdump-fortran-original -fdump-parse-tree -save-temps

🦋 Options controlling Fortran dialect 🦋

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

  • -ffree-form, -ffixed-form — Specify the layout used by the source file. The free form layout was introduced in Fortran 90. Fixed form was traditionally used in older Fortran programs. When neither option is specified, the source form is determined by the file extension.
  • -fall-intrinsics — This option causes all intrinsic procedures (including the GNU-specific extensions) to be accepted. This can be useful with -std= to force standard compliance but get access to the full range of intrinsics available with gfortran. As a consequence, -Wintrinsics-std is ignored and no user-defined procedure with the same name as any intrinsic is called except when it is explicitly declared EXTERNAL.
  • -fallow-argument-mismatch — Some code contains calls to external procedures with mismatches between the calls and the procedure definition, or with mismatches between different calls. Such code is nonconforming, and is usually flagged with an error. This options degrades the error to a warning that can only be disabled by disabling all warnings via -w. Only a single occurrence per argument is flagged by this warning. -fallow-argument-mismatch is implied by -std=legacy.
Using this option is strongly discouraged. It is possible to provide standard-conforming code that allows different types of arguments by using an explicit interface and TYPE(*).
  • -fallow-invalid-boz — A BOZ literal constant can occur in a limited number of contexts in standard conforming Fortran. This option degrades an error condition to a warning, and allows a BOZ literal constant to appear where the Fortran standard would otherwise prohibit its use.
  • -fd-lines-as-code, -fd-lines-as-comments — Enable special treatment for lines beginning with d or D in fixed form sources. If the -fd-lines-as-code option is given they are treated as if the first column contained a blank. If the -fd-lines-as-comments option is given, they are treated as comment lines.
  • -fdec — DEC compatibility mode. Enables extensions and other features that mimic the default behavior of older compilers (such as DEC). These features are nonstandard and should be avoided at all costs. For details on GNU Fortran’s implementation of these extensions see the full documentation.

Other flags enabled by this switch are: -fdollar-ok -fcray-pointer -fdec-char-conversions -fdec-structure -fdec-intrinsic-ints -fdec-static -fdec-math -fdec-include -fdec-blank-format-item -fdec-format-defaults

If -fd-lines-as-code/-fd-lines-as-comments are unset, then -fdec also sets -fd-lines-as-comments.

  • -fdec-char-conversions — Enable the use of character literals in assignments and DATA statements for non-character variables.
  • -fdec-structure — Enable DEC STRUCTURE and RECORD as well as UNION, MAP, and dot (.) as a member separator (in addition to %). This is provided for compatibility only; Fortran 90 derived types should be used instead where possible.
  • -fdec-intrinsic-ints — Enable B/I/J/K kind variants of existing integer functions (e.g. BIAND, IIAND, JIAND, etc...). For a complete list of intrinsics see Intrinsic Procedures.
  • -fdec-math — Obsolete flag. The purpose of this option was to enable legacy math intrinsics such as COTAN and degree-valued trigonometric functions (e.g. TAND, ATAND, etc...) for compatibility with older code. This option is no longer operable. The trigonometric functions are now either part of Fortran 2023 or GNU extensions.
  • -fdec-static — Enable DEC-style STATIC and AUTOMATIC attributes to explicitly specify the storage of variables and other objects.
  • -fdec-include — Enable parsing of INCLUDE as a statement in addition to parsing it as INCLUDE line. When parsed as INCLUDE statement, INCLUDE does not have to be on a single line and can use line continuations.
  • -fdec-format-defaults — Enable format specifiers F, G and I to be used without width specifiers; default widths are used instead.
  • -fdec-blank-format-item — Enable a blank format item at the end of a format specification i.e. nothing following the final comma.
  • -fdollar-ok — Allow $ as a valid non-first character in a symbol name. Symbols that start with $ are rejected since it is unclear which rules to apply to implicit typing as different vendors implement different rules. Using $ in IMPLICIT statements is also rejected.
  • -fbackslash — Change the interpretation of backslashes in string literals from a single backslash character to “C-style” escape characters. The following combinations are expanded: \a, \b, \f, \n, \r, \t, \v, \\, and \0 to the ASCII characters alert, backspace, form feed, newline, carriage return, horizontal tab, vertical tab, backslash, and NUL, respectively. Additionally, \xnn, \unnnn and \Unnnnnnnn (where each n is a hexadecimal digit) are translated into the Unicode characters corresponding to the specified code points. All other combinations of a character preceded by \ are unexpanded.
  • -fmodule-private — Set the default accessibility of module entities to PRIVATE. Use-associated entities are not accessible unless they are explicitly declared as PUBLIC.
  • -ffixed-line-length-n — Set column after which characters are ignored in typical fixed-form lines in the source file, and, unless -fno-pad-source, 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.

okay so we're still deep in OPTIONS, bestie — grab a snack, because this batch is long and lowkey iconic. every flag matters, so let's go 💅

  • -fno-pad-source — by default, fixed-form lines have spaces assumed after the ends of short fixed-form lines (as if padded to that length). this is not done if you use -ffixed-line-length-0, -ffixed-line-length-none, or -fno-pad-source. with any of those options, continued character constants never have implicit spaces appended to them to fill out the line.
  • -ffree-line-length-n — sets the 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 the entire line is meaningful. -ffree-line-length-0 means the same thing as -ffree-line-length-none.
  • -fmax-identifier-length=n — specify the maximum allowed identifier length. typical values are 31 (Fortran 95) and 63 (Fortran 2003 and later).
  • -fimplicit-none — specify that no implicit typing is allowed, unless overridden by explicit IMPLICIT statements. this is the equivalent of adding implicit none to the start of every procedure. no sneaky implicit vibes allowed ✨
  • -fcray-pointer — enable the Cray pointer extension, which provides C-like pointer functionality.
  • -fopenacc — enable handling of OpenACC directives !$acc in free-form Fortran and !$acc, c$acc and *$acc in fixed-form Fortran. when -fopenacc is specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.6 https://www.openacc.org. this option implies -pthread, and thus is only supported on targets that have support for -pthread. the option -fopenacc implies -frecursive.
  • -fopenmp — enable handling of OpenMP directives !$omp in Fortran. it additionally enables the conditional compilation sentinel !$ in Fortran. in fixed source form Fortran, the sentinels can also start with c or *. when -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5 https://www.openmp.org. this option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd and -frecursive.
  • -fopenmp-allocators — enables handling of allocation, reallocation and deallocation of Fortran allocatable and pointer variables that are allocated using the !$omp allocators and !$omp allocate constructs. files containing either directive have to be compiled with this option in addition to -fopenmp. additionally, all files that might deallocate or reallocate a variable that has been allocated with an OpenMP allocator have to be compiled with this option. this includes intrinsic assignment to allocatable variables when reallocation may occur and deallocation due to either of the following: end of scope, explicit deallocation, intent(out), deallocation of allocatable components etc. files not changing the allocation status or only for components of a derived type that have not been allocated using those two directives do not need to be compiled with this option. nor do files that handle such variables after they have been deallocated or allocated by the normal Fortran allocator.
  • -fopenmp-simd — enable handling of OpenMP's simd, declare simd, declare reduction, assume, ordered, scan and loop directive, and of combined or composite directives with simd as constituent with !$omp in Fortran. it additionally enables the conditional compilation sentinel !$ in Fortran. in fixed source form Fortran, the sentinels can also start with c or *. other OpenMP directives are ignored. unless -fopenmp is additionally specified, the loop region binds to the current task region, independent of the specified bind clause.
  • -fno-range-check — disable range checking on results of simplification of constant expressions during compilation. for example, GNU Fortran gives an error at compile time when simplifying a = 1. / 0. with this option, no error is given and a is assigned the value +Infinity. if an expression evaluates to a value outside of the relevant range of ["-HUGE()":HUGE()], then the expression is replaced by -Inf or +Inf as appropriate. similarly, DATA i/Z'FFFFFFFF'/ results in an integer overflow on most systems, but with -fno-range-check the value "wraps around" and i is initialized to -1 instead.
  • -fdefault-integer-8 — set 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-8 — set the default real type to an 8 byte wide type. this option also affects the kind of non-double real constants like 1.0. this option promotes the default width of DOUBLE PRECISION and double real constants like 1.d0 to 16 bytes if possible. if -fdefault-double-8 is given along with -fdefault-real-8, DOUBLE PRECISION and double real constants are not promoted. unlike -freal-4-real-8, -fdefault-real-8 does not promote variables with explicit kind declarations.
  • -fdefault-real-10 — set the default real type to a 10 byte wide type. this option also affects the kind of non-double real constants like 1.0. this option promotes the default width of DOUBLE PRECISION and double real constants like 1.d0 to 16 bytes if possible. if -fdefault-double-8 is given along with -fdefault-real-10, DOUBLE PRECISION and double real constants are not promoted. unlike -freal-4-real-10, -fdefault-real-10 does not promote variables with explicit kind declarations.
  • -fdefault-real-16 — set the default real type to a 16 byte wide type. this option also affects the kind of non-double real constants like 1.0. this option promotes the default width of DOUBLE PRECISION and double real constants like 1.d0 to 16 bytes if possible. if -fdefault-double-8 is given along with -fdefault-real-16, DOUBLE PRECISION and double real constants are not promoted. unlike -freal-4-real-16, -fdefault-real-16 does not promote variables with explicit kind declarations.
  • -fdefault-double-8 — set the DOUBLE PRECISION type and double real constants like 1.d0 to an 8 byte wide type. do nothing if this is already the default. this option prevents -fdefault-real-8, -fdefault-real-10, and -fdefault-real-16 from promoting DOUBLE PRECISION and double real constants like 1.d0 to 16 bytes.
  • -finteger-4-integer-8 — promote all INTEGER(KIND=4) entities to INTEGER(KIND=8) entities. if KIND=8 is unavailable, then an error is 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. like, fr fr be careful with this one 😭
  • -freal-4-real-8, -freal-4-real-10, -freal-4-real-16, -freal-8-real-4, -freal-8-real-10, -freal-8-real-16 — promote all REAL(KIND=M) entities to REAL(KIND=N) entities. if REAL(KIND=N) is unavailable, then an error is issued. the -freal-4- flags also affect the default real kind and the -freal-8- flags also the double-precision real kind. all other real-kind types are unaffected by this option. the promotion is also applied to real literal constants of default and double-precision kind and a specified kind number of 4 or 8, respectively. however, -fdefault-real-8, -fdefault-real-10, -fdefault-real-10, and -fdefault-double-8 take precedence for the default and double-precision real kinds, both for real literal constants and for declarations without a kind number. note that for REAL(KIND=KIND(1.0)) the literal may get promoted and then the result may get promoted again. 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 and calls to intrinsic procedures when passing a value to the kind= dummy argument. inspection of the intermediate representation of the translated Fortran code, produced by -fdump-fortran-original or -fdump-tree-original, is suggested. NOTE: these options are incompatible with user defined derived type I/O (DTIO). 💀
  • -std=std — specify the standard to which the program is expected to conform, which may be one of f95, f2003, f2008, f2018, f2023, gnu, or legacy. the default value for std is gnu, which specifies a superset of the latest Fortran standard that includes all of the extensions supported by GNU Fortran, although warnings are 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 nonstandard programs. the f95, f2003, f2008, f2018, and f2023 values specify strict conformance to the Fortran 95, Fortran 2003, Fortran 2008, Fortran 2018 and Fortran 2023 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. the deprecated option -std=f2008ts acts as an alias for -std=f2018. it is only present for backwards compatibility with earlier gfortran versions and should not be used any more. -std=f202y acts as an alias for -std=f2023 and enables proposed features for testing Fortran 202y. as the Fortran 202y standard develops, implementation might change or the experimental new features might be removed.
  • -ftest-forall-temp — enhance test coverage by forcing most forall assignments to use temporary.
  • -funsigned — allow the experimental unsigned extension.

💅 Enable and customize preprocessing 🎀

Okay bestie, so here's the tea: lots of Fortran compilers (GNU Fortran included) let you run your source code through a C preprocessor (CPP — sometimes also called the Fortran preprocessor, or FPP) so you can do conditional compilation. With GNU Fortran specifically, it's the GNU C Preprocessor running in traditional mode. On systems where file names preserve case, the preprocessor gets auto-invoked when your extension is .F, .FOR, .FTN, .fpp, .FPP, .F90, .F95, .F03 or .F08. Wanna manually invoke it on any old file? Use -cpp. Wanna turn it off for files where it'd run automatically? Use -nocpp. Slay.

When you're compiling a file that's already been preprocessed, use -fpreprocessed — it skips the C preprocessor entirely. No cap.

Heads up though 🫶: if a preprocessed file pulls in another file with the Fortran "INCLUDE" statement, that included file does not get preprocessed. If you want included files preprocessed, use the equivalent preprocessor statement "#include" instead.

When GNU Fortran invokes the preprocessor, __GFORTRAN__ gets defined. And you can use the macros __GNUC__, __GNUC_MINOR__ and __GNUC_PATCHLEVEL__ to figure out the compiler's version. See Top,,Overview,cpp,The C Preprocessor for all the deets. ✨

GNU Fortran also supports a bunch of INTEGER and REAL kind types on top of the ones the Fortran standard requires — but whether any given kind type is available depends on your architecture (iykyk). These predefined preprocessor macros let you conditionally include code for those extra kind types: __GFC_INT_1__, __GFC_INT_2__, __GFC_INT_8__, __GFC_INT_16__, __GFC_REAL_10__, and __GFC_REAL_16__.

So CPP is the de facto standard for preprocessing Fortran code, but Part 3 of the Fortran 95 standard (ISO/IEC 1539-3:1998) defines something called Conditional Compilation — and ngl, it's not widely used, and the GNU Fortran compiler doesn't directly support it. Periodt.

These options control preprocessing of Fortran code:

  • -cpp, -nocpp — enable preprocessing. The preprocessor gets automatically invoked if the file extension is .fpp, .FPP, .F, .FOR, .FTN, .F90, .F95, .F03 or .F08. Use this option to manually enable preprocessing on literally any kind of Fortran file. To disable preprocessing for files with any of those extensions above, use the negative form: -nocpp. The preprocessor runs in traditional mode, so any restrictions of the file format apply — especially the limits on line length, which apply to preprocessed output too 😭 — so it might be advisable to use the -ffree-line-length-none or -ffixed-line-length-none options.
  • -dM — instead of the normal output, generate a list of '#define' directives for all the macros defined while the preprocessor runs, predefined macros included. It's giving "how do I find out what's predefined in my version of the preprocessor" energy 💅. Assuming you have no file foo.f90, this command:
touch foo.f90; gfortran -cpp -E -dM foo.f90

shows you all the predefined macros.

  • -dD — like -dM except in two respects: it does not include the predefined macros, and it outputs both the "#define" directives and the result of preprocessing. Both kinds of output go to standard output. Iconic.
  • -dN — like -dD, but emits only the macro names, not their expansions.
  • -dU — like dD, except only macros that are expanded (or whose definedness gets tested in preprocessor directives) are output; the output is delayed until the macro is used or tested; and '#undef' directives are also output for macros tested but undefined at the time.
  • -dI — output '#include' directives in addition to the result of preprocessing.
  • -fworking-directory — enable generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. With this enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory (when it's present in the preprocessed input) as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but you can inhibit it with the negated form -fno-working-directory. If the -P flag is present on the command line, this option has no effect — since no "#line" directives get emitted whatsoever. 🫠
  • -idirafter dir — search dir for include files, but do it after all directories specified with -I and the standard system directories have been exhausted. dir is treated as a system include directory. If dir begins with =, then the = is replaced by the sysroot prefix; see --sysroot and -isysroot.
  • -imultilib dir — use dir as a subdirectory of the directory containing target-specific C++ headers.
  • -iprefix prefix — specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /. 💖
  • -isysroot dir — this option is like the --sysroot option, but applies only to header files. See the --sysroot option for more info.
  • -iquote dir — search dir only for header files requested with "#include "file""; they're not searched for "#include <file>", before all directories specified by -I and before the standard system directories. If dir begins with =, the = is replaced by the sysroot prefix; see --sysroot and -isysroot.
  • -isystem dir — search dir for header files, after all directories specified by -I but before the standard system directories. Marks it as a system directory, so it gets the same special treatment as the standard system directories ✨. If dir begins with =, the = is replaced by the sysroot prefix; see --sysroot and -isysroot.
  • -nostdinc — don't search the standard system directories for header files. Only the directories you've specified with -I options get searched (and the directory of the current file, if appropriate).
  • -undef — don't predefine any system-specific or GCC-specific macros. The standard predefined macros stay defined, tho.
  • -Apredicate=answer — make an assertion with the predicate predicate and answer answer. This form is preferred over the older -A predicate(answer) form, which is still supported, because it doesn't use shell special characters. 🐚
  • -A-predicate=answer — cancel an assertion with the predicate predicate and answer answer.
  • -C — do not discard comments. All comments get passed through to the output file, except comments in processed directives, which are deleted along with the directive. You should be prepared for side effects when using -C 😳 — it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a '#'.
⚠️ Warning: this currently handles C-Style comments only. The preprocessor does not yet recognize Fortran-style comments. 💀
  • -CC — do not discard comments, including during macro expansion. This is like -C, except comments contained within macros also get passed through to the output file where the macro is expanded. On top of the side-effects of -C, 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. 💀
  • -D name — predefine name as a macro, with definition 1.
  • -D name=definition — the contents of definition are tokenized and processed as if they appeared during translation phase three in a '#define' directive. In particular, the definition gets truncated by embedded newline characters 👀. If you're 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 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 need to quote the option. With sh and csh, "-D'name(args...)=definition'" works. And -D and -U options are processed in the order they're given on the command line — all -imacros file and -include file options get processed after all -D and -U options.
  • -H — print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the '#include' stack it is. 🪆
  • -P — inhibit generation of linemarkers in the output from the preprocessor. Might be useful when you're running the preprocessor on something that isn't C code, and it gets sent to a program that might be confused by the linemarkers.
  • -U name — cancel any previous definition of name, whether built in or provided with a -D option.

😭 Options to request or suppress errors and warnings 🔥

Errors are the diagnostic messages that report that the GNU Fortran compiler cannot compile the relevant piece of source code. The compiler keeps processing the program in an attempt to report further errors and help with debugging, but it does not produce any compiled output. Full stop. 💀

Warnings are diagnostic messages that report constructions that aren't inherently erroneous but that are risky or suggest there's likely 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 requests 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. iykyk ✨

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

  • -fmax-errors=n — limits the maximum number of error messages to n, at which point GNU Fortran bails out rather than attempting to continue processing the source code. If n is 0, there's no limit on how many error messages get produced. 🏃‍♀️💨
  • -fsyntax-only — vibe check only, bestie 💅 she reads your code and flags syntax errors but does not actually compile it. One thing though: it does generate module files for each module present in the code — but no other output file. So like, it's not fully ghosting you.
  • -Wpedantic, -pedantic — issues warnings for uses of extensions to Fortran. And -pedantic also applies to C-language constructs where they occur in GNU Fortran source files, like using \e in a character constant inside a directive like #include. ✨

Valid Fortran programs should compile properly with or without this option. BUT — without this option, certain GNU extensions and traditional Fortran features are supported as well. With this option, many of them are rejected. It's giving strict-mode energy.

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 (fr fr, contributions open).

This should be used in conjunction with -std=f95, -std=f2003, -std=f2008, -std=f2018 or -std=f2023. Don't use it solo, that's not the vibe.

  • -pedantic-errors — like -pedantic, except that errors are produced rather than warnings. Same girl, louder voice 🔥.
  • -Wall — enables the commonly used warning options pertaining to usage that we recommend avoiding and that we believe are easy to avoid. Currently includes -Waliasing, -Wampersand, -Wconversion, -Wsurprising, -Wc-binding-type, -Wintrinsics-std, -Wtabs, -Wintrinsic-shadow, -Wline-truncation, -Wtarget-lifetime, -Winteger-division, -Wreal-q-constant, -Wunused and -Wundefined-do-loop. One flag, whole squad 👑.
  • -Waliasing — warns 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 triggers the warning 👇

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

call bar(a,a)
  • -Wampersand — warns about a missing ampersand in continued character constants. The warning is given with -Wampersand, -pedantic, -std=f95, -std=f2003, -std=f2008, -std=f2018 and -std=f2023.
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. Yes that's unhinged, no we don't make the rules 🙈.
  • -Warray-temporaries — warns about array temporaries generated by the compiler. The info this warning spits out is sometimes useful in optimization, so you can avoid such temporaries 🎀.
  • -Wc-binding-type — warns if a variable might not be C interoperable. In particular, it warns 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-truncation — warns when a character assignment truncates the assigned string. Your string got cut off mid-sentence, and she will tell you 😭.
  • -Wline-truncation — warns when a source code line is 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. Yeah, not a suggestion bestie 💀.
  • -Wconversion — warns about implicit conversions that are likely to change the value of the expression after conversion. Implied by -Wall.
  • -Wconversion-extra — warns about implicit conversions between different types and kinds. This option does not imply -Wconversion (she's independent, she has her own life 💅).
  • -Wdeprecated-openmp — warns for usage of deprecated OpenMP code. It's giving "this is so 2015", and by 2015 I mean deprecated ✨.
  • -Wexternal-argument-mismatch — warns about argument mismatches for dummy external procedures. This is implied by -fc-prototypes-external because generation of a valid C23 interface is not possible in such a case. Also implied by -Wall.
  • -Wextra — enables some warning options for usages of language features that may be problematic. This currently includes -Wcompare-reals, -Wunused-parameter and -Wdo-subscript.
  • -Wfrontend-loop-interchange — warns when using -ffrontend-loop-interchange for performing loop interchanges.
  • -Wimplicit-interface — warns 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. So like, she's checking the vibe, not the receipts 👀.
  • -Wimplicit-procedure — warns if a procedure is called that has neither an explicit interface nor has been declared as "EXTERNAL".
  • -Winteger-division — warns if a constant integer division truncates its result. As an example, 3/5 evaluates to 0. Math said what it said 🍓.
  • -Wintrinsics-std — warns if gfortran finds a procedure named like an intrinsic not available in the currently selected standard (with -std) and treats it as an "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.
  • -Wno-overwrite-recursive — do not warn when -fno-automatic is used with -frecursive. Recursion is broken if the relevant local variables do not have the attribute "AUTOMATIC" explicitly declared. This option can be used to suppress the warning when it is known that recursion is not broken. Useful for build environments that use -Werror — iykyk 🧚.
  • -Wreal-q-constant — produces a warning if a real-literal-constant contains a "q" exponent-letter. Q?? In this economy?? 💀
  • -Wsurprising — produces a warning when "suspicious" code constructs are encountered. While technically legal, these usually indicate that an error has been made. This currently produces a warning under the following circumstances:
    • An "INTEGER"-typed "SELECT CASE" construct has a "CASE" that can never be matched as its lower value is greater than its upper value.
    • A "LOGICAL"-typed "SELECT CASE" construct has three "CASE" statements.
    • A "TRANSFER" specifies a source that is shorter than the destination.
    • The type of a function result is declared more than once with the same type. If -pedantic or standard-conforming mode is enabled, this is an error.
    • A "CHARACTER" variable is declared with negative length.
    • With -fopenmp, for fixed-form source code, when an "omx" vendor-extension sentinel is encountered. (The equivalent "ompx", used in free-form source code, is diagnosed by default.)
    • With -fopenacc, when using named constants with clauses that take a variable, as doing so has no effect.
  • -Wtabs — by default, tabs are accepted as whitespace, but tabs are not members of the Fortran Character Set. For continuation lines, a tab followed by a digit between 1 and 9 is supported. -Wtabs causes a warning to be issued if a tab is encountered. Note, -Wtabs is active for -pedantic, -std=f95, -std=f2003, -std=f2008, -std=f2018, -std=f2023 and -Wall. Tab girlies, she sees you 🌸.
  • -Wundefined-do-loop — warns if a "DO" loop with step either 1 or -1 yields an underflow or an overflow during iteration of an induction variable of the loop. This option is implied by -Wall.
  • -Wunderflow — produces a warning when numerical constant expressions that yield an underflow are encountered during compilation. Enabled by default ⭐.
  • -Wintrinsic-shadow — warns 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. Name collision, main character energy 👑
  • -Wuse-without-only — warns if a "USE" statement has no "ONLY" qualifier and thus implicitly imports all public entities of the used module. Boundaries, bestie, set them 💅.
  • -Wunused-dummy-argument — warns about unused dummy arguments. This option is implied by -Wall.
  • -Wunused-parameter — Contrary 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. Yes it's confusing, no we're not over it 😭.
  • -Walign-commons — By default, gfortran warns about any occasion of variables being padded for proper alignment inside a "COMMON" block. This warning can be turned off via -Wno-align-commons. See also -falign-commons.
  • -Wfunction-elimination — warns if any calls to impure functions are eliminated by the optimizations enabled by the -ffrontend-optimize option. This option is implied by -Wextra. She deleted your function call, and honestly? Rude 🔥.
  • -Wrealloc-lhs — warns 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 optimizes reallocation checks away. For instance, when the right-hand side contains the same variable multiplied by a scalar. See also -frealloc-lhs.
  • -Wrealloc-lhs-all — warns when the compiler inserts code to for allocation or reallocation of an allocatable variable; this includes scalars and derived types. The full sweep, no survivors 🎀.
  • -Wcompare-reals — warns when comparing real or complex types for equality or inequality. This option is implied by -Wextra. Comparing floats for equality?? In this economy?? 🙈
  • -Wtarget-lifetime — warns if the pointer in a pointer assignment might be longer than the its target. This option is implied by -Wall. Outliving your target is a choice, and she's flagging it 🦋.
  • -Wzerotrip — warns if a "DO" loop is known to execute zero times at compile time. This option is implied by -Wall. Girl the loop is empty, she knows before you even run it 💀.
  • -Wdo-subscript — warns if an array subscript inside a "DO" loop could lead to an out-of-bounds access even if the compiler cannot prove that the statement is actually executed, in cases like
real a(3)
do i=1,4
  if (condition(i)) then
    a(i) = 1.2
  end if
end do

This option is implied by -Wextra.

  • -Werror — turns all warnings into errors. Gloves off, no more gentle parenting 💜.

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

🐛 Options for debugging your program

okay so GNU Fortran has a whole lil toolkit of special options just for debugging your program, and honestly? underrated. let's go 💅

  • -fdebug-aux-vars — this one renames the internal variables that the gfortran front end creates and makes them accessible to a debugger. the name of those internal variables then starts with uppercase letters followed by an underscore. it's useful for debugging the compiler's code generation together with -fdump-tree-original, and for enabling debugging of the executable program with -g or -ggdb3. 🫶
  • -ffpe-trap=list — specifies 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 gets sent and the program being aborted, producing a core file useful for debugging. list is a (possibly empty) comma-separated list of either none (to clear the set of exceptions to be trapped), or of these 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
    • 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 👑

if the option is used more than once in the command line, the lists are joined: "ffpe-trap="list1 "ffpe-trap="list2 is equivalent to "ffpe-trap="list1,list2.

heads up bestie: once enabled an exception cannot be disabled (no negative form), except by clearing all traps by specifying none. no take-backsies 😭

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. fr fr, don't bother.

by default no exception traps are enabled.

  • -ffpe-summary=list — specifies a list of floating-point exceptions whose flag status gets 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 ✨)

if the option is used more than once in the command line, only the last one is used.

by default, a summary for all exceptions but inexact is shown.

  • -fno-backtrace — when 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 💀

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

it also affects the search paths used by cpp when used to preprocess Fortran source 🦋

  • -Idir — these affect interpretation of the INCLUDE directive (as well as of the #include directive of the cpp preprocessor).

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

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

  • -Jdir — this option specifies where to put .mod files for compiled modules. it is also added to the list of directories to searched by an USE statement. (yes, "to searched", the manpage said it like that, we don't gatekeep 🫶)

the default is the current directory.

  • -fintrinsic-modules-path dir — this option specifies the location of pre-compiled intrinsic modules, if they are not in the default location expected by the compiler. ✨

🔗 Influencing the linking step

these options come into play when the compiler links object files into an executable output file. they are meaningless if the compiler is not doing a link step — like, literally nothing, don't even try 😭

  • -static-libgfortran — on systems that provide libgfortran as a shared and a static library, this option forces the use of the static version. if no shared version of libgfortran was built when the compiler was configured, this option has no effect.
  • -static-libquadmath — on systems that provide libquadmath as a shared and a static library, this option forces the use of the static version. if no shared version of libquadmath was built when the compiler was configured, this option has no effect.
🌸 please note that the libquadmath runtime library is licensed under the GNU Lesser General Public License (LGPL), and linking it statically introduces requirements when redistributing the resulting binaries. read that again, bestie, it's giving legal homework 📚

⚡ Influencing runtime behavior

these options affect the runtime behavior of programs compiled with GNU Fortran 💜

  • -fconvert=conversion — specifies the representation of data for unformatted files. valid values for conversion on most systems 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.

on POWER systems that support -mabi=ieeelongdouble, there are additional options, which can be combined with others with commas. those are:

  • -fconvert=r16_ieee — use IEEE 128-bit format for REAL(KIND=16).
  • -fconvert=r16_ibm — use IBM long double format for REAL(KIND=16).

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. iykyk 👑

  • -frecord-marker=length — specifies the length of record markers for unformatted files. valid values for length are 4 and 8. default is 4. this is different from previous versions of gfortran, which specified a default record marker length of 8 on most systems. if you want to read or write files compatible with earlier versions of gfortran, use -frecord-marker=8. ⭐
  • -fmax-subrecord-length=length — specifies the maximum length for a subrecord. the maximum permitted value for length is 2147483639, which is also the default. only really useful for use by the gfortran testsuite 🧚
  • -fsign-zero — when enabled, floating point numbers of value zero with the sign bit set are written as negative number in formatted output and treated as negative in the SIGN intrinsic. -fno-sign-zero does not print the negative sign of zero values (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. 🌈

🛠 GNU Fortran Developer Options

GNU Fortran has various special options that are used for debugging the GNU Fortran compiler. compiler-ception, we love to see it 💅

  • -fdump-fortran-global — outputs a list of the global identifiers after translating into middle-end representation. mostly useful for debugging the GNU Fortran compiler itself. the output generated by this option might change between releases. this option may also generate internal compiler errors for features that have only recently been added 🙈
  • -fdump-fortran-optimized — outputs the parse tree after front-end optimization. mostly useful for debugging the GNU Fortran compiler itself. the output generated by this option might change between releases. this option may also generate internal compiler errors for features that have only recently been added.
  • -fdump-fortran-original — outputs the internal parse tree after translating the source program into internal representation. this option is mostly useful for debugging the GNU Fortran compiler itself. the output generated by this option might change between releases. this option may also generate internal compiler errors for features that have only recently been added.
  • -fdump-parse-tree — outputs the internal parse tree after translating the source program into internal representation. mostly useful for debugging the GNU Fortran compiler itself. the output generated by this option might change between releases. this option may also generate internal compiler errors for features that have only recently been added.
💀 this option is deprecated; use -fdump-fortran-original instead. we're moving on, she's retired.
  • -save-temps — stores the usual "temporary" intermediate files permanently; names them as auxiliary output files, as specified described under GCC -dumpbase and -dumpdir. (again: "as specified described", iconic, we stan a typo 🎀)
gfortran -save-temps -c foo.F90

preprocesses input file foo.F90 to foo.fii, compiles to an intermediate foo.s, and then assembles to the (implied) output file foo.o, whereas:

gfortran -save-temps -S foo.F

saves the preprocessor output in foo.fi, and then compiles to the (implied) output file foo.s. 🦋

🧬 Options for code generation conventions

these machine-independent options control the interface conventions used in code generation ✨

most of them have both positive and negative forms; the negative form of -ffoo would be -fno-foo. in the table below, only one of the forms is listed---the one that is not the default. you can figure out the other form by either removing no- or adding it. yes you can, i believe in you 🫶

  • -fno-automatic — treats 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. 👑
local variables or arrays having an explicit SAVE attribute are silently ignored unless the -pedantic option is added. silently!! the audacity 😭

🌈 -ff2c

okay so this one is literally the "make it talk to the old kids" option 💫 — it generates code designed to be compatible with code generated by g77 and f2c.

Here's the tea ☕ on why this matters: 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.

Also, 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 that were compiled with different calling conventions will break at execution time. 💀
⚠️ Caution: this breaks code that passes intrinsic functions of type default REAL or COMPLEX as actual arguments, as the library implementations use the -fno-f2c calling conventions.

🎀 -fno-underscoring

Do not transform names of entities specified in the Fortran source file by appending underscores to them. no cap, that's the whole thing 💅

With -funderscoring in effect, GNU Fortran appends one underscore to external names. This is done to ensure compatibility with code produced by many UNIX Fortran compilers. Note this does not apply to names declared with C binding, or within a module. 👑

⚠️ 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). like, it's giving "I know what I'm doing" energy only 🫣

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

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

is implemented as something akin to the C code:

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. super handy 🌸

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. 😭 the vibes are not aligned, and the linker can't even tell

Also, note that with -fno-underscoring, the lack of appended underscores introduces the very real possibility that a user-defined external name conflicts 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. 🔥 nightmare fuel fr fr

Also note that declaring symbols as bind(C) is a more robust way to interface with code written in other languages or compiled with different Fortran compilers than the command-line options documented in this section. 🫶 trust

🦋 -fsecond-underscore

By default, GNU Fortran appends an underscore to external names. If this option is used, GNU Fortran appends two underscores to names with underscores and one underscore to names with no underscores. double the underscore, double the chaos ✌️

For example, 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.

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

⭐ -fcoarray=<keyword>

okay so coarrays, the parallel era 👑 the keyword tells it which flavor:

  • none — Disable coarray support; using coarray declarations and image-control statements produces a compile-time error. (Default)
  • single — Single-image mode, i.e. num_images() is always one. solitary queen behavior 💅
  • shared — Use shared-memory multithreading parallelization.
  • lib — Library-based coarray parallelization; a suitable GNU Fortran coarray library such as <http://opencoarrays.org> needs to be linked. Alternatively, GCC's libcaf_single library can be linked, albeit it only supports a single image.

💜 -fcheck=<keyword>

Enable the generation of run-time checks; the argument shall be a comma-delimited list of the following keywords. Prefixing a check with no- disables it if it was activated by a previous specification. so you can turn them on and then be like "actually no" 😌

  • all — Enable all run-time test of -fcheck. the full buffet 🍓
  • array-temps — Warns at run time when for passing an actual argument a temporary array had to be generated. The information generated by this warning is sometimes useful in optimization, in order to avoid such temporaries. Note: The warning is only printed once per location.
  • bits — Enable generation of run-time checks for invalid arguments to the bit manipulation intrinsics.
  • bounds — Enable generation of run-time checks for array subscripts and against the declared minimum and maximum values. It also checks array indices for assumed and deferred shape arrays against the actual allocated bounds and ensures that all string lengths are equal for character array constructors without an explicit typespec. Some checks require that -fcheck=bounds is set for the compilation of the main program. Note: In the future this may also include other forms of checking, e.g., checking substring references.
  • do — Enable generation of run-time checks for invalid modification of loop iteration variables. hands off the loop counter bestie 🚫
  • mem — Enable generation of run-time checks for memory allocation. Note: This option does not affect explicit allocations using the ALLOCATE statement, which are always checked.
  • pointer — Enable generation of run-time checks for pointers and allocatables.
  • recursion — Enable generation of run-time checks for recursively called subroutines and functions that 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.

Example ✨ Assuming you have a file foo.f90, the command

gfortran -fcheck=all,no-array-temps foo.f90

compiles the file with all checks enabled as specified above except warnings for generated array temporaries.

🌸 -fbounds-check

Deprecated alias for -fcheck=bounds. it's giving legacy, use the main character option instead 💅

🐚 -ftail-call-workaround, -ftail-call-workaround=n

Some C interfaces to Fortran codes violate the gfortran ABI by omitting the hidden character length arguments as described in … This can lead to crashes because pushing arguments for tail calls can overflow the stack. 💀

To provide a workaround for existing binary packages, this option disables tail call optimization for gfortran procedures with character arguments. With -ftail-call-workaround=2 tail call optimization is disabled in all gfortran procedures with character arguments, with -ftail-call-workaround=1 or equivalent -ftail-call-workaround only in gfortran procedures with character arguments that call implicitly prototyped procedures.

Using this option can lead to problems including crashes due to insufficient stack space. 😭

It is very strongly recommended to fix the code in question. The -fc-prototypes-external option can be used to generate prototypes that conform to gfortran's ABI, for inclusion in the source code.

Support for this option will likely be withdrawn in a future release of gfortran. ⏳

The negative form, -fno-tail-call-workaround or equivalent -ftail-call-workaround=0, can be used to disable this option.

Default is currently -ftail-call-workaround, this will change in future releases.

🧚 -fcheck-array-temporaries

Deprecated alias for -fcheck=array-temps. same old story ✨

🌈 -fmax-array-constructor=n

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

program test
implicit none
integer j
integer, parameter :: n = 100000
integer, parameter :: i(n) = (/ (2*j, j = 1, n) /)
print '(10(I0,1X))', i
end program test
⚠️ Caution: this option can lead to long compile times and excessively large object files. your patience could never 💀

The default value for n is 65535.

💖 -fmax-stack-var-size=n

This option specifies the size in bytes of the largest array that is 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 that do not have a RECURSIVE attribute or for parallel programs. Use -fno-automatic to never use the stack. stack space is precious and it knows it 👑

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 65536.

  • -fstack-arrays — this one tells the Fortran compiler to yeet every array of unknown size and every array temporary onto stack memory 💅. Heads up though: if your program has straight-up huge local arrays, you might have to bump up your runtime limits for stack memory on some operating systems. It's on by default at optimization level -Ofast… unless you also pass -fmax-stack-var-size. iykyk.
  • -fpack-derived — tells GNU Fortran to pack derived type members as tightly as physically possible 🎀. Lowkey a warning though: code compiled with this option is likely to be incompatible with code compiled without it, and it may run slower. Not me making your build sad 😭.
  • -frepack-arrays — okay so sometimes GNU Fortran passes assumed shape array sections via a descriptor that describes a noncontiguous area of memory. This option adds code to the function prologue that repacks the data into one contiguous block at runtime ✨.

The payoff? Faster accesses to the array. The cost? It can add significant overhead to the function call, especially when the passed data is noncontiguous. Trade-offs, bestie 🦋.

  • -fshort-enums — literally here for interoperability with C code that was compiled with the -fshort-enums option 🫶. It makes GNU Fortran pick the smallest INTEGER kind that a given enumerator set fits in, and then gives all its enumerators that kind. Slay, size optimization.
  • -finline-arg-packing — so when you pass an assumed-shape argument of a procedure as an actual argument to an assumed-size or explicit size thing, or as an argument to a procedure that doesn't have an explicit interface, the argument may have to be packed — like, put into contiguous memory. Example time:
          subroutine foo(a)
             real, dimension(*) :: a
          end subroutine foo
          subroutine bar(b)
             real, dimension(:) :: b
             call foo(b)
          end subroutine bar

When -finline-arg-packing is in effect, that packing is done by inline code 👑. Which means more optimization, but bigger code size.

-finline-arg-packing is implied by any of the -O options — except when you're optimizing for size via -Os. And if your code has a very large number of arguments that have to be packed, code size and compilation time may get excessive 😩. If that happens, it might be better to disable this option. You can find instances of packing using -Warray-temporaries.

  • -fexternal-blas — makes gfortran generate calls to BLAS functions for some matrix operations like MATMUL, instead of using our own algorithms 🧚 — but only if the size of the matrices involved is larger than a given limit (peep -fblas-matmul-limit). This may be profitable if you've got an optimized vendor BLAS library available. The BLAS library has to be specified at link time. This option specifies a BLAS library with integer arguments of default kind (32 bits). It cannot be used together with -fexternal-blas64 💀.
  • -fexternal-blas64 — same energy as above: makes 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). May be profitable if you've got an optimized vendor BLAS library. The BLAS library has to be specified at link time. This option specifies a BLAS library with integer arguments of KIND=8 (64 bits). It cannot be used together with -fexternal-blas, and it requires a 64-bit system 🙈. This option also requires -ffrontend-optimize.
  • -fblas-matmul-limit=n — only significant when -fexternal-blas or -fexternal-blas64 are in effect 🫶. Matrix multiplication of matrices with size larger than or equal to n gets performed by calls to BLAS functions, while everyone else gets handled by gfortran internal algorithms. If the matrices involved aren't square, the size comparison is performed using the geometric mean of the dimensions of the argument and result matrices. Math, but make it fair ✨.

The default value for n is 30.

  • -finline-intrinsics, -finline-intrinsics=intr1,intr2,... — prefer generating inline code over calls to libgfortran functions to implement intrinsics 🔥.

Usage of intrinsics can be implemented either by generating a call to the libgfortran library function or by directly generating inline code. For most intrinsics, only a single variant is available, and there's no choice of implementation. But some intrinsics can use a library function or inline code — and inline code typically offers opportunities for additional optimization over a library function 💅. With -finline-intrinsics=... or -fno-inline-intrinsics=..., the choice applies only to the intrinsics present in the comma-separated list provided as argument.

For each intrinsic, if no choice of implementation was made through either of the flag variants, a default behavior is chosen depending on optimization: library calls are generated when not optimizing or when optimizing for size; otherwise inline code is preferred.

The set of intrinsics allowed as argument to -finline-intrinsics= is currently limited to MAXLOC and MINLOC 🎀. The effect of the flag is moreover limited to calls of those intrinsics without DIM argument and with ARRAY of a non-CHARACTER type. The case of rank-1 argument and DIM argument present, i.e. MAXLOC(A(:),DIM=1) or MINLOC(A(:),DIM=1) is inlined unconditionally for numeric rank-1 array argument A.

  • -finline-matmul-limit=n — when front-end optimization is active, some calls to the MATMUL intrinsic function get inlined 🌸. Which may result in code size increase if the size of the matrix can't be determined at compile time, because code for both cases is generated. Setting -finline-matmul-limit=0 disables inlining in all cases. Setting this option with a value of n produces inline code for matrices with size up to n. If the matrices involved aren't 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. And psst — the -fblas-matmul-limit can be used to change this value 👀.

  • -frecursive — allow indirect recursion by forcing all local arrays to be allocated on the stack 🧚. This flag cannot be used together with -fmax-stack-var-size= or -fno-automatic. No cap, but also no combining, bestie.
  • -finit-local-zero, -finit-derived, -finit-integer=n, -finit-real=<zero|inf|-inf|nan|snan>, -finit-logical=<true|false>, -finit-character=n — the whole init squad ✨. Here's the tea:

The -finit-local-zero option instructs the compiler to initialize local INTEGER, REAL, and COMPLEX variables to zero, LOGICAL variables to false, and CHARACTER variables to a string of null bytes. Finer-grained initialization options are provided by the -finit-integer=n, -finit-real=<zero|inf|-inf|nan|snan> (which also initializes the real and imaginary parts of local COMPLEX variables), -finit-logical=<true|false>, and -finit-character=n (where n is an ASCII character value) options 💖.

With -finit-derived, components of derived type variables are initialized according to these flags. Components whose type is not covered by an explicit -finit-* flag are treated as described above with -finit-local-zero.

These options do not initialize:

  • objects with the POINTER attribute
  • allocatable arrays
  • variables that appear in an 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 — but note, however, that compile-time optimizations may convert them into quiet NaN and that trapping needs to be enabled (e.g. via -ffpe-trap) 😭.

The -finit-integer option parses the value into an integer of type INTEGER(kind=C_LONG) on the host. Said value is then assigned to the integer variables in the Fortran code, which might result in wraparound if the value is too large for the kind 💀.

Finally — and this is lowkey important — enabling any of the -finit-* options silences warnings that would have been emitted by -Wuninitialized for the affected local variables 🌈.

  • -falign-commons — by default, gfortran enforces proper alignment of all variables in a COMMON block by padding them as needed 👑. On certain platforms this is mandatory, on others it increases performance. But if a COMMON block isn't 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 🫶. And to avoid potential alignment issues in COMMON blocks, it's recommended to order objects from largest to smallest.
  • -fno-protect-parens — by default the parentheses in expression are honored for all optimization levels, such that the compiler doesn't do any reassociation 🙅. Using -fno-protect-parens lets the compiler reorder REAL and COMPLEX expressions to produce faster code 🔥. But note: for the reassociation 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-lhs — an allocatable left-hand side of an intrinsic assignment is automatically (re)allocated if it's 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-elimination — functions with identical argument lists get eliminated within statements, regardless of whether these functions are marked PURE or not 👀. For example, in
          a = f(b,c) + f(b,c)

there's only a single call to f. Iconic behavior, honestly. This option only works if -ffrontend-optimize is in effect.

  • -ffrontend-optimize — this option performs front-end optimization, based on manipulating parts of the Fortran parse tree 🌳✨. Enabled by default by any -O option except -O0 and -Og. Optimizations enabled by this option include:
    • inlining calls to MATMUL,
    • elimination of identical function calls within expressions,
    • removing unnecessary calls to TRIM in comparisons and assignments,
    • replacing TRIM(a) with a(1:LEN_TRIM(a)) and
    • short-circuiting of logical operators (.AND. and .OR.).

It can be deselected by specifying -fno-frontend-optimize.

  • -ffrontend-loop-interchange — bestie, this one tries to interchange loops in the Fortran front end when it’s profitable. It’s enabled by default by any -O option. At the moment, this option only affects FORALL and DO CONCURRENT statements with several forall triplets. 💅

🌿 ENVIRONMENT 🌿

Okay so real talk: the gfortran compiler currently does not make use of any environment variables to control its operation above and beyond those that affect the operation of gcc. That’s the whole vibe. ✨

🐛 BUGS 🐛

For instructions on reporting bugs, see <https://gcc.gnu.org/bugs/>. Go off, report responsibly, and include the deets. 🫶

🔗 SEE ALSO 🔗

Lowkey the whole friend group is here: gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), gcc(1), as(1), ld(1), gdb(1), dbx(1) and the Info entries for gcc, cpp, gfortran, as, ld, binutils and gdb. Periodt. 💖

👑 AUTHOR 👑

See the Info entry for gfortran for contributors to GCC and GNU Fortran. Iconic behavior, honestly. 🌟

Copyright (c) 2004-2026 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "Funding Free Software", the Front-Cover Texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the gfdl(7) man page.

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.

This lil page is from gcc-16.2.0, dated 2026-08-07 — GFORTRAN(1), signing off 💅✨

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