Fortran Dialect Options (The GNU Fortran Compiler)

### 2.2 Options controlling Fortran dialect ¶

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

`-ffree-form` ¶
`-ffixed-form`

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

`-fall-intrinsics` ¶

  This option causes all intrinsic procedures (including the GNU-specific extensions) to be accepted. This can be useful with `-std=` 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`.

`-fno-pad-source` ¶

  By default fixed-form lines have spaces assumed (as if padded to that length) after the ends of short fixed-form lines. This is not done either if `-ffixed-line-length-0`, `-ffixed-line-length-none` or if `-fno-pad-source` option is used. With any of those options continued character constants never have implicit spaces appended to them to fill out the line.

`-ffree-line-length-n` ¶

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

`-fmax-identifier-length=n` ¶

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

`-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 an 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 an 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 an `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.

`-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.