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.
-fallow-argument-mismatchSome code contains calls to external procedures with mismatches between the calls and the procedure definition, or with mismatches between different calls. Such code is non-conforming, and will usually be flagged with an error. This options degrades the error to a warning, which 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 which allows different types of
arguments by using an explicit interface and TYPE(*).
-fallow-invalid-bozA 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-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.
-fdecDEC compatibility mode. Enables extensions and other features that mimic the default behavior of older compilers (such as DEC). These features are non-standard 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-conversionsEnable the use of character literals in assignments and DATA statements
for non-character variables.
-fdec-structureEnable 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-intsEnable B/I/J/K kind variants of existing integer functions (e.g. BIAND, IIAND, JIAND, etc...). For a complete list of intrinsics see the full documentation.
-fdec-mathEnable legacy math intrinsics such as COTAN and degree-valued trigonometric functions (e.g. TAND, ATAND, etc...) for compatability with older code.
-fdec-staticEnable DEC-style STATIC and AUTOMATIC attributes to explicitly specify the storage of variables and other objects.
-fdec-includeEnable 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-defaultsEnable format specifiers F, G and I to be used without width specifiers, default widths will be used instead.
-fdec-blank-format-itemEnable a blank format item at the end of a format specification i.e. nothing following the final comma.
-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, 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-sourceBy 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-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.
-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. 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-10Set 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-16Set 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-8Set 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-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.
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.
-std=stdSpecify the standard to which the program is expected to conform, which may be one of ‘f95’, ‘f2003’, ‘f2008’, ‘f2018’, ‘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 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’, ‘f2008’, and ‘f2018’ values specify strict conformance to the Fortran 95, Fortran 2003, Fortran 2008 and Fortran 2018 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.
-ftest-forall-tempEnhance test coverage by forcing most forall assignments to use temporary.