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2.2 Options controlling Fortran dialect

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

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.
Accept all of the intrinsic procedures provided in libgfortran without regard to the setting of -std. In particular, this option can be quite useful with -std=f95. Additionally, gfortran will ignore -Wnonstd-intrinsics.
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.
Set the DOUBLE PRECISION type to an 8 byte wide type.
Set the default integer and logical types to an 8 byte wide type. Do nothing if this is already the default.
Set the default real type to an 8 byte wide type. Do nothing if this is already the default.
Allow `$' as a valid character in a symbol name.
Change the interpretation of backslashes in string literals from “C-style” escape characters to a single backslash character.
Set column after which characters are ignored in typical fixed-form lines in the source file, and through which spaces are assumed (as if padded to that length) after the ends of short fixed-form lines.

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

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.
Specify the maximum allowed identifier length. Typical values are 31 (Fortran 95) and 63 (Fortran 2003).
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.
Enable the Cray pointer extension, which provides C-like pointer functionality.
Enable the OpenMP extensions. This includes OpenMP !$omp directives in free form and c$omp, *$omp and !$omp directives in fixed form, !$ conditional compilation sentinels in free form and c$, *$ and !$ sentinels in fixed form, and when linking arranges for the OpenMP runtime library to be linked in.
Enable range checking on results of simplification of constant expressions during compilation. For example, by default, GNU Fortran will give an overflow error at compile time when simplifying a = EXP(1000). With -fno-range-check, no error will be given and the variable a will be assigned the value +Infinity. 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.
Specify the standard to which the program is expected to conform, which may be one of `f95', `f2003', `gnu', or `legacy'. The default value for std is `gnu', which specifies a superset of the Fortran 95 standard that includes all of the extensions supported by GNU Fortran, although warnings will be given for obsolete extensions not recommended for use in new code. The `legacy' value is equivalent but without the warnings for obsolete extensions, and may be useful for old non-standard programs. The `f95' and `f2003' values specify strict conformance to the Fortran 95 and Fortran 2003 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.