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7.4.2 Argument passing conventions

Subroutines do not return a value (matching C99’s void) while functions either return a value as specified in the platform ABI or the result variable is passed as hidden argument to the function and no result is returned. A hidden result variable is used when the result variable is an array or of type CHARACTER.

Arguments are passed according to the platform ABI. In particular, complex arguments might not be compatible to a struct with two real components for the real and imaginary part. The argument passing matches the one of C99’s _Complex. Functions with scalar complex result variables return their value and do not use a by-reference argument. Note that with the -ff2c option, the argument passing is modified and no longer completely matches the platform ABI. Some other Fortran compilers use f2c semantic by default; this might cause problems with interoperablility.

GNU Fortran passes most arguments by reference, i.e. by passing a pointer to the data. Note that the compiler might use a temporary variable into which the actual argument has been copied, if required semantically (copy-in/copy-out).

For arguments with ALLOCATABLE and POINTER attribute (including procedure pointers), a pointer to the pointer is passed such that the pointer address can be modified in the procedure.

For dummy arguments with the VALUE attribute: Scalar arguments of the type INTEGER, LOGICAL, REAL and COMPLEX are passed by value according to the platform ABI. (As vendor extension and not recommended, using %VAL() in the call to a procedure has the same effect.) For TYPE(C_PTR) and procedure pointers, the pointer itself is passed such that it can be modified without affecting the caller.

For Boolean (LOGICAL) arguments, please note that GCC expects only the integer value 0 and 1. If a GNU Fortran LOGICAL variable contains another integer value, the result is undefined. As some other Fortran compilers use -1 for .TRUE., extra care has to be taken – such as passing the value as INTEGER. (The same value restriction also applies to other front ends of GCC, e.g. to GCC’s C99 compiler for _Bool or GCC’s Ada compiler for Boolean.)

For arguments of CHARACTER type, the character length is passed as a hidden argument at the end of the argument list. For deferred-length strings, the value is passed by reference, otherwise by value. The character length has the C type size_t (or INTEGER(kind=C_SIZE_T) in Fortran). Note that this is different to older versions of the GNU Fortran compiler, where the type of the hidden character length argument was a C int. In order to retain compatibility with older versions, one can e.g. for the following Fortran procedure 

subroutine fstrlen (s, a)
   character(len=*) :: s
   integer :: a
   print*, len(s)
end subroutine fstrlen

define the corresponding C prototype as follows: 

#if __GNUC__ > 7
typedef size_t fortran_charlen_t;
#else
typedef int fortran_charlen_t;
#endif

void fstrlen_ (char*, int*, fortran_charlen_t);

In order to avoid such compiler-specific details, for new code it is instead recommended to use the ISO_C_BINDING feature.

Note with C binding, CHARACTER(len=1) result variables are returned according to the platform ABI and no hidden length argument is used for dummy arguments; with VALUE, those variables are passed by value.

For OPTIONAL dummy arguments, an absent argument is denoted by a NULL pointer, except for scalar dummy arguments of type INTEGER, LOGICAL, REAL and COMPLEX which have the VALUE attribute. For those, a hidden Boolean argument (logical(kind=C_bool),value) is used to indicate whether the argument is present.

Arguments which are assumed-shape, assumed-rank or deferred-rank arrays or, with -fcoarray=lib, allocatable scalar coarrays use an array descriptor. All other arrays pass the address of the first element of the array. With -fcoarray=lib, the token and the offset belonging to nonallocatable coarrays dummy arguments are passed as hidden argument along the character length hidden arguments. The token is an opaque pointer identifying the coarray and the offset is a passed-by-value integer of kind C_PTRDIFF_T, denoting the byte offset between the base address of the coarray and the passed scalar or first element of the passed array.

The arguments are passed in the following order

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