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
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
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
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
For arguments of
CHARACTER type, the character length is passed as hidden argument. For deferred-length strings, the value is passed by reference, otherwise by value. The character length has the type
INTEGER(kind=4). 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.
OPTIONAL dummy arguments, an absent argument is denoted by a NULL pointer, except for scalar dummy arguments of type
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 oparque 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
CHARACTERand no C binding is used
CHARACTERor a nonallocatable coarray dummy argument, followed by the hidden arguments of the next dummy argument of such a type
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