The GNU Fortran Compiler: Argument passing conventions

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#### 6.4.2 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

- Result variable, when the function result is passed by reference
- Character length of the function result, if it is a of type `CHARACTER` and no C binding is used
- The arguments in the order in which they appear in the Fortran declaration
- The the present status for optional arguments with value attribute, which are internally passed by value
- The character length and/or coarray token and offset for the first argument which is a `CHARACTER` or a nonallocatable coarray dummy argument, followed by the hidden arguments of the next dummy argument of such a type

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