The GNU Fortran Compiler: Working with Pointers

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#### 7.1.5 Working with Pointers

C pointers are represented in Fortran via the special opaque derived type `type(c_ptr)` (with private components). Thus one needs to use intrinsic conversion procedures to convert from or to C pointers.

For some applications, using an assumed type (`TYPE(*)`) can be an alternative to a C pointer; see Further Interoperability of Fortran with C.

For example,

      use iso_c_binding
      type(c_ptr) :: cptr1, cptr2
      integer, target :: array(7), scalar
      integer, pointer :: pa(:), ps
      cptr1 = c_loc(array(1)) ! The programmer needs to ensure that the
                              ! array is contiguous if required by the C
                              ! procedure
      cptr2 = c_loc(scalar)
      call c_f_pointer(cptr2, ps)
      call c_f_pointer(cptr2, pa, shape=[7])

When converting C to Fortran arrays, the one-dimensional `SHAPE` argument has to be passed.

If a pointer is a dummy-argument of an interoperable procedure, it usually has to be declared using the `VALUE` attribute. `void*` matches `TYPE(C_PTR), VALUE`, while `TYPE(C_PTR)` alone matches `void**`.

Procedure pointers are handled analogously to pointers; the C type is `TYPE(C_FUNPTR)` and the intrinsic conversion procedures are `C_F_PROCPOINTER` and `C_FUNLOC`.

Let us consider two examples of actually passing a procedure pointer from C to Fortran and vice versa. Note that these examples are also very similar to passing ordinary pointers between both languages. First, consider this code in C:

    /* Procedure implemented in Fortran.  */
    void get_values (void (*)(double));

    /* Call-back routine we want called from Fortran.  */
    void
    print_it (double x)
    {
      printf ("Number is %f.\n", x);
    }

    /* Call Fortran routine and pass call-back to it.  */
    void
    foobar ()
    {
      get_values (&print_it);
    }

A matching implementation for `get_values` in Fortran, that correctly receives the procedure pointer from C and is able to call it, is given in the following `MODULE`:

    MODULE m
      IMPLICIT NONE

      ! Define interface of call-back routine.
      ABSTRACT INTERFACE
        SUBROUTINE callback (x)
          USE, INTRINSIC :: ISO_C_BINDING
          REAL(KIND=C_DOUBLE), INTENT(IN), VALUE :: x
        END SUBROUTINE callback
      END INTERFACE

    CONTAINS

      ! Define C-bound procedure.
      SUBROUTINE get_values (cproc) BIND(C)
        USE, INTRINSIC :: ISO_C_BINDING
        TYPE(C_FUNPTR), INTENT(IN), VALUE :: cproc

        PROCEDURE(callback), POINTER :: proc

        ! Convert C to Fortran procedure pointer.
        CALL C_F_PROCPOINTER (cproc, proc)

        ! Call it.
        CALL proc (1.0_C_DOUBLE)
        CALL proc (-42.0_C_DOUBLE)
        CALL proc (18.12_C_DOUBLE)
      END SUBROUTINE get_values

    END MODULE m

Next, we want to call a C routine that expects a procedure pointer argument and pass it a Fortran procedure (which clearly must be interoperable!). Again, the C function may be:

    int
    call_it (int (*func)(int), int arg)
    {
      return func (arg);
    }

It can be used as in the following Fortran code:

    MODULE m
      USE, INTRINSIC :: ISO_C_BINDING
      IMPLICIT NONE

      ! Define interface of C function.
      INTERFACE
        INTEGER(KIND=C_INT) FUNCTION call_it (func, arg) BIND(C)
          USE, INTRINSIC :: ISO_C_BINDING
          TYPE(C_FUNPTR), INTENT(IN), VALUE :: func
          INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
        END FUNCTION call_it
      END INTERFACE

    CONTAINS

      ! Define procedure passed to C function.
      ! It must be interoperable!
      INTEGER(KIND=C_INT) FUNCTION double_it (arg) BIND(C)
        INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
        double_it = arg + arg
      END FUNCTION double_it

      ! Call C function.
      SUBROUTINE foobar ()
        TYPE(C_FUNPTR) :: cproc
        INTEGER(KIND=C_INT) :: i

        ! Get C procedure pointer.
        cproc = C_FUNLOC (double_it)

        ! Use it.
        DO i = 1_C_INT, 10_C_INT
          PRINT *, call_it (cproc, i)
        END DO
      END SUBROUTINE foobar

    END MODULE m

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