7.6.2 Wisdom String Export/Import from Fortran

Dealing with FFTW's C string export/import is a bit more painful. In particular, the fftw_export_wisdom_to_string function requires you to deal with a dynamically allocated C string. To get its length, you must define an interface to the C strlen function, and to deallocate it you must define an interface to C free:

       use, intrinsic :: iso_c_binding
       interface
         integer(C_INT) function strlen(s) bind(C, name='strlen')
           import
           type(C_PTR), value :: s
         end function strlen
         subroutine free(p) bind(C, name='free')
           import
           type(C_PTR), value :: p
         end subroutine free
       end interface

Given these definitions, you can then export wisdom to a Fortran character array:

       character(C_CHAR), pointer :: s(:)
       integer(C_SIZE_T) :: slen
       type(C_PTR) :: p
       p = fftw_export_wisdom_to_string()
       if (.not. c_associated(p)) stop 'error exporting wisdom'
       slen = strlen(p)
       call c_f_pointer(p, s, [slen+1])
       ...
       call free(p)

Note that slen is the length of the C string, but the length of the array is slen+1 because it includes the terminating null character. (You can omit the ‘+1’ if you don't want Fortran to know about the null character.) The standard c_associated function checks whether p is a null pointer, which is returned by fftw_export_wisdom_to_string if there was an error.

To import wisdom from a string, use fftw_import_wisdom_from_string as usual; note that the argument of this function must be a character(C_CHAR) that is terminated by the C_NULL_CHAR character, like the s array above.