7.6.3 Wisdom Generic Export/Import from Fortran

The most generic wisdom export/import functions allow you to provide an arbitrary callback function to read/write one character at a time in any way you want. However, your callback function must be written in a special way, using the bind(C) attribute to be passed to a C interface.

In particular, to call the generic wisdom export function fftw_export_wisdom, you would write a callback subroutine of the form:

       subroutine my_write_char(c, p) bind(C)
         use, intrinsic :: iso_c_binding
         character(C_CHAR), value :: c
         type(C_PTR), value :: p
         ...write c...
       end subroutine my_write_char

Given such a subroutine (along with the corresponding interface definition), you could then export wisdom using:

       call fftw_export_wisdom(c_funloc(my_write_char), p)

The standard c_funloc intrinsic converts a Fortran bind(C) subroutine into a C function pointer. The parameter p is a type(C_PTR) to any arbitrary data that you want to pass to my_write_char (or C_NULL_PTR if none). (Note that you can get a C pointer to Fortran data using the intrinsic c_loc, and convert it back to a Fortran pointer in my_write_char using c_f_pointer.)

Similarly, to use the generic fftw_import_wisdom, you would define a callback function of the form:

       integer(C_INT) function my_read_char(p) bind(C)
         use, intrinsic :: iso_c_binding
         type(C_PTR), value :: p
         character :: c
         ...read a character c...
         my_read_char = ichar(c, C_INT)
       end function my_read_char
     
       ....
     
       integer(C_INT) :: ret
       ret = fftw_import_wisdom(c_funloc(my_read_char), p)
       if (ret .eq. 0) stop 'error importing wisdom'

Your function can return -1 if the end of the input is reached. Again, p is an arbitrary type(C_PTR that is passed through to your function. fftw_import_wisdom returns 0 if an error occurred and nonzero otherwise.