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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.