c_f_string Subroutine

   SUBROUTINE c_f_string(c_str, str)
      USE, INTRINSIC :: iso_c_binding, ONLY: c_ptr, c_f_pointer, c_char
      TYPE(c_ptr), INTENT(in) :: c_str
      CHARACTER(kind=c_char), POINTER :: arr(:)
      CHARACTER(:, kind=c_char), ALLOCATABLE, INTENT(out) :: str
      INTEGER(8) :: n, i
      INTERFACE
         ! steal std c library function rather than writing our own.
         FUNCTION strlen(s) bind(c, name='strlen')
            USE, INTRINSIC :: iso_c_binding, ONLY: c_ptr, c_size_t
            IMPLICIT NONE
            !----
            TYPE(c_ptr), INTENT(in), value :: s
            INTEGER(c_size_t) :: strlen
         END FUNCTION strlen
      END INTERFACE
      n = strlen(c_str)
      !****
      CALL c_f_pointer(c_str, arr, [n])
      ALLOCATE (CHARACTER(len=n) :: str)
      DO i = 1, n
         str(i:i) = arr(i)
      END DO
   END SUBROUTINE c_f_string