get_unit_number Function

   FUNCTION get_unit_number(file_name) RESULT(unit_number)
      !! Returns the first logical unit that is not preconnected
      !! @note
      !! -1 if no free unit exists

      CHARACTER(LEN=*), INTENT(IN), OPTIONAL             :: file_name
      INTEGER                                            :: unit_number

      INTEGER                                            :: ic, istat, nc
      LOGICAL                                            :: exists, is_open

      IF (PRESENT(file_name)) THEN
         nc = SIZE(preconnected)
         ! Check for preconnected units
         DO ic = 3, nc ! Exclude the preconnected system units (< 3)
            IF (TRIM(preconnected(ic)%file_name) == TRIM(file_name)) THEN
               unit_number = preconnected(ic)%unit_number
               RETURN
            END IF
         END DO
      END IF

      ! Get a new unit number
      DO unit_number = 1, max_unit_number
         IF (ANY(unit_number == preconnected(:)%unit_number)) CYCLE
         INQUIRE (UNIT=unit_number, EXIST=exists, OPENED=is_open, IOSTAT=istat)
         IF (exists .AND. (.NOT. is_open) .AND. (istat == 0)) RETURN
      END DO

      unit_number = -1

   END FUNCTION get_unit_number