MODULE free_unit_mod

  PRIVATE 

  INTEGER, PARAMETER :: start_unit=1000

  PUBLIC :: free_unit

CONTAINS

  FUNCTION free_unit()
    INTEGER :: free_unit
    LOGICAL :: out
    
    out=.FALSE.
    free_unit=start_unit
    
    DO WHILE (.NOT. out)
      free_unit=free_unit+1
      INQUIRE(unit=free_unit, exist=out)
    END DO
    
  END FUNCTION free_unit
  
END MODULE free_unit_mod