c  $Header: /usr/people/flittner/radtran/tomrad/pv2.2/src/RCS/get_lun.f,v 2.22 2000/08/30 16:38:09 flittner Exp $
      subroutine get_lun (ivalue)
C======================================================================C
C                                                                      C
C     SUBROUTINE : GET_LUN                                             C
C                                                                      C
C     PURPOSE: To keep logical unit numbers straight, and issues new   C
C              ones when necessary. The entry point FREE_LUN closes    C
C              file and allows the logical unit number to be reused.   C
C                                                                      C
C     USE:                                                             C
C            CALL GET_LUN (IUNIT)                                      C
C            CALL FREE_LUN (IUNIT)                                     C
C                                                                      C
C     PARAMETERS :                                                     C
C                                                                      C
C            IUNIT --- In GET_LUN, it is the returned logical unit     C
C                      number. In FREE_LUN, it is the input LUN to     C
C                      close and to be reused.                         C
C                                                                      C
C     SOURCE:            DONALD J. RICHARDSON                          C
C                        HUGHES STX                                    C
C                                                                      C
C     LANGUAGE:          FORTRAN 77                                    C
C                                                                      C
C     DATE STARTED :     September 9, 1994                             C                                              C
C                                                                      C
C     LATEST REVISION:   September 12, 1994                            C
C                                                                      C
C                                                                      C
C======================================================================C
      implicit none

      integer*4 min_lun, max_lun
      parameter (min_lun = 10, max_lun=100)

      logical open_stat
      integer*4 ivalue
      integer*4 i

      do i = min_lun, max_lun
        inquire (i, opened = open_stat)
        if (.not. open_stat) then
          ivalue = i
          return
        end if
      end do

      write (0,*) 'All the Logical Unit Numbers between ',min_lun,
     *        ' and ', max_lun, ' are all in use.'
      ivalue = -1
      return

      entry free_lun (ivalue)

      close (ivalue)

      return
      end