ESMF_UtilIOUnitGet Subroutine

  subroutine ESMF_UtilIOUnitGet(unit, keywordEnforcer, rc)
!
! !ARGUMENTS:
    integer, intent(out)           :: unit
type(ESMF_KeywordEnforcer), optional:: keywordEnforcer ! must use keywords below
    integer, intent(out), optional :: rc
!
! !STATUS:
! \begin{itemize}
! \item\apiStatusCompatibleVersion{5.2.0r}
! \end{itemize}
!
! !DESCRIPTION:
!   Scan for, and return, a free Fortran I/O unit number.
!   By default, the range of unit numbers returned is between 50 and 99
!   (parameters {\tt ESMF\_LOG\_FORTRAN\_UNIT\_NUMBER} and {\tt ESMF\_LOG\_UPPER}
!   respectively.) When integrating ESMF into an application where these values
!   conflict with other usages, the range of values may be moved by setting the
!   optional {\tt IOUnitLower} and {\tt IOUnitUpper} arguments in the initial
!   {\tt ESMF\_Initialize()} call with values in a safe, alternate, range.
!
!   The Fortran unit number which is returned is not reserved in any way.
!   Successive calls without intervening {\tt OPEN} or {\tt CLOSE} statements
!   (or other means of connecting to units), might not return a unique unit
!   number.  It is recommended that an {\tt OPEN} statement immediately follow
!   the call to {\tt ESMF\_IOUnitGet()} to activate the unit.
!
!     The arguments are:
!     \begin{description}
!     \item[unit]
!       A Fortran I/O unit number.
!     \item[{[rc]}]
!       Return code; equals {\tt ESMF\_SUCCESS} if there are no errors.
!     \end{description}
!EOP

    integer :: i
    integer :: localstat
    logical :: inuse
  
    if (present(rc)) rc = ESMF_FAILURE
  
    do, i=ESMF_UtilIOUnitLower, ESMF_UtilIOUnitUpper
      inquire (unit=i, opened=inuse, iostat=localstat)
      if (.not. inuse .and. localstat == 0) exit
    end do
  
    if (i <= ESMF_UtilIOUnitUpper) then
      unit = i
      if (present (rc)) rc = ESMF_SUCCESS
    else
      unit = -1
    end if

  end subroutine ESMF_UtilIOUnitGet