Path: utzoo!attcan!uunet!husc6!bbn!uwmcsd1!ig!agate!ucbvax!NUACC.ACNS.NWU.EDU!MCGEE
From: MCGEE@NUACC.ACNS.NWU.EDU (Randy McGee)
Newsgroups: comp.os.vms
Subject: RE: anyone have any clues why this doesn't work
Message-ID: <8205201128.aa01478@accuvax.NWU.Edu>
Date: 19 May 88 16:29:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 78

>        I am trying to whip up a quick and dirty program which will
>        emulate the Unix tail program, ie, read the last n lines of
>        a file. I wrote up the following code, which seems like it
>        should work, but it doesn't seem to position the file at EOF.
>        any clues what i'm doing wrong?
     
Your program is attempting to use BACKSPACE illegaly.  The following was 
extracted from the "Programming in VAX FORTRAN" reference manual (V4.0, order 
number AA-D034D-TE) page 13-27:

   "You must no issue a BACKSPACE statement for a file that is open for 
   direct, keyed, or append access.  Backspacing from record n is done by
   rewinding to the start of the file and then performin n-1 successive reads
   to reach the previous record.  For direct, keyed, and append access, the
   current record count (n) is not available to the FORTRAN I/O system." 

The same thing is true when you use the USEROPEN option to position the 
file.  The following modified version of your program will work.

V(AX/VMS W)izard
Randy McGee (MAGOO) <mcgee@nuacc.acns.nwu.edu>     Phone:    (312) 491 4079
Academic Computing and Network Services            USPost:   2129 Sheridan Road
Northwestern University                                      Evanston, IL  60208

---[ TAIL.FOR ]---------------------------------------------------------------

        program tail

        implicit none

        integer*4 max_lines
        parameter (max_lines = 100)

        character*80 inline(max_lines)
        integer*4 lines, nback, P, I

        open( 1, name='SYS$MANAGER:LPR_SERVER.LOG', type='OLD', READONLY
     -      ,  SHARED )

        print 100, max_lines
100     format('$Number of records from end (<', I3, ')? ')
        accept*,nback
        if ( nback .gt. max_lines ) then
           type 101, max_lines
           nback = max_lines
        end if
101     format( ' Number greater than maximum allowed.  Using ', I3 )

        lines = 0
        do while( .true. )
           read(1,'(a)', end=5) inline( mod( lines, nback ) + 1 )
           lines = lines + 1
        end do
5       continue

        lines = lines - 1
        P = mod( lines, nback ) + 1

        if ( lines .eq. 0 ) then
           print *, 'File is empty'
        else
           if ( lines .gt. nback ) then
              if ( nback .ne. P ) then
                 do I = P + 1, nback
                    print *, inline( I )
                 end do
              end if
           end if
           do I = 1, P
              print *, inline( I )
           end do
        end if

        close(unit=1)

        call exit

        end