CHNOPN

Check whether a channel is in use

WTSupported in traditional Synergy on Windows
WNSupported in Synergy .NET on Windows
USupported on UNIX
VSupported on OpenVMS
xcall CHNOPN(channel, status)

Arguments

channel

The expression that contains the channel you want to check. (n)

status

Returned with one of the following values: (n)

1 = The channel is in use.

0 = The channel is closed.

Discussion

The CHNOPN subroutine checks whether a channel is in use by an open file. CHNOPN is functionally identical to DBL$CHOPEN.

Examples

The following program opens a specified file on the first available channel.

.define TTCHN           ,1
record
    filename            ,a20            ;Filename to open
    chn                 ,d4             ;Channel to open
proc
    open(TTCHN, o, "tt:")
    xcall flags(4020)
    do forever
      begin
        display(TTCHN, "Enter a filename to open: ")
        reads(TTCHN, filename)
        xcall openit(filename, chn)
        if (chn)
          writes(TTCHN, "Opened " + filename +
  &              "on channel #" + %a(chn))
      end                               ;End of do forever loop
end                                     ;End of main routine

subroutine openit                       ;Find an available channel
    a_file              ,a              ;Contains name of file to open
    a_rchn              ,d              ;Returned with next available channel
record
    stat                ,d1             ;Used with XCALL CHNOPN
proc
    for a_rchn from 2 thru 1024
      begin
        xcall chnopn(a_rchn, stat)
        if (.not.stat)
          begin
            open(a_rchn, i, a_file)  [err=nofile]
            xreturn
          end
      end
operr,
    writes(TTCHN, "Exceeds maximum channels " + a_file)
    goto oprtn
nofile,
    writes(TTCHN, "Cannot open " + a_file)
oprtn,
    clear a_rchn
    xreturn
endsubroutine