Path: utzoo!attcan!uunet!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!sdd.hp.com!decwrl!mcnc!uvaarpa!mmdf
From: eichin@apollo.com
Newsgroups: comp.lang.perl
Subject: "comparing bitmasks"
Message-ID: <1990Oct26.182652.13422@uvaarpa.Virginia.EDU>
Date: 26 Oct 90 18:26:52 GMT
Sender: mmdf@uvaarpa.Virginia.EDU (Uvaarpa Mail System)
Reply-To: eichin@apollo.com
Organization: The Internet
Lines: 38






I'm using select to wait for a collection of file descriptors. I build
masks $rin and $win:
  @fdnames=(F1,F2,F3,F4,F5,F6,F7,F8,F9,F10);
  foreach(@fdnames) {
    $tin = ''; vec($tin,fileno($_),1) = 1; 
    if($mode{$_} eq "writewait") { $win = $win | $tin; }
    if($mode{$_} eq "readwait")  { $rin = $rin | $tin; }
  }
  $nfound = select($rin,$win,undef,undef);

The question: how do I check which bits are set (ie which filehandles
are available for read or write)? I have a similar loop, which does
($win & $tin) and checks it, but what is the right way to check? 
($win & $tin) is always true. In fact, I tried a few tests (setting
$xx=$foo & $bar), and found the following were *always* true: 
  if($xx)
  if($xx == 0)
  if($xx eq "")
  if(defined($xx))

throwing in a ($vxx)=unpack("c",$xx) made 
  if($vxx)
true as long as the bit being masked was less than 8 (ie in the first
byte, which make sense...)

What I finally got to work was testing
  if(($rin & $tin) eq $tin)
It seems to work even when vec($tin,$x,1)=1 for $x>8, so it isn't
bothered by intervening nulls. However, this behavior isn't hinted at
in the manual page (it should probably be mentioned in the select
example.)
				_Mark_  <eichin@apollo.hp.com>
					<eichin@athena.mit.edu>