$? outside SIGCHLD handler

kenshin has asked for the wisdom of the Perl Monks concerning the following question:

Hello.

I have a script for downloading bunch of data from some servers.

For each server I start ssh via IPC::Open2 like this:

my $pid = open2($rfh, $wfh, "ssh $opts $hostspec $cmd");
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Then I download all data in loop via select() (4 arg version). When some ssh dies, I need to modify some program structures (same applies for INT/TERM signal). So I decided I will do it right after select() via self-pipe trick (described in NOTES in linux select(2) manpage).

### Self-pipe init.
pipe(SIGRD, SIGWR);
# Make SIGRD non-blocking.
vec($rin, fileno SIGRD, 1) = 1;
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CHLD handler does just this (INT/TERM handlers sets $term variable in addition):

my $byte = 1;
syswrite(SIGWR, $byte, 1);
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This is top of the main while loop.

my $nfound = select($rout = $rin, $wout = $win, undef, undef);
# Check $nfound for errors.
if(vec($rout, fileno SIGRD, 1)) {
  my $byte;
  while(sysread(SIGRD, $byte, 1)) {
  if($term) {
    # INT/TERM arrived, kill sshs, modify some data, ...
    $term = 0;
  }
  else {
    # CHLD arrived, do waitpid(), modify some data, ...
  }
}
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After I do waitpid() I need to check $? if ssh exited with error. However in description of $? in perlvar is stated: If you have installed a signal handler for "SIGCHLD", the value of $? will usually be wrong outside that handler. My question is why it will be wrong when I will use it just after waitpid()? Thank you.

Comment on $? outside SIGCHLD handler Select or Download Code

Replies are listed 'Best First'.
Re: $? outside SIGCHLD handler by ikegami (Patriarch) on Sep 04, 2009 at 19:37 UTC
ug, use IO::Select. It's a thin wrapper over `select` that's much easier to use than the underlying function. You didn't show your signal handler. If it calls `wait` or `waitpid`, don't call `wait` or `waitpid` outside of the handler. Save the result from the handler, if you need to. `my %reaped; $SIG{CHLD} = sub { my $pid = wait(); $reaped{$pid} = $?; }; # When reading from read ready handle returns eof, my $exit_code = delete $reaped{$pid_associated_with_handle};` [download]	[reply] [d/l] [select]
Re^2: $? outside SIGCHLD handler by Anonymous Monk on Sep 08, 2009 at 18:24 UTC
Hello. Sorry for long delay, I had no access to internet during weekend. I'll probably switch to IO::Select later, select() code works fine so far. Code for CHLD handler is after "CHLD handler does just this ..." in my original post (No wait()/waitpid() calls inside). I was just confused by sentence from "perlvar" (see end of orig post).	[reply]
Re^3: $? outside SIGCHLD handler by kenshin (Initiate) on Sep 08, 2009 at 18:26 UTC
I forgot to log in. The reply above is mine.	[reply]