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.

In reply to $? outside SIGCHLD handler by kenshin

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