Programming Perl, 4th Edition

Blocking Signals

Now and then, you’d like to delay receipt of a signal during some critical section of code. You don’t want to blindly ignore the signal, but what you’re doing is too important to interrupt. Perl’s %SIG hash doesn’t implement signal blocking, but the POSIX module does, through its interface to the sigprocmask(2) syscall:

use POSIX qw(:signal_h);
$sigset   = POSIX::SigSet–>new;
$blockset = POSIX::SigSet–>new(SIGINT, SIGQUIT, SIGCHLD);
sigprocmask(SIG_BLOCK, $blockset, $sigset)
    || die "Could not block INT,QUIT,CHLD signals: $!\n";

Once the three signals are all blocked, you can do whatever you want without fear of being bothered. When you’re done with your critical section, unblock the signals by restoring the old signal mask:

sigprocmask(SIG_SETMASK, $sigset)
    || die "Could not restore INT,QUIT,CHLD signals: $!\n";

If any of the three signals came in while blocked, they are delivered immediately. If two or more different signals are pending, the order of delivery is not defined. Additionally, no distinction is made between having received a particular signal once while blocked and having received it many times.^[157] For example, if nine child processes exited while you were blocking CHLD signals, your handler (if you had one) would still be called only once after you unblocked. That’s why, when you reap zombies, you should always loop until they’re all gone.

^[157] Traditionally, that is. Countable signals may be implemented on some real-time systems according to the latest ...