Perl Cookbook

Blocking Signals

Problem

You’d like to delay the reception of a signal, possibly to prevent unpredictable behavior from signals that can interrupt your program at any point.

Solution

Use the POSIX module’s interface to the sigprocmask system call. This is only available if your system is POSIX conformant.

To block a signal around an operation:

use POSIX qw(:signal_h);

$sigset = POSIX::SigSet->new(SIGINT);    # define the signals to block
$old_sigset = POSIX::SigSet->new;        # where the old sigmask will be kept

unless (defined sigprocmask(SIG_BLOCK, $sigset, $old_sigset)) {
    die "Could not block SIGINT\n";
}

To unblock:

unless (defined sigprocmask(SIG_UNBLOCK, $old_sigset)) {
    die "Could not unblock SIGINT\n";
}

Discussion

The POSIX standard introduced sigaction and sigprocmask to give you better control over how signals are delivered. The sigprocmask function controls delayed delivery of signals and sigaction installs handlers. If available, Perl uses sigaction when you change %SIG.

To use sigprocmask, first build a signal set using POSIX::SigSet->new. This takes a list of signal numbers. The POSIX module exports functions named after the signals, which return their signal numbers.

use POSIX qw(:signal_h);

$sigset = POSIX::SigSet->new( SIGINT, SIGKILL );

Pass the POSIX::SigSet object to sigprocmask with the SIG_BLOCK flag to delay signal delivery, SIG_UNBLOCK to restore delivery of the signals, or SIG_SETMASK to block only signals in the POSIX::SigSet. The most paranoid of programmers block ...