NAME

alarm - schedule a SIGALRM


SYNOPSIS

alarm SECONDS

alarm


DESCRIPTION

Arranges to have a SIGALRM delivered to this process after the specified number of seconds have elapsed. If SECONDS is not specified, the value stored in $_ is used. (On some machines, unfortunately, the elapsed time may be up to one second less than you specified because of how seconds are counted.) Only one timer may be counting at once. Each call disables the previous timer, and an argument of 0 may be supplied to cancel the previous timer without starting a new one. The returned value is the amount of time remaining on the previous timer.

For delays of finer granularity than one second, you may use Perl's syscall() interface to access setitimer(2) if your system supports it, or else see select(). It is usually a mistake to intermix alarm() and sleep() calls.

If you want to use alarm() to time out a system call you need to use an eval/die pair. You can't rely on the alarm causing the system call to fail with $! set to EINTR because Perl sets up signal handlers to restart system calls on some systems. Using eval/die always works.

    eval {
        local $SIG{ALRM} = sub { die "alarm\n" };       # NB \n required
        alarm $timeout;
        $nread = sysread SOCKET, $buffer, $size;
        alarm 0;
    };
    die if $@ && $@ ne "alarm\n";       # propagate errors
    if ($@) {
        # timed out
    }
    else {
        # didn't
    }