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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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20.1 Signals

Perl uses a simple signal handling model: the %SIG hash contains names or references of user-installed signal handlers. These handlers will be called with an argument which is the name of the signal that triggered it. A signal may be generated intentionally from a particular keyboard sequence like control-C or control-Z, sent to you from another process, or triggered automatically by the kernel when special events transpire, like a child process exiting, your process running out of stack space, or hitting file size limit.

For example, to trap an interrupt signal, set up a handler like this:

sub catch_zap {
    my $signame = shift;
    die "Somebody sent me a SIG$signame";
$SIG{INT} = 'catch_zap';  # could fail in modules
$SIG{INT} = \&catch_zap;  # best strategy

Prior to Perl 5.7.3 it was necessary to do as little as you possibly could in your handler; notice how all we do is set a global variable and then raise an exception. That's because on most systems, libraries are not re-entrant; particularly, memory allocation and I/O routines are not. That meant that doing nearly anything in your handler could in theory trigger a memory fault and subsequent core dump - see 20.2.1 below.

The names of the signals are the ones listed out by kill -l on your system, or you can retrieve them from the Config module. Set up an @signame list indexed by number to get the name and a %signo table indexed by name to get the number:

use Config;
defined $Config{sig_name} || die "No sigs?";
foreach $name (split(' ', $Config{sig_name})) {
    $signo{$name} = $i;
    $signame[$i] = $name;

So to check whether signal 17 and SIGALRM were the same, do just this:

print "signal #17 = $signame[17]\n";
if ($signo{ALRM}) {
    print "SIGALRM is $signo{ALRM}\n";

You may also choose to assign the strings 'IGNORE' or 'DEFAULT' as the handler, in which case Perl will try to discard the signal or do the default thing.

On most Unix platforms, the CHLD (sometimes also known as CLD) signal has special behavior with respect to a value of 'IGNORE'. Setting $SIG{CHLD} to 'IGNORE' on such a platform has the effect of not creating zombie processes when the parent process fails to wait() on its child processes (i.e. child processes are automatically reaped). Calling wait() with $SIG{CHLD} set to 'IGNORE' usually returns -1 on such platforms.

Some signals can be neither trapped nor ignored, such as the KILL and STOP (but not the TSTP) signals. One strategy for temporarily ignoring signals is to use a local() statement, which will be automatically restored once your block is exited. (Remember that local() values are "inherited" by functions called from within that block.)

sub precious {
    local $SIG{INT} = 'IGNORE';
sub more_functions {
    # interrupts still ignored, for now...

Sending a signal to a negative process ID means that you send the signal to the entire Unix process-group. This code sends a hang-up signal to all processes in the current process group (and sets $SIG{HUP} to IGNORE so it doesn't kill itself):

    local $SIG{HUP} = 'IGNORE';
    kill HUP => -$$;
    # snazzy writing of: kill('HUP', -$$)

Another interesting signal to send is signal number zero. This doesn't actually affect a child process, but instead checks whether it's alive or has changed its UID.

unless (kill 0 => $kid_pid) {
    warn "something wicked happened to $kid_pid";

When directed at a process whose UID is not identical to that of the sending process, signal number zero may fail because you lack permission to send the signal, even though the process is alive. You may be able to determine the cause of failure using %!.

unless (kill 0 => $pid or $!{EPERM}) {
    warn "$pid looks dead";

You might also want to employ anonymous functions for simple signal handlers:

$SIG{INT} = sub { die "\nOutta here!\n" };

But that will be problematic for the more complicated handlers that need to reinstall themselves. Because Perl's signal mechanism is currently based on the signal(3) function from the C library, you may sometimes be so unfortunate as to run on systems where that function is "broken", that is, it behaves in the old unreliable SysV way rather than the newer, more reasonable BSD and POSIX fashion. So you'll see defensive people writing signal handlers like this:

sub REAPER {
    $waitedpid = wait;
    # loathe SysV: it makes us not only reinstate
    # the handler, but place it after the wait
    $SIG{CHLD} = \&REAPER;
# now do something that forks...

or better still:

use POSIX ":sys_wait_h";
sub REAPER {
    my $child;
    # If a second child dies while in the signal handler caused by the
    # first death, we won't get another signal. So must loop here else
    # we will leave the unreaped child as a zombie. And the next time
    # two children die we get another zombie. And so on.
    while (($child = waitpid(-1,WNOHANG)) > 0) {
        $Kid_Status{$child} = $?;
    $SIG{CHLD} = \&REAPER;  # still loathe SysV
# do something that forks...

Signal handling is also used for timeouts in Unix, While safely protected within an eval{} block, you set a signal handler to trap alarm signals and then schedule to have one delivered to you in some number of seconds. Then try your blocking operation, clearing the alarm when it's done but not before you've exited your eval{} block. If it goes off, you'll use die() to jump out of the block, much as you might using longjmp() or throw() in other languages.

Here's an example:

eval {
    local $SIG{ALRM} = sub { die "alarm clock restart" };
    alarm 10;
    flock(FH, 2);   # blocking write lock
    alarm 0;
if ($@ and $@ !~ /alarm clock restart/) { die }

If the operation being timed out is system() or qx(), this technique is liable to generate zombies. If this matters to you, you'll need to do your own fork() and exec(), and kill the errant child process.

For more complex signal handling, you might see the standard POSIX module. Lamentably, this is almost entirely undocumented, but the t/lib/posix.t file from the Perl source distribution has some examples in it.

ISBN 9781906966027Perl Language Reference ManualSee the print edition