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The hash %SIG is used to set signal handlers for various signals. Example:

    sub handler {	# 1st argument is signal name
	my($sig) = @_;
	print "Caught a SIG$sig--shutting down\n";

    $SIG{'INT'}  = \&handler;
    $SIG{'QUIT'} = \&handler;
    $SIG{'INT'} = 'DEFAULT';	# restore default action
    $SIG{'QUIT'} = 'IGNORE';	# ignore SIGQUIT

Using a value of 'IGNORE' usually has the effect of ignoring the signal, except for the CHLD signal. See perlipc for more about this special case.

The %SIG array contains values for only the signals actually set within the Perl script. Here are some other examples:

$SIG{"PIPE"} = Plumber;     # SCARY!!
$SIG{"PIPE"} = "Plumber";   # assumes main::Plumber (not recommended)
$SIG{"PIPE"} = \&Plumber;   # just fine; assume current Plumber
$SIG{"PIPE"} = Plumber();   # oops, what did Plumber() return??

The one marked scary is problematic because it's a bareword, which means sometimes it's a string representing the function, and sometimes it's going to call the subroutine call right then and there! Best to be sure and quote it or take a reference to it. *Plumber works too. See perlsub.

If your system has the sigaction() function then signal handlers are installed using it. This means you get reliable signal handling. If your system has the SA_RESTART flag it is used when signals handlers are installed. This means that system calls for which it is supported continue rather than returning when a signal arrives. If you want your system calls to be interrupted by signal delivery then do something like this:

use POSIX ':signal_h';

my $alarm = 0;
sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
	or die "Error setting SIGALRM handler: $!\n";


Certain internal hooks can be also set using the %SIG hash. The routine indicated by $SIG{__WARN__} is called when a warning message is about to be printed. The warning message is passed as the first argument. The presence of a __WARN__ hook causes the ordinary printing of warnings to STDERR to be suppressed. You can use this to save warnings in a variable, or turn warnings into fatal errors, like this:

local $SIG{__WARN__} = sub { die $_[0] };
eval $proggie;

The routine indicated by $SIG{__DIE__} is called when a fatal exception is about to be thrown. The error message is passed as the first argument. When a __DIE__ hook routine returns, the exception processing continues as it would have in the absence of the hook, unless the hook routine itself exits via a goto, a loop exit, or a die(). The __DIE__ handler is explicitly disabled during the call, so that you can die from a __DIE__ handler. Similarly for __WARN__.

Note that the $SIG{__DIE__} hook is called even inside eval()ed blocks/strings. See "die" in perlfunc and "$^S" in perlvar for how to circumvent this.

Note that __DIE__/__WARN__ handlers are very special in one respect: they may be called to report (probable) errors found by the parser. In such a case the parser may be in inconsistent state, so any attempt to evaluate Perl code from such a handler will probably result in a segfault. This means that calls which result/may-result in parsing Perl should be used with extreme caution, like this:

require Carp if defined $^S;
Carp::confess("Something wrong") if defined &Carp::confess;
die "Something wrong, but could not load Carp to give backtrace...
     To see backtrace try starting Perl with -MCarp switch";

Here the first line will load Carp unless it is the parser who called the handler. The second line will print backtrace and die if Carp was available. The third line will be executed only if Carp was not available.

See "die" in perlfunc, "warn" in perlfunc and "eval" in perlfunc for additional info.