#!/usr/bin/perl

use warnings FATAL => qw( all );
use strict;

use POSIX qw( :signal_h );

$SIG{ALRM} = sub { print "SIGARLM: main handler\n" };

sub f {
        my $sigset = POSIX::SigSet->new;
        my $blockset = POSIX::SigSet->new( SIGALRM );
        my $old_handler = $SIG{ALRM};

        # Install new handler
        sigprocmask(SIG_BLOCK, $blockset, $sigset );
        $SIG{ALRM} = sub { print "SIGALRM\n" }; # line 15
        sigprocmask(SIG_SETMASK, $sigset );

        # spin a bit.
        my $x;
        $x = $_ for 1..10;  

        # Restore old handler.
        sigprocmask(SIG_BLOCK, $blockset, $sigset );
        $SIG{ALRM} = $old_handler;
        sigprocmask(SIG_SETMASK, $sigset );
}

if (fork) {
        # parent
        f while 1;
} else {
        # child
        sleep 1;
        print "child starting\n";
        1 while kill ALRM => getppid;
}