This behaviour is all quite predictable given that SIGALRM is sometimes set to SIG_DFL.

Yes, naturally.

The question is, why is SIGALRM being set to SIG_DFL then back to perl's signal handler each time the local %SIG is set?

Thank you for your detailed analysis. This agrees with my observation that local $SIG{...} increases the chance of "something breaking".

The following code doesn't die immediately and doesn't set the handler to SIG_DFL anywhere (except once during what looks like early startup). I'm sure I have already tried it, but everybody makes mistakes :) I'll give it a more intensive test tomorrow.

#!/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 );

        sigprocmask(SIG_BLOCK, $blockset, $sigset );
        $SIG{ALRM} = sub { print "SIGARLM\n" };
        sigprocmask(SIG_SETMASK, $sigset );
        # do something useful here
        sigprocmask(SIG_BLOCK, $blockset, $sigset );
        $SIG{ALRM} = sub { print "SIGARLM: main handler\n" };
        sigprocmask(SIG_SETMASK, $sigset );
}

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

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In reply to Re^2: Setting signal handlers considered unsafe? by gnosek
in thread Setting signal handlers considered unsafe? by gnosek

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