Which signals are unsafe?

[Sprad]

Are all signals unsafe? Or just the alarm and interrupt signals? What about warn?

I wrote a handler for warnings to give me some more info when things break. But if the warning handler makes things break as well, then obviously I don't want to use it. Is this code bad?

Update: This is under ActiveState Perl 5.6.1, on WinXP

$SIG{__WARN__} = \&mywarn;
# ...
sub mywarn {
    my $warning = shift;
    my $i = 0;
    open (WRNLOG, ">>Warnings.log") || warn "Can't open warnings log f
+ile: $!";
    # If open failed, output will be discarded
    print STDOUT "WARNING: $warning";
    print WRNLOG "WARNING: $warning";
    print STDOUT "         Stack Trace:\n";
    print WRNLOG "         Stack Trace:\n";
    while (caller($i)) {
        my (undef, $filename, $line, $sub) = caller($i);
        print STDOUT "         $filename ($line) --- $sub\n";
        print WRNLOG "         $filename ($line) --- $sub\n";
        $i++;
    }
    print STDOUT "\n";
    print WRNLOG "\n";
}

[BrentDax]

$SIG{__WARN__} isn't a real signal, so it's safe. I believe that all signals have been safe since 5.8.

Have you looked at the Carp module? It can be used with $SIG{__WARN__} to print backtraces. Perhaps you could do something like this:

use Carp 'cluck';
$SIG{__WARN__}=\&cluck;
open(STDERR, "|tee -a Warnings.log") or die "Can't open STDERR to tee:
+ $!";

It's not exactly the same as what you have; most notably, all STDERR output would go to Warnings.log, not just warnings. Fixing this is left as an exercise for the reader. ;^)

If nothing else, Carp's longmess sub can handle the backtracing for you.

[etcshadow]

All real signals are potentially unsafe (in older perls). Warn, however, is not a real signal... it's a psuedo signal, and it's always been safe.