$? stuck returning -1...

cmv has asked for the wisdom of the Perl Monks concerning the following question:

I seem to have found a way to get $? to always return -1 (even on the date command)! What's up with this?

Try this test program on *nix and see if it happens to you too...

my $ret1 = `date`;
my $exit1 = $?;
print STDERR "exit1=$exit1 ret1=$ret1";

my $pid;
if($pid = fork) {
    $SIG{CHLD} = sub {wait};
    print STDERR "$$: parent pid=$pid\n";
}elsif (defined $pid) {
    print STDERR "$$: child\n";
    setpgrp(0, $$);
    exec "exec sleep 500 &" || die "Bad exec $!";
    exit;
}

foreach my $i (1..5) {
    my $ret2 = `date`;
    my $exit2 = $?;
    print STDERR "exit2=$exit2 ret2=$ret2";
}
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Comment on $? stuck returning -1... Download Code

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Re: $? stuck returning -1... by jettero (Monsignor) on Jun 20, 2007 at 15:52 UTC
~~The back ticks don't set `$?` (see perlop). That is the correct behavior.~~ If you want the return value, look at system() instead. It turns out that I'm wrong about that. According to perlvar it is indeed supposed to set `$?`, but it's not the return value, it's more. `$exit = $? >> 8` and `$sig = $? & 128`... -Paul	[reply] [d/l] [select]
Re^2: $? stuck returning -1... by cmv (Chaplain) on Jun 20, 2007 at 16:05 UTC
Paul- I'm confused. If it is supposed to set $?, then doesn't the sample program show that it's not working? My output looks like this: `exit1=0 ret1=Wed Jun 20 15:09:04 CDT 2007 24862: parent pid=24864 24864: child exit2=-1 ret2=Wed Jun 20 15:09:04 CDT 2007 exit2=-1 ret2=Wed Jun 20 15:09:04 CDT 2007 exit2=-1 ret2=Wed Jun 20 15:09:04 CDT 2007 exit2=-1 ret2=Wed Jun 20 15:09:04 CDT 2007 exit2=-1 ret2=Wed Jun 20 15:09:04 CDT 2007` [download] Thanks -Craig	[reply] [d/l]
Re^3: $? stuck returning -1... by Joost (Canon) on Jun 20, 2007 at 20:25 UTC
You're setting $SIG{CHLD} yourself so the wait() implicit in the backticks command will report there are no children. Set $SIG{CHLD} to undef before the foreach loop to see that that fixes the problem. update: $\|=1; my $ret1 = `date`; my $exit1 = $?; print STDERR "exit1=$exit1 ret1=$ret1"; $SIG{CHLD} = sub {print "Sigchld\n";wait}; foreach my $i (1..5) { my $ret2 = `date`; my $exit2 = $?; print STDERR "exit2=$exit2 ret2=$ret2"; } [download] output: `exit1=0 ret1=Wed Jun 20 22:29:13 CEST 2007 Sigchld exit2=-1 ret2=Wed Jun 20 22:29:13 CEST 2007 Sigchld exit2=-1 ret2=Wed Jun 20 22:29:13 CEST 2007 Sigchld exit2=-1 ret2=Wed Jun 20 22:29:13 CEST 2007 Sigchld exit2=-1 ret2=Wed Jun 20 22:29:13 CEST 2007 Sigchld exit2=-1 ret2=Wed Jun 20 22:29:13 CEST 2007` [download] "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l] [select]
Re: $? stuck returning -1... by ikegami (Patriarch) on Jun 20, 2007 at 16:31 UTC
`$? == -1` means the system call to launch the command failed. The error code/msg is in `$!`.	[reply] [d/l] [select]
Re^2: $? stuck returning -1... by cmv (Chaplain) on Jun 20, 2007 at 20:27 UTC
If this were true, then I shouldn't be getting a valid date output, but I am. This is the confusing part. Why am I getting $?=-1 after the backtick command works just fine? Aaaahhh, joost: explains all. Thanks!	[reply]