Why would waitpid in Perl return wrong exit code?

Question

I get wrong exit code from waitpid and I can't figure out why. Could someone give me some ideas?

Here what I do:

I start my child process with open2
then I wait for it to finish with waitpid
get exit code using $?

It always returns with -1 no mater what I return from child process. I check with VS debugger that my program returns an exit code of 0. VS says something like this:

The program '[3256] Test.exe: Native' has exited with code 0 (0x0).

I made sure that pids match.

Any ideas?

Could you post your perl code, using "sleep" instead of your program? — , Dec 30 '09 at 15:17

Paulius Liekis · Accepted Answer · 2009-12-31T09:31:56.343

7

I just figured it out. waitpid has 3 stages:

 1. process is running:    waitpid returns 0;   $? is -1
 2. process is exiting:    waitpid returns pid; $? is actual exit code
 3. process doesn't exist: waitpid returns -1;  $? is -1

so when doing something like while(waitpid($pid, WNOHANG) >= 0) exit code must be retrieved once cycle before that.

edited Dec 31 '09 at 09:31

answered Dec 30 '09 at 15:32

Paulius Liekis

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Or you could just call waitpid($pid, 0) and it will wait until the program has exited. – Paul Tomblin Dec 30 '09 at 16:03
Ah - yes waitpid only reports the result once. – Douglas Leeder Dec 30 '09 at 16:40
Paul: yes that would work, but I can't block on that, because I need to read log files while the process is running. – Paulius Liekis Dec 31 '09 at 09:30

score 3 · Answer 2 · edited Dec 30 '09 at 15:58

3

From the waitpid man page:

Note that on some systems, a return value of "-1" could mean that child processes are being automatically reaped. See perlipc for details, and for other examples.

edited Dec 30 '09 at 15:58

Brad Gilbert

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answered Dec 30 '09 at 15:24

Paul Tomblin

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score 0 · Answer 3 · answered Dec 30 '09 at 15:26

0

Rather than using waitpid, you should just close the filehandle. (I assume the "open2" in your question is a typo, and you meant "open")

answered Dec 30 '09 at 15:26

William Pursell

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open2 (see IPC::Open2) is a call that allows you to send input to and receive output from the same program – mob Dec 30 '09 at 15:29

score 0 · Answer 4 · answered Jan 23 '15 at 15:03

Works for me (Windows):

use IPC::Open3;
use POSIX ':sys_wait_h';
use Time::HiRes;

$|++;

my ($fin, $fh, $pid);
$pid = open3($fin, $fh, 0, 'ping', '8.8.8.8') or die('error');

my @lines = ();
while (1) {
    while (my $line = <$fh>) {
        push(@lines, $line);
        print('+');
    }
    print("\nret: `$?`\n"), last if waitpid($pid, WNOHANG) <= 0;
    Time::HiRes::usleep(100000);
    $fh->clearerr();
}
waitpid($pid, 0);
print("\nret: `$?`\n");

Will output:

++++++++++++
ret: `-1`

ret: `0`

Why would waitpid in Perl return wrong exit code?

4 Answers4