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?

Answer 1

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.

Answer 2

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.

Answer 3

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

Answer 4

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, $line);
        print( + );
    }
    print("
ret: `$?`
"), last if waitpid($pid, WNOHANG) <= 0;
    Time::HiRes::usleep(100000);
    $fh->clearerr();
}
waitpid($pid, 0);
print("
ret: `$?`
");

Will output:

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

ret: `0`

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