Strawberry Perl negative child pid when forked

Question

So I understand that Windows does not support the Unix fork-exec model and instead spawns processes. However, Strawberry Perl's fork emulation produces children with negative PIDs. These PIDs appear to be consistent, but I don't understand why they are negative or, really, how Perl is emulating Unix fork.

use strict;
use warnings;

my $cpid = fork();

if ($cpid == 0) {
    printf "%s\n", "I'm the child, pid is $$";
} else {
    printf "%s\n", "I'm the parent, pid is $$, cpid is $cpid";
}

This produces something similar to:

I'm the parent, pid is 3428, cpid is -2600
I'm the child, pid is -2600

score 5 · Accepted Answer · answered Nov 02 '15 at 21:31

5

Most of the details are in perlfork, but to answer your specific questions, Pseudo-processes for Perl on Windows are actually implemented as threads. You should interpret a positive PID to be the actual PID of the original thread and negative PIDs are actually thread IDs (negated, of course).

answered Nov 02 '15 at 21:31

tjd

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"You should interpret a positive PID to be the actual PID of the original thread and negative PIDs are actually thread IDs" Is this documented anywhere? – ThisSuitIsBlackNot Nov 02 '15 at 22:09
I read perlfork and couldn't tell where the negative numbers are coming from, only that mysterious things happen with `wait` if you get a pseudo-pid of -1 – Greg Nisbet Nov 02 '15 at 22:36

score 3 · Answer 2 · answered Nov 03 '15 at 12:50

3

If positive PIDs were used for the fake processes created by fork (which are actually threads), they could conflict with PIDs of actual processes.

answered Nov 03 '15 at 12:50

ikegami

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Strawberry Perl negative child pid when forked

2 Answers2