I wasn't aware that Windows has a process (message) queue. So far, I was under the impression that for a message queue you always needed a window handle.

Um. I get a little fuzzy with my terminology. Threads can(*) have message queues, not processes. Of course, in a single threaded process, the two are (roughly) equivalent.

(*)From PostThreadMessage(): "The system creates a thread's message queue when the thread makes its first call to one of the User or GDI functions.".

In Perl, this (probably) occurs when Win32_create_message_window() is called:

## win32.c

HWND
win32_create_message_window()
{
    /* "message-only" windows have been implemented in Windows 2000 an
+d later.
     * On earlier versions we'll continue to post messages to a specif
+ic
     * thread and use hwnd==NULL.  This is brittle when either an embe
+dding
     * application or an XS module is also posting messages to hwnd=NU
+LL
     * because once removed from the queue they cannot be delivered to
+ the
     * "right" place with DispatchMessage() anymore, as there is no Wi
+ndowProc
     * if there is no window handle.
     */
    if (!IsWin2000())
        return NULL;

    return CreateWindow("Static", "", 0, 0, 0, 0, 0, HWND_MESSAGE, 0, 
+0, NULL);
}
[download]

See also win32_kill() in the same file.

If you use the fork emulation, to start a non-perl, child process, then a thread is spawned to act as a 'placeholder' for the actual process. This then waits for the alien process to terminate and so can (could?) post (raise) a SIGCHLD to the main or spawning thread. The problem is that signals sent to the child pseudo-process' pseudo-pid do not necessarially reflect the state of, or affect, the real alien process. And that's where the emulation falls down.