DllExport unsigned int
win32_alarm(unsigned int sec)
{
    /*
     * the 'obvious' implentation is SetTimer() with a callback
     * which does whatever receiving SIGALRM would do
     * we cannot use SIGALRM even via raise() as it is not
     * one of the supported codes in <signal.h>
     */
    dTHX;

    if (w32_message_hwnd == INVALID_HANDLE_VALUE)
        w32_message_hwnd = win32_create_message_window();

    if (sec) {
        if (w32_message_hwnd == NULL)
            w32_timerid = SetTimer(NULL, w32_timerid, sec*1000, NULL);
        else {
  	    w32_timerid = 1;
            SetTimer(w32_message_hwnd, w32_timerid, sec*1000, NULL);
        }
    }
    else {
    	if (w32_timerid) {
            KillTimer(w32_message_hwnd, w32_timerid);
  	    w32_timerid = 0;
    	}
    }
    return 0;
}

##</code><code>##

static LRESULT
win32_process_message(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    /* BEWARE. The context retrieved using dTHX; is the context of the
     * 'parent' thread during the CreateWindow() phase - i.e. for all messages
     * up to and including WM_CREATE.  If it ever happens that you need the
     * 'child' context before this, then it needs to be passed into
     * win32_create_message_window(), and passed to the WM_NCCREATE handler
     * from the lparam of CreateWindow().  It could then be stored/retrieved
     * using [GS]etWindowLongPtr(... GWLP_USERDATA ...), possibly eliminating
     * the dTHX calls here. */
    /* XXX For now it is assumed that the overhead of the dTHX; for what
     * are relativley infrequent code-paths, is better than the added
     * complexity of getting the correct context passed into
     * win32_create_message_window() */
    dTHX;

    switch(msg) {

#ifdef USE_ITHREADS
        case WM_USER_MESSAGE: {
            long child = find_pseudo_pid(aTHX_ (int)wParam);
            if (child >= 0) {
                w32_pseudo_child_message_hwnds[child] = (HWND)lParam;
                return 1;
            }
            break;
        }
#endif

        case WM_USER_KILL: {
            /* We use WM_USER_KILL to fake kill() with other signals */
            int sig = (int)wParam;
            if (do_raise(aTHX_ sig))
                sig_terminate(aTHX_ sig);

            return 1;
        }

        case WM_TIMER: {
            /* alarm() is a one-shot but SetTimer() repeats so kill it */
            if (w32_timerid && w32_timerid==(UINT)wParam) {
                KillTimer(w32_message_hwnd, w32_timerid);
                w32_timerid=0;

                /* Now fake a call to signal handler */
                if (do_raise(aTHX_ 14))
                    sig_terminate(aTHX_ 14);

                return 1;
            }
            break;
        }

        default:
            break;

    } /* switch */

    /* Above or other stuff may have set a signal flag, and we may not have
     * been called from win32_async_check() (e.g. some other GUI's message
     * loop.  BUT DON'T dispatch signals here: If someone has set a SIGALRM
     * handler that die's, and the message loop that calls here is wrapped
     * in an eval, then you may well end up with orphaned windows - signals
     * are dispatched by win32_async_check() */

    return 0;
}

##</code><code>##

>perl lockdemo.pl
locker: flock LOCK_EX
locker: locked
locker: sleep 10
main: alarm 5
main: flock LOCK_EX                          ### The lock is acquired here
main: timeout at lockdemo.pl line 30.        ### before the timeout occurs.

main: 5 seconds have passed
locker: flock LOCK_UN
locker: unlocked