Linux X11 - Global Keyboard Hook

Is it possible (or how) to create a mechanism (in Linux X11, C++) that works like a global hook in windows (SetWindowsHookEx())?

I would like to be able to catch the key event but with the possibility of further propagation. I'm trying to use a XGrabKey solution (like in xbindkeys) but when I set capturing the key event, this event is "consumed".

Requirements for this mechanism are the following:

1. Global / system-wide - catching events regardless of the window that has focus
2. The possibility of "catch-hold" and "catch-pass through"
3. It must be quite fast

Sample code looks like this:

bool myFlagIsSet = false;
XEvent event;
while (true) {
    while (XPending(display) > 0) {
        usleep(SLEEP_TIME);
    }

    XNextEvent(display, &event);
    switch (e.type) {
        case KeyPress:
            if (myFlagIsSet) {
                //do not propagate
            }
            // propagate
            break;
        case KeyRelease:
            if (myFlagIsSet) {
                //do not propagate
            }
            // propagate
            break;
    }
}

On Windows I simply wrote:

if(event.isConsumed()) {
    return LRESULT(1);
}
//...
return CallNextHookEx(hookHandle, nCode, wParam, lParam);

I've also tried using XUngrabKey and XSendEvent:

switch (event.type) {
    case KeyPress:
        if (myFlagIsSet) {
            //do not propagate
        }
        // propagate
        XUngrabKey(...);
        XSendEvent(..., &event);
        XGrabKey(...);
        break;
    case KeyRelease:
        ...
    }

Unfortunately XSendEvent for unknown reasons to me - do not send this event even if XGrabKey line is commented.

Is it possible to successfully complete this approach?

Please suggest some other approach if I am condemned to failure

EDIT

I would like to implement this on Ubuntu Gnome using Compiz Window Manager