X11/Xlib: Window always on top

Question

A window should stay on top of all other windows. Is this somehow possible with plain x11/xlib? Googling for "Always on top" and "x11" / "xlib" didn't return anything useful.

I'd avoid toolkits like GTK+, if somehow possible.

I'm using Ubuntu with gnome desktop. In the window menu, there's an option "Always On Top". Is this provided by the X server or the window manager? If the second is the case, is there a general function that can be called for nearly any wm? Or how to do this in an "X11-generic" way?

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Edit: I implemented fizzer's answer, now having following code:

XSelectInput(this->display, this->window,
    ButtonPressMask |
    StructureNotifyMask |
    ExposureMask |
    KeyPressMask |
    PropertyChangeMask |
    VisibilityChangeMask ); 
// ...
// In a loop:
if (XPending(this->display) >= 0)
{
    XNextEvent(this->display, &ev);
    switch(ev.type) {
    // ...
    case VisibilityNotify:
        XRaiseWindow(this->display, this->window);
        XFlush(this->display);
    break;
    // ...
    }
}

But the eventhandling and raising nearly never gets executed even my mask is correct?!

Answer 1

#define _NET_WM_STATE_REMOVE        0    // remove/unset property
#define _NET_WM_STATE_ADD           1    // add/set property
#define _NET_WM_STATE_TOGGLE        2    // toggle property
...
...
Atom wmStateAbove = XInternAtom( display, "_NET_WM_STATE_ABOVE", 1 );
if( wmStateAbove != None ) {
    printf( "_NET_WM_STATE_ABOVE has atom of %ld\n", (long)wmStateAbove );
} else {
    printf( "ERROR: cannot find atom for _NET_WM_STATE_ABOVE !\n" );
}

Atom wmNetWmState = XInternAtom( display, "_NET_WM_STATE", 1 );
if( wmNetWmState != None ) {
    printf( "_NET_WM_STATE has atom of %ld\n", (long)wmNetWmState );
} else {
    printf( "ERROR: cannot find atom for _NET_WM_STATE !\n" );
}
// set window always on top hint
if( wmStateAbove != None ) {
    XClientMessageEvent xclient;
    memset( &xclient, 0, sizeof (xclient) );
    //
    //window  = the respective client window
    //message_type = _NET_WM_STATE
    //format = 32
    //data.l[0] = the action, as listed below
    //data.l[1] = first property to alter
    //data.l[2] = second property to alter
    //data.l[3] = source indication (0-unk,1-normal app,2-pager)
    //other data.l[] elements = 0
    //
    xclient.type = ClientMessage;
    xclient.window = mywin; // GDK_WINDOW_XID(window);
    xclient.message_type = wmNetWmState; //gdk_x11_get_xatom_by_name_for_display( display, "_NET_WM_STATE" );
    xclient.format = 32;
    xclient.data.l[0] = _NET_WM_STATE_ADD; // add ? _NET_WM_STATE_ADD : _NET_WM_STATE_REMOVE;
    xclient.data.l[1] = wmStateAbove; //gdk_x11_atom_to_xatom_for_display (display, state1);
    xclient.data.l[2] = 0; //gdk_x11_atom_to_xatom_for_display (display, state2);
    xclient.data.l[3] = 0;
    xclient.data.l[4] = 0;
    //gdk_wmspec_change_state( FALSE, window,
    //  gdk_atom_intern_static_string ("_NET_WM_STATE_BELOW"),
    //  GDK_NONE );
    XSendEvent( display,
      //mywin - wrong, not app window, send to root window!
      root, // !! DefaultRootWindow( display ) !!!
      False,
      SubstructureRedirectMask | SubstructureNotifyMask,
      (XEvent *)&xclient );
  }