Question

I’m writing a code to get the pressed key and block the keyboard input. Example: If the Super_L or Super_R key is pressed, prevent them to open a application menu. The code works while the program is opened, but when the program is closed the keys are sent to system. Example: If user press the Super_L key nothing happens while the program is running but when the program is closed, automatically the Ubuntu application menu is opened. How can i fix this issue?

void hook()
{
    Display *dpy = XOpenDisplay(0);
    XEvent ev;

    if(!dpy)
    {
        qDebug() << "Error";
        return;
    }

    XGrabKeyboard(dpy, DefaultRootWindow(dpy), false, GrabModeAsync, GrabModeAsync, CurrentTime);

    forever
    {
        XNextEvent(dpy, &ev);
        switch (ev.type)
        {
        case KeyPress:
            qDebug() << "KeyPress" << XKeysymToString(XKeycodeToKeysym(dpy, ev.xkey.keycode, 0));
            break;
        case KeyRelease:
            qDebug() << "KeyRelease" << XKeysymToString(XKeycodeToKeysym(dpy, ev.xkey.keycode, 0));
            break;
        }
    }
}

Answer 1

Quoth the manual:

When the X server’s connection to a client is closed either by an
explicit call to XCloseDisplay() or by a process that exits, the X
server performs the following automatic operations:

• It disowns all selections owned by the client (see XSetSelectionOwner()).
• It performs an XUngrabPointer() and XUngrabKeyboard() if the client has actively grabbed the pointer or the keyboard.
• It performs an XUngrabServer() if the client has grabbed the server.
• It releases all passive grabs made by the client.

Grabs are requested by clients and granted to clients. For the duration of a grab, all relevant events are delivered to the grabbing client. No client, no grab.

I don’t see what’s to fix here. If you want a piece of functionality working, make sure the application that implements it is running.