Register hotkey with modifiers only on Linux

Question

I use this sample to set a hotkey in my program on the Linux X11 graphics system. The problem is that I don’t understand how to set hotkey combinations like Ctrl+ Altand Ctrl+ Shift, i.e. Without any keys, only modifiers. I try like this:

KeyCode key = XKeysymToKeycode(display, 0); //no key code
XGrabKey(display, key, ControlMask | ShiftMask, grabWin, true, GrabModeAsync, GrabModeAsync);

But it does not work. However, it works like this (view):

 KeyCode key = XKeysymToKeycode(display, XK_Alt_L); //Alt key
 XGrabKey(display, key, ControlMask, grabWin, true, GrabModeAsync, GrabModeAsync);

I do not like this solution because:

• Logically wrong
• It only works when Ctrl+ is Altpressed, not Alt+ Ctrl, i.e. important to press order
• It blocks all other combinations in the windows with the Ctrland keys Alt.

What am I doing wrong?