Shift + d to trigger more "d"s

key.c

#include <unistd.h>
#include <stdio.h>
#include <X11/Xlib.h>
#include <X11/Intrinsic.h>
#include <X11/extensions/XTest.h>

int main() {
  Display *dpy    = XOpenDisplay(NULL);
  KeyCode keycode = XKeysymToKeycode(dpy, XK_D);

  XEvent  ev;
  Window  window     = DefaultRootWindow(dpy);
  Bool    keyPressed = False;

  unsigned int  key_modifier = ShiftMask;
  unsigned int  modifiers    = AnyModifier;

  XGrabKey(dpy, keycode, modifiers, window, False, GrabModeAsync, GrabModeAsync);

  XSelectInput(dpy, window, KeyPressMask);

  // Wait for hotkey
  while(True) {
    XNextEvent(dpy, &ev);
    switch(ev.type) {
      case KeyPress:
        // Check Shift + d hotkey
        if ((ev.xkey.state & key_modifier) == (key_modifier)) {
          printf("Hotkey pressed!\n");
          keyPressed = True;
          XUngrabKey(dpy, keycode, modifiers, window);
        }
      default: break;
    }
    if(keyPressed)
      break;
  }

  printf("Charging ...\n");
  sleep(2);

  for (int i=0; i<10; i++) {
    XTestFakeKeyEvent(dpy, keycode, True, 0);
    XTestFakeKeyEvent(dpy, keycode, False, 0);
    XFlush(dpy);
    sleep(1);
  }
  XCloseDisplay(dpy);

  printf(" ... Done!\n");

  return 0;
}