Ignore auto repeat in X11 applications

Question

If you press and hold a key in X11 while AutoRepeat is enabled, you continuously receive KeyPress and KeyRelease events. I know that AutoRepeat can be disabled using the function XAutoRepeatOff(), but this changes the setting for the whole X server. Is there a way to either disable AutoRepeat for a single application or to ignore repeated keystrokes?

What I'm looking for is a single KeyPress event when a key is pressed and a single KeyRelease event when a key is released, without interfering with the X server's AutoRepeat setting.

Here's a minimal example to get you going (mostly from the Beginner Xlib Tutorial):

#include <stdio.h>
#include <stdlib.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <X11/Xos.h>
#include <X11/Xatom.h>
#include <X11/keysym.h>

Display *dis;
Window win;
XEvent report;

int main ()
{
  dis = XOpenDisplay (NULL);
  // XAutoRepeatOn(dis);
  win = XCreateSimpleWindow (dis, RootWindow (dis, 0), 1, 1, 500, 500,
        0, BlackPixel (dis, 0), BlackPixel (dis, 0));
  XSelectInput (dis, win, KeyPressMask | KeyReleaseMask);
  XMapWindow (dis, win);
  XFlush (dis);

  while (1)
    {
      XNextEvent (dis, &report);
      switch (report.type)
 {
 case KeyPress:
   fprintf (stdout, "key #%ld was pressed.\n",
     (long) XLookupKeysym (&report.xkey, 0));
   break;
 case KeyRelease:
   fprintf (stdout, "key #%ld was released.\n",
     (long) XLookupKeysym (&report.xkey, 0));
   break;
 }
    }

  return (0);
}

Answer 1

When you receive a key release and the next event is a key press of the same key combination, then it's auto-repeat and the key wasn't acutally released. You can use code like this to peek next event

if (event->type == KeyRelease && XEventsQueued(disp, QueuedAfterReading)) {   XEvent nev;   XPeekEvent(disp, &nev);        if (nev.type == KeyPress && nev.xkey.time == event->xkey.time &&       nev.xkey.keycode == event->xkey.keycode)   {     /* Key wasn’t actually released */   } } 

Answer 2

You can use the XkbSetDetectableAutorepeat function to tell the X server to only send KeyRelease events when the user actually releases the key - when you don't want autorepeat events, then you discard any KeyPress without matching KeyRelease.

Answer 3

For your reference, here's a working minimal example that deletes auto-repeated KeyPress events. Thank you, kralyk!

#include <stdio.h>
#include <stdlib.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <X11/Xos.h>
#include <X11/Xatom.h>
#include <X11/keysym.h>

Display *dis;
Window win;
XEvent report;

int main ()
{
  dis = XOpenDisplay (NULL);
  // XAutoRepeatOn(dis);
  win = XCreateSimpleWindow (dis, RootWindow (dis, 0), 1, 1, 500, 500,
        0, BlackPixel (dis, 0), BlackPixel (dis, 0));
  XSelectInput (dis, win, KeyPressMask | KeyReleaseMask);
  XMapWindow (dis, win);
  XFlush (dis);

  while (1)
    {
      XNextEvent (dis, &report);
      switch (report.type)
 {
 case KeyPress:
   fprintf (stdout, "key #%ld was pressed.\n",
     (long) XLookupKeysym (&report.xkey, 0));
   break;
 case KeyRelease:
   {
     unsigned short is_retriggered = 0;

     if (XEventsQueued(dis, QueuedAfterReading))
       {
         XEvent nev;
         XPeekEvent(dis, &nev);

         if (nev.type == KeyPress && nev.xkey.time == report.xkey.time &&
             nev.xkey.keycode == report.xkey.keycode)
           {
             fprintf (stdout, "key #%ld was retriggered.\n",
               (long) XLookupKeysym (&nev.xkey, 0));

             // delete retriggered KeyPress event
             XNextEvent (dis, &report);
             is_retriggered = 1;
           }
       }

     if (!is_retriggered)
       fprintf (stdout, "key #%ld was released.\n",
         (long) XLookupKeysym (&report.xkey, 0));
   }
   break;
 }
    }

  return (0);
}