Thread: Greedy XGrabKeyboard

Hi there,

How do I passively capture keyboard input from a terminal program without locking out the system?

As it stands, my small test program (shown below) locks out all running programs of keyboard input and I've tried a number of variations to try and solve this:
- tried issuing XAllowEvents after KeyPress and KeyRelease, passing AsyncKeyboard, SyncKeyboard and ReplayKeyboard.
- tried using GrabKey with AnyKey, AnyModifier on DefaultRootWindow but that triggers an error (x_grabkey BadAccess (attempt to access private resource denied ...))

Any comments/suggestions would be greatly appreciated!

Code:

int main(int argc, char **argv)
{
   Display *dpy;
   int quit = 0;

   if((dpy = XOpenDisplay(NULL)) == NULL) {
      perror(argv[0]);
      exit(1);
   }

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

   while(!quit) {
      XEvent ev;

      XNextEvent(dpy, &ev);

      switch (ev.type) {
      case KeyPress:
        char *s;
        unsigned int kc;

        kc = ((XKeyPressedEvent*)&ev)->keycode;
        s = XKeysymToString(XKeycodeToKeysym(dpy, kc, 0));

        if(s) printf("KEY: %s\n", s);
        if(!strcmp(s, "q")) quit=~0;
      }
   }

   XUngrabKeyboard(dpy, CurrentTime);

   if (XCloseDisplay(dpy)) {
      perror(argv[0]);
      exit(1);
   }

   return 0;
}

Originally Posted by MK27

For one thing, don't use Xlib to do it. Is this a program you are writing? If so, use libtermcap or ncurses. If not, hmm, google "keylogger".

Thanks for your reply. First off, this isn't a keylogger at all

This is a program I'm writing as an introduction to Linux programming (my background is in Win32.) Its aim is to:
- run as a daemon (already done)
- capture keyboard input
- produce a sound (from a headerless uncompressed sound file) once a key is pressed via ALSA (already done)

This is why I thought of interfacing directly with X11 for keyboard capture. Was that a wrong design call?