X11 input filtering example // vertexSymphony

#include <stdio.h>
#include <stdlib.h>

#include <X11/Xlib.h>
#include <X11/keysym.h>


void injectToCurrentWindow(Display* targetDisplay, Window rootWindow, KeySym keyToInject, int keyModifiers, int keyPress)
{


    Window      xFocused;
    XKeyEvent   xInputEvent;
    int         xRevert;

    XGetInputFocus(targetDisplay, &xFocused, &xRevert);

    xInputEvent.display     = targetDisplay;
    xInputEvent.window      = xFocused;
    xInputEvent.root        = rootWindow;
    xInputEvent.subwindow   = None;
    xInputEvent.time        = CurrentTime;
    xInputEvent.x           = 1;
    xInputEvent.y           = 1;
    xInputEvent.x_root      = 1;
    xInputEvent.y_root      = 1;
    xInputEvent.same_screen = True;
    xInputEvent.keycode     = XKeysymToKeycode(targetDisplay, keyToInject);
    xInputEvent.state       = keyModifiers;

    /* Take key press or release accordingly and dispatch event */
    xInputEvent.type = keyPress?KeyPress:KeyRelease;
    XSendEvent(xInputEvent.display,xInputEvent.window, True, KeyPressMask, (XEvent*) &xInputEvent );
}


int main(void)
{
    Display*    xDisplay;
    Window      xRoot;

    KeySym      xKeyPressed;
    XEvent      xKeyEvent;

    int         runLoop = 1;


    /* Just open default display and check for errors */
    xDisplay = XOpenDisplay(0);

    if (!xDisplay){
        fprintf(stderr,"Failed to open display ! \r\n");
        exit(1);
    }

    /* Get root window and do the actual grab ... this will fail of other app has a grab */
    xRoot = DefaultRootWindow(xDisplay);
    XGrabKeyboard(xDisplay, xRoot, False, GrabModeAsync, GrabModeAsync, CurrentTime);


    while(runLoop){

        /* Peek events */
        XNextEvent(xDisplay,&xKeyEvent);

        if (xKeyEvent.type == KeyPress || xKeyEvent.type == KeyRelease){

            /* Convert and operate with KeySym ... We only allow "a" to pass, "q" stops the software  */
            xKeyPressed = XKeycodeToKeysym(xDisplay, xKeyEvent.xkey.keycode, 0);

            if (xKeyPressed == XK_a) injectToCurrentWindow(xDisplay, xRoot, XK_a, xKeyEvent.xkey.state, xKeyEvent.type == KeyPress?1:0  );
            if (xKeyPressed == XK_q) runLoop = 0;

            printf("[%s] You shall not pass !!! \r\n",XKeysymToString(xKeyPressed));
        }



    }

    XUngrabKeyboard(xDisplay,CurrentTime);
    XCloseDisplay(xDisplay);

}