How to use use XNextEvent, XPeekEvent and XPutBackEvent

Question

I am trying to develop a little Linux application to perform measurements directly on the screen i.e. to measure distances and angles in images, documents etc.

To this end I was inspired by How to listen for mouse events in Linux? and thought I could use XNextEvent or XPeekEvent and possibly XPutBackEvent. This program responds nicely to mouse events but since it uses XNextEvent the bucket stops there and the event is not sent to the target window and the mouse becomes useless and if I use XPutBackEvent the same event comes back again. To fix this I replaced XNextEvent with XPeekEvent, which is supposed to send the event up the ladder. This didn't help but made things worse sending an endless stream of the same event, filtering out equal events didn't help either.

Thanks in advance for any tip to solve this problem.

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The problem is basically that the events do not disappear and I don't know how to get rid of them in order to be able to use the mouse. The code below is copied from How to listen for mouse events in Linux? and somewhat modified.

#include <stdio.h>
#include <X11/Xlib.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
char *key_name[] = {
    "first",
    "second (or middle)",
    "third",
    "fourth",  // :D
    "fivth"    // :|
};

int main(int argc, char **argv)
{
    Display *display;
    XEvent xevent,oldevent;
    Window window;
        long unsigned int buttontime=0;
    if( (display = XOpenDisplay(NULL)) == NULL )
        return -1;


    window = DefaultRootWindow(display);
    XAllowEvents(display, AsyncBoth, CurrentTime);

    XGrabPointer(display, 
                 window,
                 1, 
                 //PointerMotionMask | 
                 ButtonPressMask | ButtonReleaseMask , 
                 GrabModeAsync,
                 GrabModeAsync, 
                 None,
                 None,
                 CurrentTime);
    while(1)
     {
        XNextEvent(display, &xevent);
        //XPeekEvent(display, &xevent);
        switch (xevent.type) {
            /*case MotionNotify:
                printf("Mouse move      : [%d, %d]\n", xevent.xmotion.x_root, xevent.xmotion.y_root);
                break;*/
            case ButtonPress:
                //if ((buttontime <= xevent.xbutton.time))
                    {
                printf("Button pressed  : %s\n", key_name[xevent.xbutton.button - 1]);
                printf("timevent %lu\n",xevent.xbutton.time);
                printf("timeold %lu\n",buttontime);
                buttontime = xevent.xbutton.time;
                buttontime++;
              }
                break;
            case ButtonRelease:
                printf("Button released : %s\n", key_name[xevent.xbutton.button - 1]);
                break;
        }
       // XPutBackEvent(display, &xevent);
        sleep(1);
    }

    return 0;
}