#include <X11/Xlib.h>

int main() {
  Display* dpy = XOpenDisplay(0);
  int scrnum = DefaultScreen(dpy);
  Window root = RootWindow(dpy, scrnum);
  long unsigned int white_pixel = WhitePixel(dpy, scrnum);
  Window win = XCreateSimpleWindow(dpy, root, 0, 0, 100, 100, 0, white_pixel, white_pixel);
  XMapWindow(dpy, win);
  XSelectInput(dpy, win, ExposureMask);

  GC gc = XCreateGC(dpy, win, 0, 0);

  while(1) {
    XEvent event;
    XNextEvent(dpy, &event);
    if(event.type == Expose) {
      XDrawLine(dpy, win, gc, 0, 0, 0, 0); // This produces either one black pixel or nothing in 0x0 coordinates of the window.
    }
  }
}