Select a specific window X11 - c

Question

I am designing an app that having the ID of an X11 window to draw a rectangle.
The problem I have is that I can not draw nothing in the window.
Code

Answer 1

One obvious error is that you did not select ExposureMask in call to XSelectInput, so you will not receive Expose event you are waiting for.
Other possible problem is not setting foreground drawing color of the GC by XSetForegroundColor, default is black. And using default gc of screen may fail if the window has different color depth or different visual. Other important GC attributes issubwindow_mode (whether to draw over child windows).
Still, after I did those changes, the program only works for me (draws a rectangle) on root window and xev, but not for xterm, no expose events.
This is my fixed version:
#include <X11/Xlib.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
int main (int argc, char *argv[]){
Window win;
Display *display;
XEvent e;
display = XOpenDisplay(NULL);
if(display==NULL){
fprintf(stderr,"Cannot open Display\n");
exit(1);
}
int s = DefaultScreen(display);
//unsigned long *id = (unsigned long*)(argv[1]);
sscanf(argv[1], "0x%x", &win);
if(!XSelectInput(display,win,StructureNotifyMask | ExposureMask)){
fprintf(stderr,"Cannot select Display\n");
return -1;
}
if(!XMapWindow(display,win)){
fprintf(stderr,"Cannot map Display\n");
return -1;
}
XGCValues gcv;
gcv.subwindow_mode = IncludeInferiors;
GC gc = XCreateGC(display, win, GCSubwindowMode,
&gcv);
XSetForeground(display, gc, 0xff00ff00);
XSetPlaneMask(display, gc, 0xffffffff);
while (1) {
puts("waiting for event\n");
XNextEvent(display, &e);
puts("got event\n");
if (e.type == Expose) {
printf("drawing\n");
XFillRectangle(display, win, gc, 20, 20, 100, 100);
}
if (e.type == KeyPress)
break;
}
return 0;
}