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X11lib

# apt-get install libx11-dev
gcc prova.c -lX11

/*
   Simple Xlib application drawing a box in a window.
   To Compile: gcc -O2 -Wall -o test test.c -L /usr/X11R6/lib -lX11 -lm
 */
  
 #include<X11/Xlib.h>
 #include <stdio.h>
 #include <stdlib.h>
 
int main() {
	Display *d;
	int s;
	Window w;
	XEvent e;

	/* open connection with the server */
	d=XOpenDisplay(NULL);

	if(d==NULL) {
		printf("Cannot open display\n");
		exit(1);
	}
	s=DefaultScreen(d);

	/* create window */
	w=XCreateSimpleWindow(d, RootWindow(d, s), 0, 0, 32, 32, 1, BlackPixel(d, s), WhitePixel(d, s));

	// Process Window Close Event through event handler so XNextEvent does Not fail
	Atom delWindow = XInternAtom( d, "WM_DELETE_WINDOW", 0 );
	XSetWMProtocols(d , w, &delWindow, 1);

	/* select kind of events we are interested in */
	XSelectInput(d, w, ExposureMask | KeyPressMask);

	/* map (show) the window */
	XMapWindow(d, w);

	/* event loop */
	while(1) {
		XNextEvent(d, &e);
		/* draw or redraw the window */
		if(e.type==Expose) {
			XFillRectangle(d, w, DefaultGC(d, s), 20, 20, 10, 10);
			XFillRectangle(d, w, DefaultGC(d, s), 2, 2, 10, 10);
		}
		/* exit on key press */
		if(e.type==KeyPress) {
			break;
		}
		// Handle Windows Close Event
		if(e.type==ClientMessage) {
			break;
		}
	}

	/* destroy our window */
	XDestroyWindow(d, w);

	/* close connection to server */
	XCloseDisplay(d);

	return 0;
}