x11-window-monitor.c

#define _GNU_SOURCE
#include <X11/Xlib.h>
#include <X11/Xatom.h>
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int get_prop_card32(Display *d, Window w, Atom p) {
	Atom actual_type;
	int actual_format;
	unsigned long nitems, bytes;
	unsigned char *prop;
	int status, value;

	status = XGetWindowProperty(d, w, p,
			/* long_offset */ 0L,
			/* long_length */ 1024L,
			/* delete */ False,
			/* req_type */ XA_CARDINAL,
			/* actual_type_return */ &actual_type,
			/* actual_format_return */ &actual_format,
			/* nitems_return */ &nitems,
			/* bytes_after_return */ &bytes,
			/* prop_return */ &prop);
	if (status != 0)
		return -1;
	if (nitems < 1)
		return -1;
	value = prop[0]
	      | prop[1] << 8
	      | prop[2] << 16
	      | prop[3] << 24;
	XFree(prop);
	return value;
}

pid_t get_window_pid(Display *d, Window w) {
	/* we could use XResQueryClientIds but this usually works fine */
	static Atom am_wm_pid;
	
	if (!am_wm_pid)
		am_wm_pid = XInternAtom(d, "_NET_WM_PID", False);
	return get_prop_card32(d, w, am_wm_pid);
}

int main(void) {
	Display *d;
	int s;
	Window rw;
	XSetWindowAttributes at;
	Atom am_wm_pid;
	XEvent ev;

	d = XOpenDisplay(NULL);
	if (!d)
		errx(1, "cannot open display");
	s = DefaultScreen(d);
	rw = RootWindow(d, s);
	XSelectInput(d, rw, SubstructureNotifyMask);
	for (;;) {
		XNextEvent(d, &ev);
		if (ev.type != CreateNotify)
			continue;
		if (ev.xcreatewindow.parent != rw)
			continue;
		Window w = ev.xcreatewindow.window;
		pid_t pid = get_window_pid(d, w);
		if (pid < 0)
			continue;
		warnx("CreateNotify for window %x pid %d", w, pid);
	}
}