Print how many clients the X server has (including this one)

numclients.c

/* Link with -lXRes -lX11 */

#include <X11/Xlib.h>
#include <X11/extensions/XRes.h>

#include <stdio.h>

int main() {
	Display *display = NULL;
	XResClient *clients = NULL;
	int num_client = 0;
	int ret = 1;
	int event_base, error_base;

	display = XOpenDisplay(NULL);
	if (!display) {
		fprintf(stderr, "Failed to open display.\n");
		goto out;
	}
	if (!XResQueryExtension(display, &event_base, &error_base)) {
		fprintf(stderr, "XRes not supported.\n");
		goto out;
	}
	if (!XResQueryClients(display, &num_client, &clients)) {
		fprintf(stderr, "QueryClients failed.\n");
		goto out;
	}
	printf("%d\n", num_client);	
	
	ret = 0;
out:
	XFree(clients);
	if (display) {
		XCloseDisplay(display);
	}
	return ret;
}