xcompcheck.c

#include <X11/Xlib.h>
#include <X11/Xatom.h>
#include <stdio.h>
#include <stdlib.h>

int main() {
    Display *d = NULL;
    d = XOpenDisplay(NULL);
    Window w;
    XEvent e;
    Atom atom;
    int screen;

    if (d == NULL) {
        printf("Error opening display\n");
        exit(1);
    }

    screen = DefaultScreen(d);

    static char net_wm_cm[] = "_NET_WM_CM_Sxx";
    snprintf (net_wm_cm, sizeof (net_wm_cm), "_NET_WM_CM_S%d", screen);
    
    atom = XInternAtom (d, net_wm_cm, False);
    w = XGetSelectionOwner(d, atom);

    if (w == None) {
        printf("Compositor isn't registered.");
    } else {
        printf("Compositor is registered.");
    }

    XCloseDisplay(d);
    return 0;
}