$ gcc -lX11 foobar.c -o foobar

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

int main(int argc, char *argv[])
{
    char *disp_name = ":0";
    Display *disp;

    if(!(disp = XOpenDisplay(disp_name))) return 1;
    while(1) {
        char *name;
        int id, ret;
        Window wid;

        XGetInputFocus(disp, &wid, &id);
        ret = XFetchName(disp, wid, &name);
        if(ret != 0) printf("Window name: %s\n", name);
        XFree(name);
        sleep(1);
    }

    return 0;
}