centre.c

//gcc centre.c -lxcb -lxcb-util -o centre

#include <stdio.h>
#include <stdlib.h>
#include <xcb/xcb.h>
#include <xcb/xcb_aux.h>

int main (int argc, char **argv)
{
    (void) argc;
    (void) argv;

    xcb_connection_t *c = xcb_connect(NULL, NULL);
    if(xcb_connection_has_error(c)){
        fputs("Failed to open display\n", stderr);
        return 1;
    }

    xcb_screen_t *s = xcb_setup_roots_iterator(xcb_get_setup(c)).data;
    if(!s){
        fputs("Couldn't get screen\n", stderr);
        return 1;
    }

    xcb_get_input_focus_reply_t *f = xcb_get_input_focus_reply(c, xcb_get_input_focus(c), NULL);
    if(!f){
        fputs("Couldn't get focused window\n", stderr);
        return 1;
    }

    xcb_window_t w = f->focus;
    free(f);

    if(w == s->root){
        return 0;
    }
    
    xcb_get_geometry_reply_t *sg = xcb_get_geometry_reply(c, xcb_get_geometry(c, s->root), NULL);
    if(!sg){
        fputs("Couldn't get root window geometry\n", stderr);
        return 1;
    }

    xcb_get_geometry_reply_t *fg = xcb_get_geometry_reply(c, xcb_get_geometry(c, w), NULL);
    if(!fg){
        fputs("Couldn't get focused window geometry\n", stderr);
        return 1;
    }

    const uint32_t v[] = {(sg->width - fg->width)/2, (sg->height - fg->height)/2};
    free(sg);
    free(fg);

    xcb_configure_window(c, w, XCB_CONFIG_WINDOW_X | XCB_CONFIG_WINDOW_Y, v);
    xcb_aux_sync(c);

    if(c){
        xcb_disconnect(c);
        return 0;
    }

    return 1;
}