How to subscribe to different events and debug them in an xcb demo application?

Question

I'm trying to create a simple application that interacts with the xcb library functions. My objective is to emulate the behavior of a simple window manager. From what I understood, I need to setup Substructure Redirection on the root window, then listen to specific events on that window in order to organize its top level windows the way I want. The problem is, I just cannot figure out how to test the solution I came up with.

This is the code responsible for doing what I explained:

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

int main() {
    xcb_connection_t *connection = xcb_connect(NULL, NULL);
    xcb_screen_t *screen = xcb_setup_roots_iterator(xcb_get_setup(connection)).data;

    /* Subscribing to the desired events */
    uint32_t mask = XCB_CW_EVENT_MASK | 
                    XCB_EVENT_MASK_SUBSTRUCTURE_NOTIFY | 
                    XCB_EVENT_MASK_SUBSTRUCTURE_REDIRECT;
    uint32_t values[3] = { XCB_KEY_PRESS, 1, 1 };

    /* This applies the values and mask to the root window I think */
    xcb_change_window_attributes(connection, screen->root, mask, values);

    xcb_flush(connection);

    xcb_generic_event_t *ev;
    while( (ev = xcb_wait_for_event(connection)) ) {
        switch(ev->response_type & ~0x80) {
            case XCB_KEY_PRESS: {
                xcb_key_press_event_t *ev = (xcb_key_press_event_t *)ev;

                if(ev->state & XCB_MOD_MASK_1) {
                    printf("Pressed the Alt key modifier\n");
                }
                printf("Key pressed: Keycode %d\n", ev->detail);

                break;
            }
            case XCB_CREATE_NOTIFY: {
                printf("CREATE NOTIFY\n");
                break;
            }
            case XCB_MAP_NOTIFY: {
                printf("MAP NOTIFY\n");
                break;
            }
        }

        free(ev);
    }

    xcb_disconnect(connection);
}

As you can see, the code is pretty simple, and I plan to expand it if I can find a solution to the problem at hand.

I'm using Xephyr to run the program. I just ran Xephyr -br -ac -noreset -screen 500x500 :1 to create a new window, and then DISPLAY=:1 ./myprogram to test it. Obviously nothing happens in the screen, but I wonder how can I get the outputs of the printf calls.

EDIT: The XCB_CREATE_NOTIFY and XCB_MAP_NOTIFY events are there because I'm expecting that when I init a new application inside this window, the X server would send that request to the window manager, which has the substructure redirection toggled. That way I could correctly alter the desired properties of the window. Please correct me if I'm wrong.

Answer 1

Call xcb_flush() before you while-loop.

By default, sent requests end up in the output buffer. You need to explicitly call xcb_flush() to have the requests actually sent.

but I wonder how can I get the outputs of the printf calls.

There is no output since no printf calls run yet, because your ChangeWindowAttributes request was not yet actually sent.

---

Actually, when doing that and running your program under x11trace shows:

000:<:0001: 16: Request(2): ChangeWindowAttributes window=0x0000051e value-list={event-mask=KeyRelease}
000:>:0001:Error 16=Length: major=2, minor=0, bad=0x0000051e, seq=0001

Your request fails. And does not send what you expect it to send.

You want:

uint32_t mask = XCB_CW_EVENT_MASK;
uint32_t values[3] = { XCB_EVENT_MASK_SUBSTRUCTURE_NOTIFY | XCB_EVENT_MASK_SUBSTRUCTURE_REDIRECT };

This still produces no output. I see events of type ConfigureRequest and MapRequest being generated, but none of these events are being printed.

I added the following to the switch to actually get some output:

default: printf("%d\n", ev->response_type);

(Also, your printf calls are missing a trailing \n.)