How does XCB's event handling detect the ESC key in C?

Question

I've been studying an XCB tutorial, but I'm stuck on this one. In this example, the program quits when you press the ESC key. But in the example code, I can't figure out how "case 9" catches the ESC key. I've searched and looked all through xcb.h and xproto.h , and the ASCII table.

I thought I understood it, until I got to "case 9". This was where I got totally lost. So basically, where do they get 9 from? And how does that correspond to ESC ?

Here's the example code I was studying:

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

#define WIDTH 300
#define HEIGHT 100 

static xcb_gc_t gc_font_get (xcb_connection_t *c,
                             xcb_screen_t     *screen,
                             xcb_window_t      window,
                             const char       *font_name);

static void text_draw (xcb_connection_t *c,
                       xcb_screen_t     *screen,
                       xcb_window_t      window,
                       int16_t           x1,
                       int16_t           y1,
                       const char       *label)
{
  xcb_void_cookie_t    cookie_gc;
  xcb_void_cookie_t    cookie_text;
  xcb_generic_error_t *error;
  xcb_gcontext_t       gc;
  uint8_t              length;

  length = strlen (label);

  gc = gc_font_get(c, screen, window, "7x13");

  cookie_text = xcb_image_text_8_checked (c, length, window, gc,
                                          x1,
                                          y1, label);
  error = xcb_request_check (c, cookie_text);
  if (error) {
    fprintf (stderr, "ERROR: can't paste text : %d\n", error->error_code);
    xcb_disconnect (c);
    exit (-1);
  }

  cookie_gc = xcb_free_gc (c, gc);
  error = xcb_request_check (c, cookie_gc);
  if (error) {
    fprintf (stderr, "ERROR: can't free gc : %d\n", error->error_code);
    xcb_disconnect (c);
    exit (-1);
  }
}

static xcb_gc_t gc_font_get (xcb_connection_t *c,
                             xcb_screen_t     *screen,
                             xcb_window_t      window,
                             const char       *font_name)
{
  uint32_t             value_list[3];
  xcb_void_cookie_t    cookie_font;
  xcb_void_cookie_t    cookie_gc;
  xcb_generic_error_t *error;
  xcb_font_t           font;
  xcb_gcontext_t       gc;
  uint32_t             mask;

  font = xcb_generate_id (c);
  cookie_font = xcb_open_font_checked (c, font,
                                       strlen (font_name),
                                       font_name);

  error = xcb_request_check (c, cookie_font);
  if (error) {
    fprintf (stderr, "ERROR: can't open font : %d\n", error->error_code);
    xcb_disconnect (c);
    return -1;
  }

  gc = xcb_generate_id (c);
  mask = XCB_GC_FOREGROUND | XCB_GC_BACKGROUND | XCB_GC_FONT;
  value_list[0] = screen->black_pixel;
  value_list[1] = screen->white_pixel;
  value_list[2] = font;
  cookie_gc = xcb_create_gc_checked (c, gc, window, mask, value_list);
  error = xcb_request_check (c, cookie_gc);
  if (error) {
    fprintf (stderr, "ERROR: can't create gc : %d\n", error->error_code);
    xcb_disconnect (c);
    exit (-1);
  }

  cookie_font = xcb_close_font_checked (c, font);
  error = xcb_request_check (c, cookie_font);
  if (error) {
    fprintf (stderr, "ERROR: can't close font : %d\n", error->error_code);
    xcb_disconnect (c);
    exit (-1);
  }

  return gc;
}

int main ()
{
  xcb_screen_iterator_t screen_iter;
  xcb_connection_t     *c;
  const xcb_setup_t    *setup;
  xcb_screen_t         *screen;
  xcb_generic_event_t  *e;
  xcb_generic_error_t  *error;
  xcb_void_cookie_t     cookie_window;
  xcb_void_cookie_t     cookie_map;
  xcb_window_t          window;
  uint32_t              mask;
  uint32_t              values[2];
  int                   screen_number;

  /* getting the connection */
  c = xcb_connect (NULL, &screen_number);
  if (!c) {
    fprintf (stderr, "ERROR: can't connect to an X server\n");
    return -1;
  }

  /* getting the current screen */
  setup = xcb_get_setup (c);

  screen = NULL;
  screen_iter = xcb_setup_roots_iterator (setup);
  for (; screen_iter.rem != 0; --screen_number, xcb_screen_next (&screen_iter))
    if (screen_number == 0)
      {
        screen = screen_iter.data;
        break;
      }
  if (!screen) {
    fprintf (stderr, "ERROR: can't get the current screen\n");
    xcb_disconnect (c);
    return -1;
  }

  /* creating the window */
  window = xcb_generate_id (c);
  mask = XCB_CW_BACK_PIXEL | XCB_CW_EVENT_MASK;
  values[0] = screen->white_pixel;
  values[1] =
    XCB_EVENT_MASK_KEY_RELEASE |
    XCB_EVENT_MASK_BUTTON_PRESS |
    XCB_EVENT_MASK_EXPOSURE |
    XCB_EVENT_MASK_POINTER_MOTION;
  cookie_window = xcb_create_window_checked (c,
                                             screen->root_depth,
                                             window, screen->root,
                                             20, 200, WIDTH, HEIGHT,
                                             0, XCB_WINDOW_CLASS_INPUT_OUTPUT,
                                             screen->root_visual,
                                             mask, values);
  cookie_map = xcb_map_window_checked (c, window);

  /* error managing */
  error = xcb_request_check (c, cookie_window);
  if (error) {
    fprintf (stderr, "ERROR: can't create window : %d\n", error->error_code);
    xcb_disconnect (c);
    return -1;
  }
  error = xcb_request_check (c, cookie_map);
  if (error) {
    fprintf (stderr, "ERROR: can't map window : %d\n", error->error_code);
    xcb_disconnect (c);
    return -1;
  }

  xcb_flush(c);

  while (1) {
    e = xcb_poll_for_event(c);
    if (e) {
      switch (e->response_type & ~0x80) {
      case XCB_EXPOSE: {
        char *text;

        text = "Press ESC key to exit...";
        text_draw (c, screen, window, 10, HEIGHT - 10, text);
        break;
      }
      case XCB_KEY_RELEASE: {
        xcb_key_release_event_t *ev;

        ev = (xcb_key_release_event_t *)e;

        switch (ev->detail) {
          /* ESC */
        case 9:
          free (e);
          xcb_disconnect (c);
          return 0;
        }
      }
      }
      free (e);
    }
  }

  return 0;
}

Thanks

Answer 1

Low-level X KeyCode s lie in the range 8..255 and represent the physical layout of the keyboard. Presumably in practice the value 9 ("1" relative to the start of the range, ie upper-left) is the Esc key.

With that said, it seems like a really bad idea to hard-code device-specific knowledge like this rather than looking up which keycode is associated with the Esc KeySym .

This link seems to provide some useful information:

https://tronche.com/gui/x/xlib/input/keyboard-encoding.html

Answer 2

I thought I understood it, until I got to "case 9". This was where I got totally lost. So basically, where do they get 9 from? And how does that correspond to ESC?

If you open a terminal and run:

xev | grep keycode

Then you press the Esc key on the xev window that pops up, you will see:

    state 0x0, keycode 9 (keysym 0xff1b, Escape), same_screen YES,
    state 0x0, keycode 9 (keysym 0xff1b, Escape), same_screen YES,

You could do crazy stuff like use xmodmap to remap your escape key, but it would still be keycode 9 , even though you mapped it to something else... Using xmodmap I reset keycode 9 to be F1, but xev can still see that I'm really using the Escape key (keycode 9)

    state 0x0, keycode 9 (keysym 0xffbe, F1), same_screen YES,
    state 0x0, keycode 9 (keysym 0xffbe, F1), same_screen YES,

Historically WHY is keycode 9 the Escape key?

If you run:

showkey --ascii
# press the Escape key
^[  27 0033 0x1b

You see that Escape maps to 27 on the ASCII table. Meanwhile to grab the key's "Scancode":

sudo showkey --scancodes
# press the Escape key
^[  0x01 0x81

Since you're working with xcb in this tutorial and only Xcb/X11 must be considered, Code 9 will always be the hardware Escape key.

You can use the setkeycodes ( kbd package) to set scancodes to map to other keycodes in Linux, but these codes WILL NOT be picked up by X.

Depending on the layer (USB hardware/kernel/X11) that you're operating at, you'll need to choose the appropriate identifier for the appropriate key.

---

I believe the tutorial that @s0s was referring to is available online at:

https://www.x.org/releases/X11R7.5/doc/libxcb/tutorial/