XCB

XCB

Developer(s)	Jamey Sharp, Josh Triplett, Bart Massey
Stable release	1.7 / August 13, 2010 (2010-08-13)
Operating system	POSIX
Type	X Window core protocol development library
License	MIT
Website	xcb.freedesktop.org

In computing, XCB (X protocol C-language Binding) is a C language binding for the X Window System. It is implemented as free software and aims to replace Xlib. The project was started in 2001 by Bart Massey.

Xlib/XCB provides application binary interface compatibility with both Xlib and XCB, providing an incremental porting path. Xlib/XCB uses the protocol layer of Xlib, but replaces the Xlib transport layer with XCB, and provides access to the underlying XCB connection for direct use of XCB. Most operating systems use Xlib/XCB for their libX11^{[citation needed]}, because it allows an application to open a single connection to the X server and use both XCB and Xlib, possibly through a mixture of libraries designed for one or the other.

Goals

The main goals of XCB are to

• reduce library size and complexity;
• provide direct access to the X11 protocol.

Secondary goals include making the C interface asynchronous, facilitating better multithreading and making it easier to implement extensions (via XML protocol descriptions).

The core and extension protocol descriptions are in XML, with a program written in Python creating the C bindings. (Previous versions used XSLT and M4.)

A further goal is to be able to use these protocol descriptions to create protocol documentation, more language bindings, and server-side stubs.

Massey and others have worked to prove key portions of XCB formally correct using Z notation. (Xlib has long been known to contain errors.)

Example

/* Simple XCB application drawing a box in a window */
/* to compile it use :
 gcc -Wall x.c -lxcb
*/
#include <xcb/xcb.h>
#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
  xcb_connection_t    *c;
  xcb_screen_t        *s;
  xcb_window_t         w;
  xcb_gcontext_t       g;
  xcb_generic_event_t *e;
  uint32_t             mask;
  uint32_t             values[2];
  int                  done = 0;
  xcb_rectangle_t      r = { 20, 20, 60, 60 };
 
                        /* open connection with the server */
  c = xcb_connect(NULL,NULL);
  if (xcb_connection_has_error(c)) {
    printf("Cannot open display\n");
    exit(1);
  }
                        /* get the first screen */
  s = xcb_setup_roots_iterator( xcb_get_setup(c) ).data;
 
                       /* create black graphics context */
  g = xcb_generate_id(c);
  w = s->root;
  mask = XCB_GC_FOREGROUND | XCB_GC_GRAPHICS_EXPOSURES;
  values[0] = s->black_pixel;
  values[1] = 0;
  xcb_create_gc(c, g, w, mask, values);
 
                       /* create window */
  w = xcb_generate_id(c);
  mask = XCB_CW_BACK_PIXEL | XCB_CW_EVENT_MASK;
  values[0] = s->white_pixel;
  values[1] = XCB_EVENT_MASK_EXPOSURE | XCB_EVENT_MASK_KEY_PRESS;
  xcb_create_window(c, s->root_depth, w, s->root,
                    10, 10, 100, 100, 1,
                    XCB_WINDOW_CLASS_INPUT_OUTPUT, s->root_visual,
                    mask, values);
 
                        /* map (show) the window */
  xcb_map_window(c, w);
 
  xcb_flush(c);
 
                        /* event loop */
  while (!done && (e = xcb_wait_for_event(c))) {
    switch (e->response_type & ~0x80) {
    case XCB_EXPOSE:    /* draw or redraw the window */
      xcb_poly_fill_rectangle(c, w, g,  1, &r);
      xcb_flush(c);
      break;
    case XCB_KEY_PRESS:  /* exit on key press */
      done = 1;
      break;
    }
    free(e);
  }
                        /* close connection to server */
  xcb_disconnect(c);
 
  return 0;
}

XCB has a comparable, but slightly lower-level API than Xlib, as can be seen with this example.

Protocol description

Creators of XCB have invented a specialized Interface description language to model X11 protocol in language-neutral way and facilitate generation of bindings to other programming languages. libxcb itself is implemented as a code generator and a tiny C stub of utility functions.

Example:

<xcb header="bigreq" extension-xname="BIG-REQUESTS"
    extension-name="BigRequests" extension-multiword="true"
    major-version="0" minor-version="0">
 
  <request name="Enable" opcode="0">
    <reply>
      <pad bytes="1" />
      <field type="CARD32" name="maximum_request_length" />
    </reply>
  </request>
</xcb>

Logo

The XCB logo was produced by Gearóid Molloy, author of the web comic Neko the Kitty, and donated to the project.

References

• Massey, Bart; Sharp, Jamey (2001-09-19). "XCB: An X Protocol C Binding" (PDF). Proceedings of the XFree86 Technical Conference. Oakland, California: USENIX. http://www.linuxshowcase.org/2001/full_papers/massey/massey.pdf. Retrieved 2008-11-07. 
• Massey, Bart; Bauer, Robert (2002). "X Meets Z: Verifying Correctness In The Presence Of POSIX Threads". Proceedings of the FREENIX Track: 2002 USENIX Annual Technical Conference. Monterey, California: USENIX. pp. 221–234. http://www.usenix.org/events/usenix02/tech/freenix/full_papers/massey/massey_html/index.html. Retrieved 2008-11-07. 
• Sharp, Jamey; Massey, Bart (2002). "XCL: A Compatibility Layer For XCB". Proceedings of the FREENIX Track: 2002 USENIX Annual Technical Conference. Monterey, California: USENIX. pp. 71–83. http://www.usenix.org/events/usenix02/tech/freenix/full_papers/sharp/sharp_html/index.html. Retrieved 2008-11-07. 

External links

• XCB wiki (freedesktop.org)
• XCB API reference - tutorial
• libxcb tutorial
• Further publications