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XCB

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Developer(s)Jamey Sharp, Josh Triplett, Bart Massey
Initial release2001
Stable release
1.10 / 22 December 2013; 10 years ago (2013-12-22)[1]
Repository
Operating systemPOSIX
TypeX Window core protocol development library
LicenseMIT
Websitexcb.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. Xlib/XCB allows an application to open a single connection to the X display server and use both XCB and Xlib, possibly through a mixture of libraries designed for one or the other.[2][3]

Goals

The role of KMS (Kernel mode-setting), Linux example
The Linux graphic stack
Illustrates the Linux graphics stack current as of 2013-08-24
Scheme: Humane-machine_interaction
XCB and Xlib are client libraries which implement a display server communications protocol
The place of certain Linux kernel modules
The display server sits between the kernel (here: Linux kernel) and its clients. It communicates with its clients over a given protocol.

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.[4] (Xlib has long been known to contain errors.[5])

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>

The XCB logo was produced by Gearóid Molloy, author of the web comic Neko the Kitty, and donated to the project.[6]

Other language binding

  • xpyb - The Python binding to the X Window System using XCB. As of June 2013, it does not support Python 3. Provided by freedesktop.org.

Notes

  1. ^ Schlachter, Uli (2013-12-22). "[Xcb] [ANNOUNCE] libxcb 1.10". xcb (Mailing list). Retrieved 2014-03-24. {{cite mailing list}}: Unknown parameter |mailinglist= ignored (|mailing-list= suggested) (help)
  2. ^ "Xlib/XCB: Xlib with XCB transport". 2008-01-11. Retrieved 2009-09-11.
  3. ^ Jamey Sharp and Josh Triplett (2006-11-26). "libx11 with Xlib/XCB now in experimental; please test with your packages". debian-devel-announce (Mailing list). Retrieved 2009-09-11. {{cite mailing list}}: Unknown parameter |mailinglist= ignored (|mailing-list= suggested) (help)
  4. ^ Massey and Bauer, 2002.
  5. ^ Sharp and Massey, 2002, §2.4. "While Xlib was designed to support threaded applications, and while that support is not unusable, there are known race conditions that cannot be eliminated without changing the Xlib interface."
  6. ^ KittyLogo (xcb.freedesktop.org)

References

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