XCB

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Original author(s)	Bart Massey
Developer(s)	Jamey Sharp, Josh Triplett, Bart Massey
Initial release	2001; 23 years ago (2001)
Stable release	1.17.0[1]  / 15 April 2024; 6 months ago (15 April 2024)
Repository	gitlab.freedesktop.org/xorg/lib/libxcb
Written in	C
Operating system	POSIX
Type	X11 client library
License	MIT License
Website	xcb.freedesktop.org

XCB (X protocol C-language Binding) is a library implementing the client-side of the X11 display server protocol. XCB is written in the C programming language and distributed under the MIT License. The project was started in 2001 by Bart Massey and aims to replace Xlib.

XCB was designed as a smaller, modernized replacement for Xlib, previously the primary C library for communicating with the X window system, coinciding with a more complete overhaul of the X implementation that took place during the early 2000s.^[2] The main goals of XCB are to:

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

The required size reduction is achieved primarily by restricting XCB's scope to handling the X protocol and omitting Xlib functionality such as its extensive utility library, much of which saw little use by applications. This results in a factor thirty reduction of the compiled library size (as of 2004).^[3] 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])

Xlib/XCB provides application binary interface compatibility with both Xlib and XCB, providing an incremental porting path.^[6] 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.^[7][8]

// Simple XCB application for opening a window and drawing a box in it  // To compile it using GNU, use: // gcc x.c -lxcb  #include <stdio.h> #include <stdlib.h> #include <xcb/xcb.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 to the server   c = xcb_connect(NULL, NULL);   if (xcb_connection_has_error(c)) {     printf("Cannot open display\n");     exit(EXIT_FAILURE);   }   // 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);    exit(EXIT_SUCCESS); } 

The bitwise and operation a->response_type & ~0x80 removes a bit that indicates where the event came from.^[9]

XCB has a comparable, but slightly lower-level API than Xlib,^[10] as can be seen with this example.

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.^{[dubious – discuss]} libxcb itself is implemented as a code generator and a tiny C stub of utility functions.

An 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.^[11]

• XCB.pm - Perl module implementing bindings to XCB.
• 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.
• xcffib - Another Python binding which supports Python 2 & 3 as well as several more X extensions than xpyb.

• Massey, Bart; Sharp, Jamey (2001-09-19). "XCB: An X Protocol C Binding" (PDF). Proceedings of the XFree86 Technical Conference. Oakland, California: USENIX. Retrieved 2012-03-12.
• 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. 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. Retrieved 2008-11-07.

• XCB wiki (freedesktop.org)
• XCB API reference, tutorial
• libxcb tutorial
• Further publications
• The X New Developer’s Guide: Xlib and XCB