XCB

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. 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). 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.)

Xlib compatibility

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.

// 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 *connection; xcb_screen_t *screen; xcb_window_t window; xcb_gcontext_t context; xcb_generic_event_t *event; uint32_t mask; uint32_t values[2]; int done = 0; xcb_rectangle_t r = { 20, 20, 60, 60 }; // open connection to the server connection = xcb_connect(NULL, NULL); if (xcb_connection_has_error(connection)) { printf("Cannot open display\n"); exit(EXIT_FAILURE); } // get the first screen screen = xcb_setup_roots_iterator( xcb_get_setup(connection) ).data; // create black graphics context context = xcb_generate_id(connection); window = screen->root; mask = XCB_GC_FOREGROUND | XCB_GC_GRAPHICS_EXPOSURES; values[0] = screen->black_pixel; values[1] = 0; xcb_create_gc(connection, context, window, mask, values); // create window window = xcb_generate_id(connection); mask = XCB_CW_BACK_PIXEL | XCB_CW_EVENT_MASK; values[0] = screen->white_pixel; values[1] = XCB_EVENT_MASK_EXPOSURE | XCB_EVENT_MASK_KEY_PRESS; xcb_create_window(connection, screen->root_depth, window, screen->root, 10, 10, 100, 100, 1, XCB_WINDOW_CLASS_INPUT_OUTPUT, screen->root_visual, mask, values); // map (show) the window xcb_map_window(connection, window); xcb_flush(connection); // event loop while (!done && (event = xcb_wait_for_event(connection))) { switch (event->response_type & ~0x80) { case XCB_EXPOSE: // draw or redraw the window xcb_poly_fill_rectangle(connection, window, context, 1, &r); xcb_flush(connection); break; case XCB_KEY_PRESS: // exit on key press done = 1; break; } free(event); } // close connection to server xcb_disconnect(connection); exit(EXIT_SUCCESS); }

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

XCB has a comparable, but slightly lower-level API than Xlib, 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. 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.

• 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.
• guile-xcb - Guile module implementing a language to treat the xcb definition as source code.