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Xlib
 Xlib (also known as libX11) is an X Window System protocol client library written in the C programming language. It contains functions for interacting with an X server. These functions allow programmers to write programs without knowing the details of the X protocol. Few applications use Xlib directly; rather, they employ other libraries that use Xlib functions to provide widget toolkits:
Xlib appeared around 1985,[citation needed] and is used in GUIs for many Unix-like operating systems. A re-implementation of Xlib was introduced in 2007 using XCB.[2] Data typesThe role of KMS (Kernel mode-setting), Linux example  Illustrates the Linux graphics stack current as of 2013-08-24   The display server sits between the kernel (here: Linux kernel) and its clients. It communicates with its clients over a given protocol.  Simple DirectMedia Layer can circumvent Xlib and write directly to framebuffer. An additional port to EGL is also available The main types of data in Xlib are the Informally, a display is a physical or virtual device where graphical operations are done. The Windows, colormaps, etc. are managed by the server, which means that the data about their actual implementation is all stored in the server. The client operates on these objects by using their identifiers. The client cannot directly operate on an object, but can only request the server to perform the operation specifying the identifier of the object. The types The identifiers are unique to the server. Most of them can be used by different applications to refer to the same objects. For example, two applications connecting with the same server use the same identifier to refer to the same window. These two applications use two different channels, and therefore have two different Display structures; however, when they request operations on the same identifier, these operations will be done on the same object. Protocol and eventsThe Xlib functions that send requests to the server usually do not send these requests immediately but store them in a buffer, called the request buffer. The term request in this case refers to the request from the client that is directed to the server: the request buffer can contain all kinds of requests to the server, not only those having a visible effect on the screen. The request buffer is guaranteed to be flushed (i.e., all requests done so far are sent to the server) after a call to the functions Xlib stores the received events in a queue. The client application can inspect and retrieve events from the queue. While the X server sends events asynchronously, applications using the Xlib library are required to explicitly call Xlib functions for accessing the events in the queue. Some of these functions may block; in this case, they also flush the request buffer. Errors are instead received and treated asynchronously: the application can provide an error handler that will be called whenever an error message from the server is received. The content of a window is not guaranteed to be preserved if the window or one of its parts are made not visible. In this case, the application is sent an FunctionsThe functions in the Xlib library can be grouped in:
Example  The following program creates a window with a little black square in it: /*
Simple Xlib application for creating a window and drawing a box in it.
gcc input.c -o output -lX11
*/
#include <X11/Xlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void)
{
Display *display;
Window window;
XEvent event;
char *msg = "Hello, World!";
int s;
// open connection to the server
display = XOpenDisplay(NULL);
if (display == NULL)
{
fprintf(stderr, "Cannot open display\n");
exit(1);
}
s = DefaultScreen(display);
// create window
window = XCreateSimpleWindow(display, RootWindow(display, s), 10, 10, 200, 200, 1,
BlackPixel(display, s), WhitePixel(display, s));
// select kind of events we are interested in
XSelectInput(display, window, ExposureMask | KeyPressMask);
// map (show) the window
XMapWindow(display, window);
// event loop
for (;;)
{
XNextEvent(display, &event);
// draw or redraw the window
if (event.type == Expose)
{
XFillRectangle(display, window, DefaultGC(display, s), 20, 20, 10, 10);
XDrawString(display, window, DefaultGC(display, s), 50, 50, msg, strlen(msg));
}
// exit on key press
if (event.type == KeyPress)
break;
}
// close connection to the server
XCloseDisplay(display);
return 0;
}
The client creates a connection with the server by calling The square is drawn by calling The drawing of the window content is therefore made inside the loop handling the events. Before entering this loop, the events the application is interested in are selected, in this case with Other librariesXlib does not provide support for buttons, menus, scrollbars, etc. Such widgets are provided by other libraries, which in turn use Xlib. There are two kinds of such libraries:
Applications using any of these widget libraries typically specify the content of the window before entering the main loop and do not need to explicitly handle The XCB library is an alternative to Xlib. Its two main aims are: reduction in library size and direct access to the X11 protocol. A modification of Xlib has been produced to use XCB as a low-level layer. References
External links The Wikibook X Window Programming has a page on the topic of: XLib
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