Event-driven programming on X Windows

The X Windows programming model is a cross-platform API, is mostly supported on POSIX systems, and has even been ported to Microsoft Windows. In fact, X is a network windowing protocol, which required a Window manager to manage the Windows stack. The screen contents are managed by the X server and the client library will pull the contents and display them on the local machine. In desktop environments, the server runs locally on the same machine. The following program will help the reader understand the gist of the XLib programming model and how events are handled in the platform:

#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;

The preceding code snippet includes the proper header files that a programmer is supposed to include to get the function prototypes provided by the XLib C library. There are some data structures that a programmer should be aware of while writing XLib programs from scratch. Nowadays, people use libraries such as Qt, WxWidgets, Gtk+, Fox toolkit, and so on to write commercial-quality X Programs.

    /* open connection with 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);

The preceding code snippet initializes the server and creates a window to certain specifications. Traditionally, most X Windows programs run under a window manager that manages the cascading windows. We selected the messages that are of interest to us by invoking the XSelectInput API call before displaying the 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;
    }

Then, the program goes to an infinite loop while polling for any events, and the appropriate Xlib API will be used to draw a string on the Window. In Windowing parlance, it is called a message loop. The retrieval of events will be done by the XNextEvent API call:

    /* close connection to server */
    XCloseDisplay(display);

    return 0;
    }

Once we are out of the infinite message loop, the connection to the server will be closed.