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Low Level X Window Programming: An Introduction by Examples

The X Window System, commonly referred to as X11, is a windowing system for bitmap displays. It provides a basic framework for creating graphical user interfaces (GUIs) on Unix-like operating systems. While many developers rely on high-level toolkits like GTK+ or Qt to build GUI applications, understanding the low-level X Window programming is essential for creating custom, efficient, and platform-specific interfaces.

In this article, we will introduce the basics of low-level X Window programming through a series of examples. We will cover the fundamental concepts, data structures, and functions required to create and manage windows, handle events, and draw graphics.

Introduction to X11

The X Window System is a client-server architecture, where the X server manages the display and input devices, and the X client is the application that interacts with the server to create and manage windows. The X11 protocol is used for communication between the client and server.

To start programming with X11, you need to include the necessary header files, typically X11/Xlib.h and X11/Xutil.h. You also need to link against the X11 library, usually done by adding the -lX11 flag during compilation.

Creating a Window

The first step in creating a GUI application is to create a window. This involves opening a connection to the X server, creating a window, and mapping it to the screen.

c

include <X11/Xlib.h>

include <stdio.h>

int main() {
// Open a connection to the X server
Display *display = XOpenDisplay(NULL);
if (!display) {
fprintf(stderr, “Could not open display\n”);
return 1;
}

// Create a window
Window window = XCreateSimpleWindow(display, RootWindow(display, 0),
                                   100, 100, 400, 300, 1,
                                   BlackPixel(display, 0), WhitePixel(display, 0));

// Map the window to the screen
XMapWindow(display, window);

// Flush the output buffer
XFlush(display);

// Keep the window open for 5 seconds
sleep(5);

// Close the connection to the X server
XCloseDisplay(display);
return 0;

}

This example creates a simple window with a white background and a black border. The XCreateSimpleWindow function is used to create the window, and XMapWindow is used to map it to the screen.

Handling Events

X11 uses an event-driven programming model, where the application receives events from the X server and responds accordingly. Events can be keyboard input, mouse movements, button clicks, or window resizing.

To handle events, you need to use the XNextEvent function, which blocks until an event is received. You can then use the XLookupKeysym function to determine the type of event and respond accordingly.

c

include <X11/Xlib.h>

include <stdio.h>

int main() {
// Open a connection to the X server
Display *display = XOpenDisplay(NULL);
if (!display) {
fprintf(stderr, “Could not open display\n”);
return 1;
}

// Create a window
Window window = XCreateSimpleWindow(display, RootWindow(display, 0),
                                   100, 100, 400, 300, 1,
                                   BlackPixel(display, 0), WhitePixel(display, 0));

// Map the window to the screen
XMapWindow(display, window);

// Flush the output buffer
XFlush(display);

// Event loop
XEvent event;
while (1) {
    XNextEvent(display, &event);
    switch (event.type) {
        case KeyPress:
            printf("Key pressed: %c\n", XLookupKeysym(&event.xkey, 0));
            break;
        case ButtonPress:
            printf("Button pressed: %d\n", event.xbutton.button);
            break;
        case Expose:
            printf("Window exposed\n");
            break;
    }
}

// Close the connection to the X server
XCloseDisplay(display);
return 0;

}

This example creates a window and enters an event loop, where it handles keyboard input, mouse button clicks, and window exposure events.

Drawing Graphics

To draw graphics in an X11 window, you need to use the XDrawLine, XDrawRectangle, and XDrawString functions. You can also use the XCreateGC function to create a graphics context, which allows you to set the drawing color, font, and other attributes.

c

include <X11/Xlib.h>

include <stdio.h>

int main() {
// Open a connection to the X server
Display *display = XOpenDisplay(NULL);
if (!display) {
fprintf(stderr, “Could not open display\n”);
return 1;
}

// Create a window
Window window = XCreateSimpleWindow(display, RootWindow(display, 0),
                                   100, 100, 400, 300, 1,
                                   BlackPixel(display, 0), WhitePixel(display, 0));

// Map the window to the screen
XMapWindow(display, window);

// Flush the output buffer
XFlush(display);

// Create a graphics context
GC gc = XCreateGC(display, window, 0, NULL);

// Set the drawing color
XSetForeground(display, gc, BlackPixel(display, 0));

// Draw a line
XDrawLine(display, window, gc, 10, 10, 390, 290);

// Draw a rectangle
XDrawRectangle(display, window, gc, 50, 50, 100, 100);

// Draw a string
XDrawString(display, window, gc, 150, 150, "Hello, World!", 13);

// Flush the output buffer
XFlush(display);

// Keep the window open for 5 seconds
sleep(5);

// Close the connection to the X server
XCloseDisplay(display);
return 0;

}

This example creates a window and draws a line, rectangle, and string using the XDrawLine, XDrawRectangle, and XDrawString functions.

Conclusion

In this article, we introduced the basics of low-level X Window programming through a series of examples. We covered the fundamental concepts, data structures, and functions required to create and manage windows, handle events, and draw graphics. While high-level toolkits like GTK+ or Qt provide a more convenient and platform-independent way to build GUI applications, understanding the low-level X Window programming is essential for creating custom, efficient, and platform-specific interfaces.