Xlib

Language: C

GUI / Windowing

Xlib was developed in the mid-1980s as the primary client library for X11. It provides a low-level interface to X server operations, allowing developers to build graphical applications directly. Although higher-level toolkits like GTK and Qt are more commonly used today, Xlib is still valuable for lightweight, custom, or embedded X11 applications.

Xlib is the standard C library for interfacing with the X Window System (X11) on Unix-like operating systems. It allows applications to create and manage windows, handle user input events, draw graphics, and communicate with the X server.

Installation

linux: sudo apt install libx11-dev

mac: Install XQuartz to get X11 headers and libraries

windows: Xlib is mainly for Unix-like systems; use Cygwin/X or similar for Windows

Usage

Xlib enables C programs to open a connection to the X server, create windows, manage events like keyboard and mouse inputs, and draw graphics primitives such as lines, rectangles, and text.

Opening a display and creating a simple window

#include <X11/Xlib.h>
#include <stdio.h>
#include <stdlib.h>

int main() {
    Display *d;
    Window w;
    XEvent e;
    int s;

    d = XOpenDisplay(NULL);
    if (d == NULL) {
        fprintf(stderr, "Cannot open display\n");
        exit(1);
    }

    s = DefaultScreen(d);
    w = XCreateSimpleWindow(d, RootWindow(d, s), 10, 10, 200, 200, 1,
                            BlackPixel(d, s), WhitePixel(d, s));
    XSelectInput(d, w, ExposureMask | KeyPressMask);
    XMapWindow(d, w);

    while (1) {
        XNextEvent(d, &e);
        if (e.type == Expose) {
            XFillRectangle(d, w, DefaultGC(d, s), 20, 20, 10, 10);
        }
        if (e.type == KeyPress)
            break;
    }

    XCloseDisplay(d);
    return 0;
}

Opens a connection to the X server, creates a window, listens for expose and keypress events, draws a rectangle, and closes the display.

Drawing graphics primitives

#include <X11/Xlib.h>

// Use functions like XDrawLine, XDrawRectangle, XDrawArc to draw shapes on a window

Demonstrates low-level drawing operations using Xlib functions to create lines, rectangles, circles, and arcs.

Handling multiple events

// Use XSelectInput to listen for multiple event types such as KeyPressMask, ButtonPressMask, ExposureMask
// Use XNextEvent to process events in an event loop

Allows handling of keyboard, mouse, and window events in a single loop efficiently.

Font and text rendering

// Use XLoadFont and XDrawString to draw text on windows with Xlib

Enables rendering text with specific fonts and coordinates in a window.

Error Handling

Cannot open display: Ensure the DISPLAY environment variable is set correctly and an X server is running.

BadWindow or BadDrawable: Verify that the window or drawable exists before performing drawing operations.

Font loading failure: Check that the specified font exists on the system and the name is correct.

Best Practices

Always check the return values of Xlib functions to catch errors early.

Use event masks efficiently to avoid unnecessary event handling overhead.

Close the Display connection using XCloseDisplay to release resources.

Consider using higher-level toolkits like GTK or Qt for complex GUI applications.

Separate event handling and drawing logic for maintainability.

Official Docs Github