XOpenDisplay

XOpenDisplay is a function in the X Window System's Xlib library that opens a connection to an X server. It returns a pointer to a Display structure, which represents the connection and is used by virtually all subsequent Xlib calls. If the connection cannot be established, XOpenDisplay returns NULL.

The function takes a display_name argument that identifies which X server to connect to. If display_name is NULL, XOpenDisplay uses the value of the DISPLAY environment variable. Common formats for display names include host:display.screen, optionally followed by a .depth. The connection may involve network sockets or local IPC, and the X server may require authentication, in which case the connection will be rejected if credentials are invalid or the server is unavailable.

On a successful connection, the Display object provides default settings such as the default screen, default colormap, and the resource manager database. The application is expected to perform cleanup by calling XCloseDisplay when the interaction with the X server is finished. If a failure occurs, the returned NULL indicates that no Display was created, and error information can be retrieved via X11 error handling mechanisms.

For multi-threaded programs, access to the Display should be serialized because Xlib is not inherently thread-safe across all versions. Some implementations offer a mechanism to enable threading support, but the recommended practice is to coordinate all Xlib calls through a single thread or protect calls with synchronization primitives. XOpenDisplay is typically paired with XCloseDisplay in well-formed Xlib programs.