# CGWindowServerCFMachPort() Returns a Core Foundation Mach port (CFMachPort) that corresponds to the macOS window server. ``` func CGWindowServerCFMachPort() -> CFMachPort? ``` ## Return Value A Core Foundation Mach port, or `NULL` if the window server is not running. When you no longer need the port, you should release it using the function `CFRelease`. ## Discussion You can use this function to detect whether the window server process exits or is not running. If this function returns `NULL`, the window server is not running. This code example shows how to register a callback function to detect when the window server exits: ```objc static void handleWindowServerDeath( CFMachPortRef port, void *info ) { printf( "Window Server port death detected!\n" ); CFRelease(port); exit(1); } static void watchForWindowServerDeath() { CFMachPortRef port = CGWindowServerCFMachPort(); CFMachPortSetInvalidationCallBack(port, handleWindowServerDeath); } ``` Note that this callback only works if your program has an active run loop. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)