atexit

Atexit is a mechanism used in several programming environments to register callbacks that will be invoked automatically when a program terminates normally. The callbacks are used to perform cleanup tasks such as releasing resources, flushing I/O buffers, or saving state. Although implemented differently across languages, the core idea is to run user-defined cleanup code during shutdown rather than at arbitrary points during execution.

In the C standard library, atexit is declared as int atexit(void (*func)(void)); It registers func to be called when the program terminates normally via exit or by returning from main. Functions are invoked in reverse order of their registration (the last registered runs first). The function may fail if the limit on registered handlers is reached; in that case it returns a nonzero value. Abnormal termination, such as by calling _Exit or abort, is not guaranteed to trigger atexit callbacks.

In Python, the atexit module provides atexit.register(func, *args, **kwargs) to add callbacks that will run when the interpreter terminates normally. Callbacks are called in reverse registration order. A function can be unregistered with atexit.unregister(func). If a callback raises an exception during shutdown, the exception is printed to standard error and the shutdown proceeds; other callbacks still run. The atexit module is commonly used for resource cleanup, such as closing files or releasing acquired resources at program exit.