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ATEXIT(3) Library Functions Manual ATEXIT(3)

atexit —
register a function to be called on exit

#include <stdlib.h>

int
atexit(void (*function)(void));

The atexit() function registers the given function to be called at program exit, whether via exit(3) or via return from the program's main(). Functions so registered are called in reverse order; no arguments are passed. At least 32 functions can always be registered, and more are allowed as long as sufficient memory can be allocated.

If a shared object is unloaded from process memory using dlclose(3), then any functions registered by calling atexit() from that shared object will be called in reverse order and unregistered. Note that it is the source of the call to atexit() that matters, not the source of the function that was registered.

atexit() is very difficult to use correctly without creating exit(3)-time races. Unless absolutely necessary, please avoid using it.

The atexit function returns the value 0 if successful; otherwise a non-zero value is returned and the global variable errno is set to indicate the error.

[]
No memory was available to add the function to the list. The existing list of functions is unmodified.

dlclose(3), exit(3)

The atexit() function conforms to ANSI X3.159-1989 (“ANSI C89”).

Setting errno on error and the behavior when a shared object is unloaded are extensions to that standard.

June 27, 2019 OpenBSD-current