atexit - Man Page

register a function to run at process termination


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <stdlib.h>

int atexit(void (*func)(void));


The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard.

The atexit() function shall register the function pointed to by func, to be called without arguments at normal program termination. At normal program termination, all functions registered by the atexit() function shall be called, in the reverse order of their registration, except that a function is called after any previously registered functions that had already been called at the time it was registered. Normal termination occurs either by a call to exit() or a return from main().

At least 32 functions can be registered with atexit().

After a successful call to any of the exec functions, any functions previously registered by atexit() shall no longer be registered.

Return Value

Upon successful completion, atexit() shall return 0; otherwise, it shall return a non-zero value.


No errors are defined.

The following sections are informative.



Application Usage

The functions registered by a call to atexit() must return to ensure that all registered functions are called.

The application should call sysconf() to obtain the value of {ATEXIT_MAX}, the number of functions that can be registered. There is no way for an application to tell how many functions have already been registered with atexit().

Since the behavior is undefined if the exit() function is called more than once, portable applications calling atexit() must ensure that the exit() function is not called at normal process termination when all functions registered by the atexit() function are called.

All functions registered by the atexit() function are called at normal process termination, which occurs by a call to the exit() function or a return from main() or on the last thread termination, when the behavior is as if the implementation called exit() with a zero argument at thread termination time.

If, at normal process termination, a function registered by the atexit() function is called and a portable application needs to stop further exit() processing, it must call the _exit() function or the _Exit() function or one of the functions which cause abnormal process termination.



Future Directions


See Also

exec, exit(), sysconf()

The Base Definitions volume of POSIX.1-2017, <stdlib.h>

Referenced By

exec(3p), exit(3p), _Exit(3p), fclose(3p), pthread_atfork(3p), stdlib.h(0p).

2017 IEEE/The Open Group POSIX Programmer's Manual