The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


atexit - register a function to run at process termination


#include <stdlib.h>

int atexit(void (*


[CX] [Option Start] 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 IEEE Std 1003.1-2001 defers to the ISO C standard. [Option End]

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().

[CX] [Option Start] After a successful call to any of the exec functions, any functions previously registered by atexit() shall no longer be registered. [Option End]


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


No errors are defined.

The following sections are informative.




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.






exit(), sysconf(), the Base Definitions volume of IEEE Std 1003.1-2001, <stdlib.h>


First released in Issue 4. Derived from the ANSI C standard.

Issue 6

Extensions beyond the ISO C standard are marked.

The DESCRIPTION is updated for alignment with the ISO/IEC 9899:1999 standard.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/19 is applied, adding further clarification to the APPLICATION USAGE section.

End of informative text.

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