NAME

atexit — register a function to be called from .I exit () or after return from .I main ()

SYNOPSIS

#include <stdlib.h>

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


DESCRIPTION

^[CX] 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.1-2024 defers to the ISO C standard.

The atexit() function shall register the function pointed to by func, to be called without arguments from exit(), or after return from the initial call to main(), or on the last thread termination. If the exit() function is called, it is unspecified whether a call to the atexit() function that does not happen before exit() is called will succeed.

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

RETURN VALUE

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

ERRORS

No errors are defined.

EXAMPLES

None.

APPLICATION USAGE

The atexit() function registrations are distinct from the at_quick_exit() registrations, so applications might need to call both registration functions with the same argument.

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 when the functions registered by the atexit() function are called.

If 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.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

at_quick_exit , exec , exit , sysconf

XBD <stdlib.h>

CHANGE HISTORY

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.

Issue 8

Austin Group Defect 1302 is applied, aligning this function with the ISO/IEC 9899:2018 standard.

Austin Group Defect 1646 is applied, removing redundant text relating to the exec family of functions.