---
name: "atexit"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/atexit"
---

# atexit(3)

register a function to be called on exit

## NAME

**atexit** – register a function to be called on exit

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<stdlib.h\>**

*int*\

**atexit**(*void (\*function)(void)*);

*int*\

**atexit\_b**(*void (^function)(void)*);

## DESCRIPTION

The **atexit**() function registers the given *function* to be called at program exit, whether via [exit(3)](/exit/3.md) or via return from the program's **main**(). Functions so registered are called in reverse order; no arguments are passed.

These functions must not call **exit**(); if it should be necessary to terminate the process while in such a function, the [\_exit(2)](/_exit/2.md) function should be used. (Alternatively, the function may cause abnormal process termination, for example by calling [abort(3)](/abort/3.md).)

At least 32 functions can always be registered, and more are allowed as long as sufficient memory can be allocated.

The **atexit\_b**() function behaves identically to **atexit**(), except that it takes a block, rather than a function pointer.

## RETURN VALUES

The **atexit()** function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

## ERRORS

- \[ENOMEM\]

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

- \[ENOSYS\]

  The **atexit\_b**() function was called by a program that did not supply a **\_Block\_copy**() implementation.

## SEE ALSO

[\_exit(2)](/_exit/2.md), [at\_quick\_exit(3)](/at_quick_exit/3.md), [exit(3)](/exit/3.md)

## STANDARDS

The **atexit**() function conforms to ISO/IEC 9899:1990 (“ISO C90”).