on_exit(3)

💖 on_exit(3) — the "run this when we log off" bestie 💖

okay so this is a page from the Library Functions Manual, and it's all about on_exit, a lil function that lets you ✨schedule a callback✨ for the exact moment your process says bye-bye. it's giving "leave a note before you head out" energy, and honestly? iconic. 🫶

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🌸 LIBRARY 🌸

Standard C library (libc, -lc) — yeah she lives in the classic place, no cap.

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✨ SYNOPSIS ✨

#include <stdlib.h>

int on_exit(typeof(void (int, void *)) *function, void *arg);

🎀 Feature Test Macro Requirements for glibc 🎀

(peep feature_test_macros(7) if you're like "...macro? bestie what")

on_exit():

• Since glibc 2.19:
  • _DEFAULT_SOURCE
• glibc 2.19 and earlier:
  • _BSD_SOURCE || _SVID_SOURCE

so like, depending on which glibc era you're in, you gotta define a different macro to even see her. vintage behavior fr fr. 💅

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💜 DESCRIPTION 💜

The on_exit() function registers the function you hand it, and that function gets called at normal process termination — meaning it runs whether you go out through exit(3) or through a return from your program's main(). either way, she's there. main character energy. 👑

When she runs, your function gets passed:

• the status argument you gave to the last call to exit(3), and
• the arg argument that you passed into on_exit().

The same function may be registered multiple times, and girl is thorough: it gets called once for each registration. no shortcuts, no deduplication, we respect the hustle. 🔥

When a child process is created via fork(2), it inherits copies of its parent's registrations — like, the whole vibe gets duplicated. but upon a successful call to one of the exec(3) functions, all registrations are removed. boom. gone. 🙈

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🌈 RETURN VALUE 🌈

on_exit() returns the value 0 if successful. otherwise, it returns a nonzero value. that's it, that's the whole story. simple girl, simple contract. 💀

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🦋 ATTRIBUTES 🦋

For an explanation of the terms used in this section, see attributes(7).

• Interface: on_exit()
• Attribute: Thread safety
• Value: MT-Safe

so she's MT-Safe, which lowkey means she plays nice with threads. very demure, very mindful. 🧚

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⭐ STANDARDS ⭐

None.

not in any standard, bestie. she's a lovable rogue. 😭

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🌸 HISTORY 🌸

SunOS 4, glibc. She was Removed in Solaris (SunOS 5). and the official advice? use the standard atexit(3) instead. like, they literally said "get a standard one, girl." 💅

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💀 CAVEATS 💀

⚠️ listen up because this one MATTERS ⚠️

By the time function is executed, stack (auto) variables may already have gone out of scope. so:

• arg should not be a pointer to a stack variable. ❌
• it may, however, be a pointer to a heap variable or a global variable. ✅

if you hand her a dangling stack pointer you're gonna have a bad time, and not the fun kind. 🫠

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👑 SEE ALSO 👑

• atexit(3)
• _exit(3)
• exit(3)

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and that's the whole page, bestie — this was on_exit(3) from Linux man-pages 6.20, dated 2026-06-05. go register your cleanup function, protect those heap pointers, and slay. 💖✨🎀