PTHREAD_ATFORK(3P)

💖 PROLOG 💖

okay so real talk before we even start: this page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (so go consult the corresponding Linux manual page for the deets on Linux behavior), or honestly the interface may not be implemented on Linux at all. 🙈

---

✨ NAME ✨

pthread_atfork — register fork handlers 🍴

---

🌸 SYNOPSIS 🌸

#include <pthread.h>

int pthread_atfork(void (*prepare)(void), void (*parent)(void),
    void (*child)(void));

---

🦋 DESCRIPTION 🦋

So pthread_atfork() declares fork handlers — little callbacks that get called before and after fork(), in the context of the thread that called fork(). Let me break the trio down for you, bestie:

• The prepare fork handler gets called before fork() processing commences.
• The parent fork handler gets called after fork() processing completes, in the parent process.
• The child fork handler gets called after fork() processing completes, in the child process.

And if you don't want handling at one or more of these three points? Just set the corresponding fork handler address(es) to NULL. Simple. 💅

🚨 real talk: if a fork() call in a multi-threaded process leads to a child fork handler calling any function that is not async-signal-safe, the behavior is undefined. no cap, that's a whole vibe killer.

The order of calls to pthread_atfork() is significant, like actually. The parent and child fork handlers get called in the order in which they were established by calls to pthread_atfork(). The prepare fork handlers? Called in the opposite order. mirror moment ✨

---

🌈 RETURN VALUE 🌈

Upon successful completion, pthread_atfork() returns a value of zero. Otherwise, an error number gets returned to indicate the error. that's it that's the tweet 💜

---

👑 ERRORS 👑

The pthread_atfork() function shall fail if:

• ENOMEM — Insufficient table space exists to record the fork handler addresses. like, there's literally no room bestie 😭

Also important: the pthread_atfork() function shall not return an error code of [EINTR].

💀 The following sections are informative. (i.e. the standard's way of saying "this is the lore, not the law")

---

🎀 EXAMPLES 🎀

None. that's right, zero, nada, the void. 💀

---

🫶 APPLICATION USAGE 🫶

okay so the original usage pattern envisaged for pthread_atfork() was: prepare fork handler locks mutexes and other locks, and the parent and child handlers unlock them. cute in theory, right? BUT — since all of the relevant unlocking functions, except sem_post(), are not async-signal-safe, this usage results in undefined behavior in the child process, unless the only such unlocking function it calls is sem_post(). so yeah, that's the catch. 🙈

---

💅 RATIONALE 💅

grab a snack, this is the long one.

There are at least two serious problems with the semantics of fork() in a multi-threaded program. Problem one is about state (for example, memory) covered by mutexes. Picture this: one thread has a mutex locked and the state covered by that mutex is inconsistent, while another thread calls fork(). In the child, the mutex is in the locked state — locked by a nonexistent thread, so it can literally never be unlocked. 💀 And having the child simply reinitialize the mutex is unsatisfactory, because that approach does not resolve the question of how to correct or otherwise deal with the inconsistent state in the child.

It is suggested that programs that use fork() call an exec function very soon afterwards in the child process, thus resetting all states. In the meantime, only a short list of async-signal-safe library routines are promised to be available.

Unfortunately, this solution does not address the needs of multi-threaded libraries. Application programs may not be aware that a multi-threaded library is in use, and they feel free to call any number of library routines between the fork() and exec calls, just as they always have. Indeed, they may be extant single-threaded programs and cannot, therefore, be expected to obey new restrictions imposed by the threads library.

On the other hand, the multi-threaded library needs a way to protect its internal state during fork() in case it is re-entered later in the child process. The problem arises especially in multi-threaded I/O libraries, which are almost sure to be invoked between the fork() and exec calls to effect I/O redirection. The solution may require locking mutex variables during fork(), or it may entail simply resetting the state in the child after the fork() processing completes.

So pthread_atfork() was intended to give multi-threaded libraries a means to protect themselves from innocent application programs that call fork(), and to give multi-threaded application programs a standard mechanism for protecting themselves from fork() calls in a library routine or the application itself.

The expected usage was that the prepare handler would acquire all mutex locks and the other two fork handlers would release them.

For example, an application could have supplied a prepare routine that acquires the necessary mutexes the library maintains, plus child and parent routines that release those mutexes — thus ensuring the child would have got a consistent snapshot of the state of the library (and that no mutexes would have been left stranded). This is good in theory, but in reality not practical. Each and every mutex and lock in the process must be located and locked. Every component of a program, including third-party components, must participate, and they must agree who is responsible for which mutex or lock. This is especially problematic for mutexes and locks in dynamically allocated memory. All mutexes and locks internal to the implementation must be locked, too. This possibly delays the thread calling fork() for a long time, or even indefinitely, since uses of these synchronization objects may not be under control of the application. A final problem to mention here is the problem of locking streams. At least the streams under control of the system (like stdin, stdout, stderr) must be protected by locking the stream with flockfile(). But the application itself could have done that, possibly in the same thread calling fork(). In this case, the process will deadlock. 💀💀

Alternatively, some libraries might have been able to supply just a child routine that reinitializes the mutexes in the library and all associated states to some known value (for example, what it was when the image was originally executed). This approach is not possible, though, because implementations are allowed to fail *_init() and *_destroy() calls for mutexes and locks if the mutex or lock is still locked. In this case, the child routine is not able to reinitialize the mutexes and locks.

When fork() is called, only the calling thread is duplicated in the child process. Synchronization variables remain in the same state in the child as they were in the parent at the time fork() was called. Thus, for example, mutex locks may be held by threads that no longer exist in the child process, and any associated states may be inconsistent. The intention was that the parent process could have avoided this by explicit code that acquires and releases locks critical to the child via pthread_atfork(). In addition, any critical threads would have needed to be recreated and reinitialized to the proper state in the child (also via pthread_atfork()).

A higher-level package may acquire locks on its own data structures before invoking lower-level packages. Under this scenario, the order specified for fork handler calls allows a simple rule of initialization for avoiding package deadlock: a package initializes all packages on which it depends before it calls the pthread_atfork() function for itself.

And as explained, there is no suitable solution for functionality which requires non-atomic operations to be protected through mutexes and locks. This is why the POSIX.1 standard since the 1996 release requires that the child process after fork() in a multi-threaded process only calls async-signal-safe interfaces. periodt. 🎀

---

🔮 FUTURE DIRECTIONS 🔮

The pthread_atfork() function may be formally deprecated (for example, by shading it OB) in a future version of this standard. so like... watch this space bestie 👀

---

💜 SEE ALSO 💜

• atexit(), exec, fork()
• The Base Definitions volume of POSIX.1‐2017, <pthread.h>, <sys_types.h>

---

📜 COPYRIGHT 📜

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

---

and that's the whole page, bestie 💖 this iconic lil manual page came to you courtesy of IEEE/The Open Group, dated 2017, from the POSIX Programmer's Manual — PTHREAD_ATFORK(3P), slay, goodbye, go fork responsibly 🍴✨