Manual browser: pthread_atfork(3)

PTHREAD_ATFORK(3) Library Functions Manual PTHREAD_ATFORK(3)


pthread_atforkregister handlers to be called when process forks


Standard C Library (libc, -lc)


#include <pthread.h>

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


The pthread_atfork() function registers the provided handler functions to be called when the fork(2) function is called. Each of the three handlers is called at a different place in the fork(2) sequence. The prepare handler is called in the parent process before the fork happens, the parent handler is called in the parent process after the fork has happened, and the child handler is called in the child process after the fork has happened. The parent and child handlers are called in the order in which they were registered, while the prepare handlers are called in reverse of the order in which they were registered.

Any of the handlers given may be NULL.

The intended use of pthread_atfork() is to provide a consistent state to a child process from a multithreaded parent process where locks may be acquired and released asynchronously with respect to the fork(2) call. Each subsystem with locks that are used in a child process should register handlers with pthread_atfork() that acquires those locks in the prepare handler and releases them in the parent handler.


The pthread_atfork() function returns 0 on success and an error number on failure.


The following error code may be returned:
Insufficient memory exists to register the fork handlers.


The pthread_atfork() function conforms to IEEE Std 1003.1c-1995 (“POSIX.1c”).


The pthread_atfork() function first appeared in NetBSD 2.0.


After calling fork(2) from a multithreaded process, it is only safe to call async-signal-safe functions until calling one of the exec(3) functions. The pthread_*() functions are not async-signal-safe, so it is not safe to use such functions in the child handler. POSIX does not mandate that pthread_mutex_unlock() be async-signal-safe, but it is in NetBSD and thus safe to use within the child handler.


There is no way to unregister a handler registered with pthread_atfork().
July 18, 2014 NetBSD 7.0