pthread_atfork — register fork handlers


#include <pthread.h>

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

Link with -pthread.


The pthread_atfork() function registers fork handlers that are to be executed when fork(2) is called by this thread. The handlers are executed in the context of the thread that calls fork(2).

Three kinds of handler can be registered:

Any of the three arguments may be NULL if no handler is needed in the corresponding phase of fork(2) processing.

Return Value

On success, pthread_atfork() returns zero. On error, it returns an error number. pthread_atfork() may be called multiple times by a thread, to register multiple handlers for each phase. The handlers for each phase are called in a specified order: the prepare handlers are called in reverse order of registration; the parent and child handlers are called in the order of registration.



Could not allocate memory to record the form handler entry.

Conforming to

POSIX.1-2001, POSIX.1-2008.


When fork(2) is called in a multithreaded process, only the calling thread is duplicated in the child process. The original intention of pthread_atfork() was to allow the calling thread to be returned to a consistent state. For example, at the time of the call to fork(2), other threads may have locked mutexes that are visible in the user-space memory duplicated in the child. Such mutexes would never be unlocked, since the threads that placed the locks are not duplicated in the child. The intent of pthread_atfork() was to provide a mechanism whereby the application (or a library) could ensure that mutexes and other process and thread state would be restored to a consistent state. In practice, this task is generally too difficult to be practicable.

After a fork(2) in a multithreaded process returns in the child, the child should call only async-signal-safe functions (see signal-safety(7)) until such time as it calls execve(2) to execute a new program.

POSIX.1 specifies that pthread_atfork() shall not fail with the error EINTR.

See Also

fork(2), atexit(3), pthreads(7)


This page is part of release 5.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at

Referenced By

clone(2), fork(2), OPENSSL_fork_prepare.3ssl(3), posix_spawn(3), pthreads(7), signal-safety(7), system(3), vfork(2).

2017-09-15 Linux Programmer's Manual