pthread_atfork - Man Page

register fork handlers


POSIX threads library (libpthread, -lpthread)


#include <pthread.h>

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


The pthread_atfork() function registers fork handlers that are to be executed when fork(2) is called by any thread in a process. 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 process to register additional handlers. 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 fork handler list entry.






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 child process 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)

Referenced By

arc4random.3bsd(3), clone(2), fork(2), OPENSSL_fork_prepare.3ossl(3), posix_spawn(3), pthreads(7), signal-safety(7), system(3), vfork(2).

2023-10-31 Linux man-pages 6.7