pthread_atfork(3)	    Library Functions Manual	       pthread_atfork(3)

NAME
     pthread_atfork - register fork handlers

LIBRARY
     POSIX threads library (libpthread, -lpthread)

SYNOPSIS
     #include <pthread.h>

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

DESCRIPTION
     The  pthread_atfork()  function registers fork handlers that are to be exe-
     cuted 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:

     *	prepare	 specifies  a handler that is executed in the parent process be-
	fore fork(2) processing starts.

     *	parent specifies a handler that is executed in the parent process  after
	fork(2) processing completes.

     *	child  specifies  a  handler that is executed in the child process after
	fork(2) processing completes.

     Any of the three arguments may be NULL if no handler is needed in the  cor-
     responding 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 reg-
     ister additional handlers.	 The handlers for each phase  are  called  in  a
     specified order: the prepare handlers are called in reverse order of regis-
     tration; the parent and child handlers are called in the order of registra-
     tion.

ERRORS
     ENOMEM
	    Could not allocate memory to record the fork handler list entry.

STANDARDS
     POSIX.1-2008.

HISTORY
     POSIX.1-2001.

NOTES
     When  fork(2) is called in a multithreaded process, only the calling thread
     is duplicated in the child process.  The original intention of  pthread_at-
     fork() 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 en-
     sure 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)

Linux man-pages 6.18		   2026-02-08		       pthread_atfork(3)