Re: pthread_atfork and PTHREAD_FORK

On Wed, 26 Sep 2001, Bill Medland wrote: NAME pthread_atfork - register handlers to be called at fork(2) time SYNOPSIS #include <pthread.h> int pthread_atfork(void (*prepare)(void), void (*par­ ent)(void), void (*child)(void)); DESCRIPTION pthread_atfork registers handler functions to be called just before and just after a new process is created with fork(2). The prepare handler will be called from the par­ ent process, just before the new process is created. The parent handler will be called from the parent process, just before fork(2) returns. The child handler will be called from the child process, just before fork(2) returns. => One or several of the three handlers prepare, parent and => child can be given as NULL, meaning that no handler needs => to be called at the corresponding point. pthread_atfork can be called several times to install sev­ eral sets of handlers. At fork(2) time, the prepare han­ dlers are called in LIFO order (last added with pthread_atfork, first called before fork), while the par­ ent and child handlers are called in FIFO order (first added, first called). To understand the purpose of pthread_atfork, recall that fork(2) duplicates the whole memory space, including mutexes in their current locking state, but only the call­ ing thread: other threads are not running in the child process. Thus, if a mutex is locked by a thread other than the thread calling fork, that mutex will remain locked forever in the child process, possibly blocking the execu­ tion of the child process. To avoid this, install handlers with pthread_atfork as follows: the prepare handler locks the global mutexes (in locking order), and the parent and child handlers unlock them (in reverse order). Alterna­ tively, prepare and parent can be set to NULL and child to a function that calls pthread_mutex_init on the global mutexes. RETURN VALUE pthread_atfork returns 0 on success and a non-zero error code on error. ERRORS ENOMEM insufficient memory available to register the han­ dlers. AUTHOR Xavier Leroy &lt;Xavier.Leroy(a)inria.fr&gt; SEE ALSO fork(2), pthread_mutex_lock(3), pthread_mutex_unlock(3). LinuxThreads PTHREAD_ATFORK(3)