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pthread_atfork(3) Library Functions Manual pthread_atfork(3) NAME pthread_atfork - Declares fork handler routines to be called when the calling thread's process forks a child process. LIBRARY Standard C Library (libc.so, libc.a) SYNOPSIS #include <pthread.h> int pthread_atfork( void (*prepare)(void), void (*parent)(void), void (*child)(void)); STANDARDS Interfaces documented on this reference page conform to industry stan- dards as follows: IEEE Std 1003.1c-1995, POSIX System Application Program Interface PARAMETERS Address of a routine that performs the fork preparation handling. This routine is called in the parent process before creating the child process. Address of a routine that performs the fork parent handling. This routine is called in the parent process after creating the child process and before returning to the caller of fork(2). Address of a routine that performs the fork child handling. This routine is called in the child process before returning to the caller of fork(2). DESCRIPTION This routine allows a main program or library to control resources dur- ing a DIGITAL UNIX fork(2) operation by declaring fork handler rou- tines, as follows: The fork handler routine specified in the prepare argument is called before fork(2) executes. The fork handler routine specified in the parent argument is called after fork(2) executes within the parent process. The fork handler routine specified in the child argument is called in the new child process after fork(2) exe- cutes. Your program (or library) can use fork handlers to ensure that program context in the child process is consistent and meaningful. Af- ter fork(2) executes, only the calling thread exists in the child process, and the state of all memory in the parent process is repli- cated in the child process, including the states of any mutexes, condi- tion variables, and so on. For example, in the new child process there might exist locked mutexes that are copies of mutexes that were locked in the parent process by threads that do not exist in the child process. Therefore, any associ- ated program state might be inconsistent in the child process. The program can avoid this problem by calling pthread_atfork to provide routines that acquire and release resources that are critical to the child process. For example, the prepare handler should lock all mutexes that you want to be usable in the child process. The parent handler just unlocks those mutexes. The child handler will also unlock them all--and might also create threads or reset any program state for the child process. If no fork handling is desired, you can set any of this routine's argu- ments to NULL. NOTES It is not legal to call pthread_atfork from within a fork handler rou- tine. Doing so could cause a deadlock. EXAMPLES For example, if your library uses a mutex my_mutex, you might provide pthread_atfork handler routines coded as follows: void my_prepare(void) { pthread_mutex_lock(&my_mutex); } void my_parent(void) { pthread_mutex_unlock(&my_mutex); } void my_child(void) { pthread_mutex_unlock(&my_mutex); /* Reinitialize state that doesn't apply...like heap owned */ /* by other threads */ } { . . . pthread_atfork(my_prepare, my_parent, my_child); . . fork(); } RETURN VALUES If an error condition occurs, this routine returns an integer value in- dicating the type of error. Possible return values are as follows: Successful completion Insufficient table space exists to record the fork handler routines' addresses. ERRORS None RELATED INFORMATION Functions: pthread_create(3) Manuals: Guide to DECthreads, Programmer's Guide delim off pthread_atfork(3)