pthread_atfork(3P)

NAME
     pthread_atfork - register fork() handlers

C SYNOPSIS
     #include <pthread.h>

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

DESCRIPTION
     The pthread_atfork() function registers three functions which are invoked
     when any thread calls fork().  If prepare is not NULL it must be a
     function that will be called prior to the actual fork() in the parent
     context.  Similarly, parent and child, if not NULL, will be called after
     fork() in the contexts of the parent and child respectively.  Multiple
     calls to pthread_atfork() are possible; prepare handlers are run in the
     opposite order to which they were registered and parent and child
     handlers in the order they were registered.

     When fork() is called from a threaded application a new process is
     created with a single thread (the caller); other threads which may be
     running in the parent do not exist in the child.  A side effect of this
     behavior is that locks which protect data in the process may be held in
     the child by threads which no longer exist.  If the single surviving
     thread were to encounter these locks it would wait indefinitely.

     Using fork() handlers an application may protect its own data by
     synchronizing its state with respect to fork().  Typically this means
     adding a prepare handler to acquire a lock and parent and child handlers
     to unlock it again, ensuring that the child may use the lock (and
     associated data) as usual.

     Although an application may protect its own data in this way, libraries
     it uses may not.  Therefore the child process should restrict itself to
     its own code and to system calls.  This restriction is less onerous than
     it appears since the most common reason for using fork() in a threaded
     application is in order to start a new process with exec().

DIAGNOSTICS
     On success pthread_atfork() returns zero; otherwise an error number is
     returned:

     [ENOMEM]       Memory cannot be allocated to record the handlers.

SEE ALSO
     fork(2), exec(2).