NAME

       pthread_atfork - register fork handlers

SYNOPSIS

       #include <pthread.h>

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

DESCRIPTION

       The  pthread_atfork()  function  shall declare fork handlers to be called before and after fork(), in the
       context of the thread that called fork().  The  prepare  fork  handler  shall  be  called  before  fork()
       processing  commences.  The  parent  fork handle shall be called after fork() processing completes in the
       parent process. The child fork handler shall be called after fork() processing  completes  in  the  child
       process.   If no handling is desired at one or more of these three points, the corresponding fork handler
       address(es) may be set to NULL.

       The order of calls to pthread_atfork() is significant. The parent and child fork handlers shall be called
       in  the  order  in  which  they were established by calls to pthread_atfork().  The prepare fork handlers
       shall be called in the opposite order.

RETURN VALUE

       Upon successful completion, pthread_atfork() shall return a value of zero;  otherwise,  an  error  number
       shall be returned to indicate the error.

ERRORS

       The pthread_atfork() function shall fail if:

       ENOMEM Insufficient table space exists to record the fork handler addresses.

       The pthread_atfork() function shall not return an error code of [EINTR].

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       None.

RATIONALE

       There  are  at  least  two serious problems with the semantics of fork() in a multi-threaded program. One
       problem has to do with state (for example, memory) covered by mutexes. Consider the case where one thread
       has a mutex locked and the state covered by that mutex is inconsistent while another thread calls fork().
       In the child, the mutex is in the locked state (locked by a nonexistent thread  and  thus  can  never  be
       unlocked).  Having the child simply reinitialize the mutex is unsatisfactory since this approach does not
       resolve the question about how to correct or otherwise deal with the inconsistent state in the child.

       It is suggested that programs that use fork() call an exec function very soon  afterwards  in  the  child
       process,  thus  resetting  all  states.  In  the meantime, only a short list of async-signal-safe library
       routines are promised to be available.

       Unfortunately, this solution does not address the needs of multi-threaded libraries. Application programs
       may  not  be  aware  that  a  multi-threaded  library is in use, and they feel free to call any number of
       library routines between the fork() and exec calls, just as they always have.  Indeed, they may be extant
       single-threaded  programs  and  cannot,  therefore,  be  expected to obey new restrictions imposed by the
       threads library.

       On the other hand, the multi-threaded library needs a way to protect its internal state during fork()  in
       case  it  is  re-entered  later in the child process. The problem arises especially in multi-threaded I/O
       libraries, which are almost sure to  be  invoked  between  the  fork()  and  exec  calls  to  effect  I/O
       redirection.  The  solution  may  require  locking mutex variables during fork(), or it may entail simply
       resetting the state in the child after the fork() processing completes.

       The pthread_atfork() function provides multi-threaded libraries with a means to protect  themselves  from
       innocent  application programs that call fork(), and it provides multi-threaded application programs with
       a standard mechanism for protecting themselves from fork() calls in a library routine or the  application
       itself.

       The  expected  usage is that the prepare handler acquires all mutex locks and the other two fork handlers
       release them.

       For example, an application can supply a prepare routine that acquires the necessary mutexes the  library
       maintains  and  supply child and parent routines that release those mutexes, thus ensuring that the child
       gets a consistent snapshot of the state  of  the  library  (and  that  no  mutexes  are  left  stranded).
       Alternatively, some libraries might be able to supply just a child routine that reinitializes the mutexes
       in the library and all associated states to some known value (for example, what it was when the image was
       originally executed).

       When  fork()  is  called,  only  the  calling thread is duplicated in the child process.  Synchronization
       variables remain in the same state in the child as they were in the parent at the time fork() was called.
       Thus,  for example, mutex locks may be held by threads that no longer exist in the child process, and any
       associated states may be inconsistent.  The parent process may avoid this by explicit code that  acquires
       and releases locks critical to the child via pthread_atfork().  In addition, any critical threads need to
       be recreated and reinitialized to the proper state in the child (also via pthread_atfork()).

       A higher-level package may acquire locks on its own data structures before invoking lower-level packages.
       Under  this  scenario,  the order specified for fork handler calls allows a simple rule of initialization
       for avoiding package deadlock: a package initializes all packages on which it depends before it calls the
       pthread_atfork() function for itself.

FUTURE DIRECTIONS

       None.

SEE ALSO

       atexit() , fork() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/types.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
       Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open  Group  Base
       Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers,
       Inc and The Open Group. In the event of any discrepancy between this version and the  original  IEEE  and
       The  Open  Group  Standard,  the  original  IEEE and The Open Group Standard is the referee document. The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .