pthread_atfork

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() process-
       ing commences. The parent fork handle shall be called after fork() pro-
       cessing  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 corre-
       sponding 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  exam-
       ple, 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 unsatis-
       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  inconsis-
       tent.  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>


IEEE/The Open Group                  2003                    PTHREAD_ATFORK(P)