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
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.
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
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.
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