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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
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 was intended to provide multi-threaded libraries with a means to protect
themselves from innocent application programs that call fork(), and to provide 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 was that the prepare handler would acquire all mutex locks and the other two fork
handlers would release them.
For example, an application could have supplied a prepare routine that acquires the necessary mutexes the
library maintains and supplied child and parent routines that release those mutexes, thus ensuring that
the child would have got a consistent snapshot of the state of the library (and that no mutexes would
have been left stranded). This is good in theory, but in reality not practical. Each and every mutex and
lock in the process must be located and locked. Every component of a program including third-party
components must participate and they must agree who is responsible for which mutex or lock. This is
especially problematic for mutexes and locks in dynamically allocated memory. All mutexes and locks
internal to the implementation must be locked, too. This possibly delays the thread calling fork() for a
long time or even indefinitely since uses of these synchronization objects may not be under control of
the application. A final problem to mention here is the problem of locking streams. At least the streams
under control of the system (like stdin, stdout, stderr) must be protected by locking the stream with
flockfile(). But the application itself could have done that, possibly in the same thread calling
fork(). In this case, the process will deadlock.
Alternatively, some libraries might have been 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). This approach is not possible, though, because implementations are
allowed to fail *_init() and *_destroy() calls for mutexes and locks if the mutex or lock is still
locked. In this case, the child routine is not able to reinitialize the mutexes and locks.
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 intention was that the parent process could have avoided this
by explicit code that acquires and releases locks critical to the child via pthread_atfork(). In
addition, any critical threads would have needed 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.
As explained, there is no suitable solution for functionality which requires non-atomic operations to be
protected through mutexes and locks. This is why the POSIX.1 standard since the 1996 release requires
that the child process after fork() in a multi-threaded process only calls async-signal-safe interfaces.
FUTURE DIRECTIONS
None.
SEE ALSO
atexit(), exec, fork()
The Base Definitions volume of POSIX.1‐2008, <pthread.h>, <sys_types.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .