pthread_atfork - register fork handlers
#include <pthread.h>
int pthread_atfork(void (*prepare)(void), void (*parent)(void),
void (*child)(void));
Link with
-pthread.
The
pthread_atfork() function registers fork handlers that are to be
executed when
fork(2) is called by this thread. The handlers are
executed in the context of the thread that calls
fork(2).
Three kinds of handler can be registered:
- *
- prepare specifies a handler that is executed before fork(2)
processing starts.
- *
- parent specifies a handler that is executed in the parent process
after fork(2) processing completes.
- *
- child specifies a handler that is executed in the child process
after fork(2) processing completes.
Any of the three arguments may be NULL if no handler is needed in the
corresponding phase of
fork(2) processing.
On success,
pthread_atfork() returns zero. On error, it returns an error
number.
pthread_atfork() may be called multiple times by a thread, to
register multiple handlers for each phase. The handlers for each phase are
called in a specified order: the
prepare handlers are called in reverse
order of registration; the
parent and
child handlers are called
in the order of registration.
- ENOMEM
- Could not allocate memory to record the form handler entry.
POSIX.1-2001, POSIX.1-2008.
When
fork(2) is called in a multithreaded process, only the calling
thread is duplicated in the child process. The original intention of
pthread_atfork() was to allow the calling thread to be returned to a
consistent state. For example, at the time of the call to
fork(2),
other threads may have locked mutexes that are visible in the user-space
memory duplicated in the child. Such mutexes would never be unlocked, since
the threads that placed the locks are not duplicated in the child. The intent
of
pthread_atfork() was to provide a mechanism whereby the application
(or a library) could ensure that mutexes and other process and thread state
would be restored to a consistent state. In practice, this task is generally
too difficult to be practicable.
After a
fork(2) in a multithreaded process returns in the child, the
child should call only async-signal-safe functions (see
signal-safety(7)) until such time as it calls
execve(2) to
execute a new program.
POSIX.1 specifies that
pthread_atfork() shall not fail with the error
EINTR.
fork(2),
atexit(3),
pthreads(7)