pthread_atfork(3) Library Functions Manual pthread_atfork(3)
NAME
pthread_atfork - register fork handlers
LIBRARY
POSIX threads library (libpthread, -lpthread)
SYNOPSIS
#include <pthread.h>
int pthread_atfork(void (*prepare)(void), void (*parent)(void),
void (*child)(void));
DESCRIPTION
The pthread_atfork() function registers fork handlers that are to be ex-
ecuted when fork(2) is called by any thread in a process. 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 in the parent process
before fork(2) processing starts.
• parent specifies a handler that is executed in the parent process af-
ter 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.
RETURN VALUE
On success, pthread_atfork() returns zero. On error, it returns an er-
ror number. pthread_atfork() may be called multiple times by a process
to register additional handlers. 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.
ERRORS
ENOMEM Could not allocate memory to record the fork handler list entry.
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001.
NOTES
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 child process to be returned to a con-
sistent 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 mem-
ory 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 sig-
nal-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.
SEE ALSO
fork(2), atexit(3), pthreads(7)
Linux man-pages 6.9.1 2024-05-02 pthread_atfork(3)
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