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| .TH PTHREAD_ATFORK 3 2020-08-13 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| pthread_atfork \- register fork handlers |
| .SH SYNOPSIS |
| .nf |
| .B #include <pthread.h> |
| .PP |
| .BI "int pthread_atfork(void (*" prepare ")(void), void (*" parent ")(void)," |
| .BI " void (*" child ")(void));" |
| .fi |
| .PP |
| Link with \fI\-pthread\fP. |
| .SH DESCRIPTION |
| The |
| .BR pthread_atfork () |
| function registers fork handlers that are to be executed when |
| .BR fork (2) |
| is called by this thread. |
| The handlers are executed in the context of the thread that calls |
| .BR fork (2). |
| .PP |
| Three kinds of handler can be registered: |
| .IP * 3 |
| .IR prepare |
| specifies a handler that is executed before |
| .BR fork (2) |
| processing starts. |
| .IP * |
| .I parent |
| specifies a handler that is executed in the parent process after |
| .BR fork (2) |
| processing completes. |
| .IP * |
| .I child |
| specifies a handler that is executed in the child process after |
| .BR fork (2) |
| processing completes. |
| .PP |
| Any of the three arguments may be NULL if no handler is needed |
| in the corresponding phase of |
| .BR fork (2) |
| processing. |
| .SH RETURN VALUE |
| On success, |
| .BR pthread_atfork () |
| returns zero. |
| On error, it returns an error number. |
| .BR 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 |
| .I prepare |
| handlers are called in reverse order of registration; the |
| .I parent |
| and |
| .I child |
| handlers are called in the order of registration. |
| .SH ERRORS |
| .TP |
| .B ENOMEM |
| Could not allocate memory to record the form handler entry. |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008. |
| .SH NOTES |
| When |
| .BR fork (2) |
| is called in a multithreaded process, |
| only the calling thread is duplicated in the child process. |
| The original intention of |
| .BR pthread_atfork () |
| was to allow the calling thread to be returned to a consistent state. |
| For example, at the time of the call to |
| .BR 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 |
| .BR 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. |
| .PP |
| After a |
| .BR fork (2) |
| in a multithreaded process returns in the child, |
| the child should call only async-signal-safe functions (see |
| .BR signal\-safety (7)) |
| until such time as it calls |
| .BR execve (2) |
| to execute a new program. |
| .PP |
| POSIX.1 specifies that |
| .BR pthread_atfork () |
| shall not fail with the error |
| .BR EINTR . |
| .SH SEE ALSO |
| .BR fork (2), |
| .BR atexit (3), |
| .BR pthreads (7) |