.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved .TH "PTHREAD_ATFORK" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual" .\" pthread_atfork .SH NAME pthread_atfork \- register fork handlers .SH SYNOPSIS .LP \fB#include .br .sp int pthread_atfork(void (*\fP\fIprepare\fP\fB)(void), void (*\fP\fIparent\fP\fB)(void), .br \ \ \ \ \ \ void (*\fP\fIchild\fP\fB)(void)); \fP \fB .br \fP .SH DESCRIPTION .LP The \fIpthread_atfork\fP() function shall declare fork handlers to be called before and after \fIfork\fP(), in the context of the thread that called \fIfork\fP(). The \fIprepare\fP fork handler shall be called before \fIfork\fP() processing commences. The \fIparent\fP fork handle shall be called after \fIfork\fP() processing completes in the parent process. The \fIchild\fP fork handler shall be called after \fIfork\fP() 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. .LP The order of calls to \fIpthread_atfork\fP() is significant. The \fIparent\fP and \fIchild\fP fork handlers shall be called in the order in which they were established by calls to \fIpthread_atfork\fP(). The \fIprepare\fP fork handlers shall be called in the opposite order. .SH RETURN VALUE .LP Upon successful completion, \fIpthread_atfork\fP() shall return a value of zero; otherwise, an error number shall be returned to indicate the error. .SH ERRORS .LP The \fIpthread_atfork\fP() function shall fail if: .TP 7 .B ENOMEM Insufficient table space exists to record the fork handler addresses. .sp .LP The \fIpthread_atfork\fP() function shall not return an error code of [EINTR]. .LP \fIThe following sections are informative.\fP .SH EXAMPLES .LP None. .SH APPLICATION USAGE .LP None. .SH RATIONALE .LP There are at least two serious problems with the semantics of \fIfork\fP() 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 \fIfork\fP(). 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. .LP It is suggested that programs that use \fIfork\fP() call an \fIexec\fP 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. .LP 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 \fIfork\fP() and \fIexec\fP 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. .LP On the other hand, the multi-threaded library needs a way to protect its internal state during \fIfork\fP() 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 \fIfork\fP() and \fIexec\fP calls to effect I/O redirection. The solution may require locking mutex variables during \fIfork\fP(), or it may entail simply resetting the state in the child after the \fIfork\fP() processing completes. .LP The \fIpthread_atfork\fP() function provides multi-threaded libraries with a means to protect themselves from innocent application programs that call \fIfork\fP(), and it provides multi-threaded application programs with a standard mechanism for protecting themselves from \fIfork\fP() calls in a library routine or the application itself. .LP The expected usage is that the \fIprepare\fP handler acquires all mutex locks and the other two fork handlers release them. .LP For example, an application can supply a \fIprepare\fP routine that acquires the necessary mutexes the library maintains and supply \fIchild\fP and \fIparent\fP 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 \fIchild\fP 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). .LP When \fIfork\fP() 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 \fIfork\fP() 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 \fIpthread_atfork\fP(). In addition, any critical threads need to be recreated and reinitialized to the proper state in the child (also via \fIpthread_atfork\fP()). .LP 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 \fIpthread_atfork\fP() function for itself. .SH FUTURE DIRECTIONS .LP None. .SH SEE ALSO .LP \fIatexit\fP() , \fIfork\fP() , the Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI\fP .SH COPYRIGHT 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 .