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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta name="generator" content="HTML Tidy, see www.w3.org"><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 --><!-- Copyright (c) 2001-2003 The Open Group, All Rights Reserved --><title>pthread_atfork</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="pthread_atfork"></a> <a name="tag_03_486"></a><!-- pthread_atfork --> <!--header start--><center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2003 Edition<br>Copyright © 2001-2003 The IEEE and The Open Group, All Rights reserved.</font></center><!--header end--><hr size="2" noshade><h4><a name="tag_03_486_01"></a>NAME</h4><blockquote>pthread_atfork - register fork handlers</blockquote><h4><a name="tag_03_486_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><div class="box"><code><tt><sup>[<a href="javascript:open_code('THR')">THR</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> #include <<a href="../basedefs/pthread.h.html">pthread.h</a>><br><br> int pthread_atfork(void (*</tt><i>prepare</i><tt>)(void), void (*</tt><i>parent</i><tt>)(void),<br> void (*</tt><i>child</i><tt>)(void)); <img src="../images/opt-end.gif" alt="[Option End]"border="0"></tt></code></div><tt><br></tt></blockquote><h4><a name="tag_03_486_03"></a>DESCRIPTION</h4><blockquote><p>The <i>pthread_atfork</i>() function shall declare fork handlers to be called before and after <a href="../functions/fork.html"><i>fork</i>()</a>, in the context of the thread that called <a href="../functions/fork.html"><i>fork</i>()</a>. The <i>prepare</i> fork handler shall be called before <a href="../functions/fork.html"><i>fork</i>()</a> processing commences. The <i>parent</i> fork handle shall be called after <a href="../functions/fork.html"><i>fork</i>()</a> processing completes in the parent process. The <i>child</i> fork handler shall becalled after <a href="../functions/fork.html"><i>fork</i>()</a> processing completes in the child process. If no handling isdesired at one or more of these three points, the corresponding fork handler address(es) may be set to NULL.</p><p>The order of calls to <i>pthread_atfork</i>() is significant. The <i>parent</i> and <i>child</i> fork handlers shall be calledin the order in which they were established by calls to <i>pthread_atfork</i>(). The <i>prepare</i> fork handlers shall be calledin the opposite order.</p></blockquote><h4><a name="tag_03_486_04"></a>RETURN VALUE</h4><blockquote><p>Upon successful completion, <i>pthread_atfork</i>() shall return a value of zero; otherwise, an error number shall be returnedto indicate the error.</p></blockquote><h4><a name="tag_03_486_05"></a>ERRORS</h4><blockquote><p>The <i>pthread_atfork</i>() function shall fail if:</p><dl compact><dt>[ENOMEM]</dt><dd>Insufficient table space exists to record the fork handler addresses.</dd></dl><p>The <i>pthread_atfork</i>() function shall not return an error code of [EINTR].</p></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_03_486_06"></a>EXAMPLES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_486_07"></a>APPLICATION USAGE</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_486_08"></a>RATIONALE</h4><blockquote><p>There are at least two serious problems with the semantics of <a href="../functions/fork.html"><i>fork</i>()</a> in amulti-threaded program. One problem has to do with state (for example, memory) covered by mutexes. Consider the case where onethread has a mutex locked and the state covered by that mutex is inconsistent while another thread calls <a href="../functions/fork.html"><i>fork</i>()</a>. In the child, the mutex is in the locked state (locked by a nonexistent thread and thuscan never be unlocked). Having the child simply reinitialize the mutex is unsatisfactory since this approach does not resolve thequestion about how to correct or otherwise deal with the inconsistent state in the child.</p><p>It is suggested that programs that use <a href="../functions/fork.html"><i>fork</i>()</a> call an <i><a href="../functions/exec.html">exec</a></i> function very soon afterwards in the child process, thus resetting all states. In themeantime, only a short list of async-signal-safe library routines are promised to be available.</p><p>Unfortunately, this solution does not address the needs of multi-threaded libraries. Application programs may not be aware thata multi-threaded library is in use, and they feel free to call any number of library routines between the <a href="../functions/fork.html"><i>fork</i>()</a> and <i><a href="../functions/exec.html">exec</a></i> 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 thethreads library.</p><p>On the other hand, the multi-threaded library needs a way to protect its internal state during <a href="../functions/fork.html"><i>fork</i>()</a> in case it is re-entered later in the child process. The problem arises especially inmulti-threaded I/O libraries, which are almost sure to be invoked between the <a href="../functions/fork.html"><i>fork</i>()</a>and <i><a href="../functions/exec.html">exec</a></i> calls to effect I/O redirection. The solution may require locking mutexvariables during <a href="../functions/fork.html"><i>fork</i>()</a>, or it may entail simply resetting the state in the child afterthe <a href="../functions/fork.html"><i>fork</i>()</a> processing completes.</p><p>The <i>pthread_atfork</i>() function provides multi-threaded libraries with a means to protect themselves from innocentapplication programs that call <a href="../functions/fork.html"><i>fork</i>()</a>, and it provides multi-threaded applicationprograms with a standard mechanism for protecting themselves from <a href="../functions/fork.html"><i>fork</i>()</a> calls in alibrary routine or the application itself.</p><p>The expected usage is that the <i>prepare</i> handler acquires all mutex locks and the other two fork handlers release them.</p><p>For example, an application can supply a <i>prepare</i> routine that acquires the necessary mutexes the library maintains andsupply <i>child</i> and <i>parent</i> routines that release those mutexes, thus ensuring that the child gets a consistent snapshotof the state of the library (and that no mutexes are left stranded). Alternatively, some libraries might be able to supply just a<i>child</i> routine that reinitializes the mutexes in the library and all associated states to some known value (for example, whatit was when the image was originally executed).</p><p>When <a href="../functions/fork.html"><i>fork</i>()</a> 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 <a href="../functions/fork.html"><i>fork</i>()</a> was called. Thus, for example, mutex locks may be held by threads that no longer existin the child process, and any associated states may be inconsistent. The parent process may avoid this by explicit code thatacquires and releases locks critical to the child via <i>pthread_atfork</i>(). In addition, any critical threads need to berecreated and reinitialized to the proper state in the child (also via <i>pthread_atfork</i>()).</p><p>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 packageinitializes all packages on which it depends before it calls the <i>pthread_atfork</i>() function for itself.</p></blockquote><h4><a name="tag_03_486_09"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_486_10"></a>SEE ALSO</h4><blockquote><p><a href="atexit.html"><i>atexit</i>()</a> , <a href="fork.html"><i>fork</i>()</a> , the Base Definitions volume ofIEEE Std 1003.1-2001, <a href="../basedefs/sys/types.h.html"><i><sys/types.h></i></a></p></blockquote><h4><a name="tag_03_486_11"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 5. Derived from the POSIX Threads Extension.</p><p>IEEE PASC Interpretation 1003.1c #4 is applied.</p></blockquote><h4><a name="tag_03_486_12"></a>Issue 6</h4><blockquote><p>The <i>pthread_atfork</i>() function is marked as part of the Threads option.</p><p>The <a href="../basedefs/pthread.h.html"><i><pthread.h></i></a> header is added to the SYNOPSIS.</p></blockquote><div class="box"><em>End of informative text.</em></div><hr><hr size="2" noshade><center><font size="2"><!--footer start-->UNIX ® is a registered Trademark of The Open Group.<br>POSIX ® is a registered Trademark of The IEEE.<br>[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href="../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>]</font></center><!--footer end--><hr size="2" noshade></body></html>⌨️ 快捷键说明
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