<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Thread Local Storage</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../thread.html" title="Chapter 22. Thread"> <link rel="prev" href="synchronization.html" title="Synchronization"> <link rel="next" href="time.html" title="Date and Time Requirements"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="synchronization.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../thread.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="time.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="thread.thread_local_storage"></a><a class="link" href="thread_local_storage.html" title="Thread Local Storage">Thread Local Storage</a> </h2></div></div></div> <div class="toc"><dl><dt><span class="section"><a href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr"> Class <code class="computeroutput"><span class="identifier">thread_specific_ptr</span></code></a></span></dt></dl></div> <a name="thread.thread_local_storage.synopsis"></a><h4> <a name="id2717032"></a> <a class="link" href="thread_local_storage.html#thread.thread_local_storage.synopsis">Synopsis</a> </h4> <p> Thread local storage allows multi-threaded applications to have a separate instance of a given data item for each thread. Where a single-threaded application would use static or global data, this could lead to contention, deadlock or data corruption in a multi-threaded application. One example is the C <code class="computeroutput"><span class="identifier">errno</span></code> variable, used for storing the error code related to functions from the Standard C library. It is common practice (and required by POSIX) for compilers that support multi-threaded applications to provide a separate instance of <code class="computeroutput"><span class="identifier">errno</span></code> for each thread, in order to avoid different threads competing to read or update the value. </p> <p> Though compilers often provide this facility in the form of extensions to the declaration syntax (such as <code class="computeroutput"><span class="identifier">__declspec</span><span class="special">(</span><span class="identifier">thread</span><span class="special">)</span></code> or <code class="computeroutput"><span class="identifier">__thread</span></code> annotations on <code class="computeroutput"><span class="keyword">static</span></code> or namespace-scope variable declarations), such support is non-portable, and is often limited in some way, such as only supporting POD types. </p> <a name="thread.thread_local_storage.portable_thread_local_storage_with__code__phrase_role__identifier__boost__phrase__phrase_role__special______phrase__phrase_role__identifier__thread_specific_ptr__phrase___code_"></a><h4> <a name="id2717142"></a> <a class="link" href="thread_local_storage.html#thread.thread_local_storage.portable_thread_local_storage_with__code__phrase_role__identifier__boost__phrase__phrase_role__special______phrase__phrase_role__identifier__thread_specific_ptr__phrase___code_">Portable thread-local storage with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">thread_specific_ptr</span></code></a> </h4> <p> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">thread_specific_ptr</span></code> provides a portable mechanism for thread-local storage that works on all compilers supported by <span class="bold"><strong>Boost.Thread</strong></span>. Each instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">thread_specific_ptr</span></code> represents a pointer to an object (such as <code class="computeroutput"><span class="identifier">errno</span></code>) where each thread must have a distinct value. The value for the current thread can be obtained using the <code class="computeroutput"><span class="identifier">get</span><span class="special">()</span></code> member function, or by using the <code class="computeroutput"><span class="special">*</span></code> and <code class="computeroutput"><span class="special">-></span></code> pointer deference operators. Initially the pointer has a value of <code class="computeroutput"><span class="identifier">NULL</span></code> in each thread, but the value for the current thread can be set using the <code class="computeroutput"><span class="identifier">reset</span><span class="special">()</span></code> member function. </p> <p> If the value of the pointer for the current thread is changed using <code class="computeroutput"><span class="identifier">reset</span><span class="special">()</span></code>, then the previous value is destroyed by calling the cleanup routine. Alternatively, the stored value can be reset to <code class="computeroutput"><span class="identifier">NULL</span></code> and the prior value returned by calling the <code class="computeroutput"><span class="identifier">release</span><span class="special">()</span></code> member function, allowing the application to take back responsibility for destroying the object. </p> <a name="thread.thread_local_storage.cleanup_at_thread_exit"></a><h4> <a name="id2717149"></a> <a class="link" href="thread_local_storage.html#thread.thread_local_storage.cleanup_at_thread_exit">Cleanup at thread exit</a> </h4> <p> When a thread exits, the objects associated with each <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">thread_specific_ptr</span></code> instance are destroyed. By default, the object pointed to by a pointer <code class="computeroutput"><span class="identifier">p</span></code> is destroyed by invoking <code class="computeroutput"><span class="keyword">delete</span> <span class="identifier">p</span></code>, but this can be overridden for a specific instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">thread_specific_ptr</span></code> by providing a cleanup routine to the constructor. In this case, the object is destroyed by invoking <code class="computeroutput"><span class="identifier">func</span><span class="special">(</span><span class="identifier">p</span><span class="special">)</span></code> where <code class="computeroutput"><span class="identifier">func</span></code> is the cleanup routine supplied to the constructor. The cleanup functions are called in an unspecified order. If a cleanup routine sets the value of associated with an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">thread_specific_ptr</span></code> that has already been cleaned up, that value is added to the cleanup list. Cleanup finishes when there are no outstanding instances of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">thread_specific_ptr</span></code> with values. </p> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="thread.thread_local_storage.thread_specific_ptr"></a><a class="link" href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr" title="Class thread_specific_ptr"> Class <code class="computeroutput"><span class="identifier">thread_specific_ptr</span></code></a> </h3></div></div></div> <div class="toc"><dl> <dt><span class="section"><a href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.default_constructor"> <code class="computeroutput"><span class="identifier">thread_specific_ptr</span><span class="special">();</span></code></a></span></dt> <dt><span class="section"><a href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.constructor_with_custom_cleanup"> <code class="computeroutput"><span class="keyword">explicit</span> <span class="identifier">thread_specific_ptr</span><span class="special">(</span><span class="keyword">void</span> <span class="special">(*</span><span class="identifier">cleanup_function</span><span class="special">)(</span><span class="identifier">T</span><span class="special">*));</span></code></a></span></dt> <dt><span class="section"><a href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.destructor"> <code class="computeroutput"><span class="special">~</span><span class="identifier">thread_specific_ptr</span><span class="special">();</span></code></a></span></dt> <dt><span class="section"><a href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.get"> <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span></code></a></span></dt> <dt><span class="section"><a href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.operator_arrow"> <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span> <span class="keyword">operator</span><span class="special">->()</span> <span class="keyword">const</span><span class="special">;</span></code></a></span></dt> <dt><span class="section"><a href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.operator_star"> <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">*()</span> <span class="keyword">const</span><span class="special">;</span></code></a></span></dt> <dt><span class="section"><a href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.reset"> <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">reset</span><span class="special">(</span><span class="identifier">T</span><span class="special">*</span> <span class="identifier">new_value</span><span class="special">=</span><span class="number">0</span><span class="special">);</span></code></a></span></dt> <dt><span class="section"><a href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.release"> <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span> <span class="identifier">release</span><span class="special">();</span></code></a></span></dt> </dl></div> <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">thread</span><span class="special">/</span><span class="identifier">tss</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">thread_specific_ptr</span> <span class="special">{</span> <span class="keyword">public</span><span class="special">:</span> <span class="identifier">thread_specific_ptr</span><span class="special">();</span> <span class="keyword">explicit</span> <span class="identifier">thread_specific_ptr</span><span class="special">(</span><span class="keyword">void</span> <span class="special">(*</span><span class="identifier">cleanup_function</span><span class="special">)(</span><span class="identifier">T</span><span class="special">*));</span> <span class="special">~</span><span class="identifier">thread_specific_ptr</span><span class="special">();</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span> <span class="identifier">T</span><span class="special">*</span> <span class="keyword">operator</span><span class="special">->()</span> <span class="keyword">const</span><span class="special">;</span> <span class="identifier">T</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">*()</span> <span class="keyword">const</span><span class="special">;</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">release</span><span class="special">();</span> <span class="keyword">void</span> <span class="identifier">reset</span><span class="special">(</span><span class="identifier">T</span><span class="special">*</span> <span class="identifier">new_value</span><span class="special">=</span><span class="number">0</span><span class="special">);</span> <span class="special">};</span> </pre> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="thread.thread_local_storage.thread_specific_ptr.default_constructor"></a><a class="link" href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.default_constructor" title="thread_specific_ptr();"> <code class="computeroutput"><span class="identifier">thread_specific_ptr</span><span class="special">();</span></code></a> </h4></div></div></div> <div class="variablelist"> <p class="title"><b></b></p> <dl> <dt><span class="term">Requires:</span></dt> <dd><p> <code class="computeroutput"><span class="keyword">delete</span> <span class="keyword">this</span><span class="special">-></span><span class="identifier">get</span><span class="special">()</span></code> is well-formed. </p></dd> <dt><span class="term">Effects:</span></dt> <dd><p> Construct a <code class="computeroutput"><span class="identifier">thread_specific_ptr</span></code> object for storing a pointer to an object of type <code class="computeroutput"><span class="identifier">T</span></code> specific to each thread. The default <code class="computeroutput"><span class="keyword">delete</span></code>-based cleanup function will be used to destroy any thread-local objects when <code class="computeroutput"><span class="identifier">reset</span><span class="special">()</span></code> is called, or the thread exits. </p></dd> <dt><span class="term">Throws:</span></dt> <dd><p> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">thread_resource_error</span></code> if an error occurs. </p></dd> </dl> </div> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="thread.thread_local_storage.thread_specific_ptr.constructor_with_custom_cleanup"></a><a class="link" href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.constructor_with_custom_cleanup" title="explicit thread_specific_ptr(void (*cleanup_function)(T*));"> <code class="computeroutput"><span class="keyword">explicit</span> <span class="identifier">thread_specific_ptr</span><span class="special">(</span><span class="keyword">void</span> <span class="special">(*</span><span class="identifier">cleanup_function</span><span class="special">)(</span><span class="identifier">T</span><span class="special">*));</span></code></a> </h4></div></div></div> <div class="variablelist"> <p class="title"><b></b></p> <dl> <dt><span class="term">Requires:</span></dt> <dd><p> <code class="computeroutput"><span class="identifier">cleanup_function</span><span class="special">(</span><span class="keyword">this</span><span class="special">-></span><span class="identifier">get</span><span class="special">())</span></code> does not throw any exceptions. </p></dd> <dt><span class="term">Effects:</span></dt> <dd><p> Construct a <code class="computeroutput"><span class="identifier">thread_specific_ptr</span></code> object for storing a pointer to an object of type <code class="computeroutput"><span class="identifier">T</span></code> specific to each thread. The supplied <code class="computeroutput"><span class="identifier">cleanup_function</span></code> will be used to destroy any thread-local objects when <code class="computeroutput"><span class="identifier">reset</span><span class="special">()</span></code> is called, or the thread exits. </p></dd> <dt><span class="term">Throws:</span></dt> <dd><p> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">thread_resource_error</span></code> if an error occurs. </p></dd> </dl> </div> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="thread.thread_local_storage.thread_specific_ptr.destructor"></a><a class="link" href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.destructor" title="~thread_specific_ptr();"> <code class="computeroutput"><span class="special">~</span><span class="identifier">thread_specific_ptr</span><span class="special">();</span></code></a> </h4></div></div></div> <div class="variablelist"> <p class="title"><b></b></p> <dl> <dt><span class="term">Effects:</span></dt> <dd><p> Calls <code class="computeroutput"><span class="keyword">this</span><span class="special">-></span><span class="identifier">reset</span><span class="special">()</span></code> to clean up the associated value for the current thread, and destroys <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>. </p></dd> <dt><span class="term">Throws:</span></dt> <dd><p> Nothing. </p></dd> </dl> </div> <div class="note"><table border="0" summary="Note"> <tr> <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> <th align="left">Note</th> </tr> <tr><td align="left" valign="top"><p> Care needs to be taken to ensure that any threads still running after an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">thread_specific_ptr</span></code> has been destroyed do not call any member functions on that instance. </p></td></tr> </table></div> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="thread.thread_local_storage.thread_specific_ptr.get"></a><a class="link" href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.get" title="T* get() const;"> <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span></code></a> </h4></div></div></div> <div class="variablelist"> <p class="title"><b></b></p> <dl> <dt><span class="term">Returns:</span></dt> <dd><p> The pointer associated with the current thread. </p></dd> <dt><span class="term">Throws:</span></dt> <dd><p> Nothing. </p></dd> </dl> </div> <div class="note"><table border="0" summary="Note"> <tr> <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> <th align="left">Note</th> </tr> <tr><td align="left" valign="top"><p> The initial value associated with an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">thread_specific_ptr</span></code> is <code class="computeroutput"><span class="identifier">NULL</span></code> for each thread. </p></td></tr> </table></div> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="thread.thread_local_storage.thread_specific_ptr.operator_arrow"></a><a class="link" href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.operator_arrow" title="T* operator->() const;"> <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span> <span class="keyword">operator</span><span class="special">->()</span> <span class="keyword">const</span><span class="special">;</span></code></a> </h4></div></div></div> <div class="variablelist"> <p class="title"><b></b></p> <dl> <dt><span class="term">Returns:</span></dt> <dd><p> <code class="computeroutput"><span class="keyword">this</span><span class="special">-></span><span class="identifier">get</span><span class="special">()</span></code> </p></dd> <dt><span class="term">Throws:</span></dt> <dd><p> Nothing. </p></dd> </dl> </div> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="thread.thread_local_storage.thread_specific_ptr.operator_star"></a><a class="link" href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.operator_star" title="T& operator*() const;"> <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">*()</span> <span class="keyword">const</span><span class="special">;</span></code></a> </h4></div></div></div> <div class="variablelist"> <p class="title"><b></b></p> <dl> <dt><span class="term">Requires:</span></dt> <dd><p> <code class="computeroutput"><span class="keyword">this</span><span class="special">-></span><span class="identifier">get</span></code> is not <code class="computeroutput"><span class="identifier">NULL</span></code>. </p></dd> <dt><span class="term">Returns:</span></dt> <dd><p> <code class="computeroutput"><span class="special">*(</span><span class="keyword">this</span><span class="special">-></span><span class="identifier">get</span><span class="special">())</span></code> </p></dd> <dt><span class="term">Throws:</span></dt> <dd><p> Nothing. </p></dd> </dl> </div> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="thread.thread_local_storage.thread_specific_ptr.reset"></a><a class="link" href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.reset" title="void reset(T* new_value=0);"> <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">reset</span><span class="special">(</span><span class="identifier">T</span><span class="special">*</span> <span class="identifier">new_value</span><span class="special">=</span><span class="number">0</span><span class="special">);</span></code></a> </h4></div></div></div> <div class="variablelist"> <p class="title"><b></b></p> <dl> <dt><span class="term">Effects:</span></dt> <dd><p> If <code class="computeroutput"><span class="keyword">this</span><span class="special">-></span><span class="identifier">get</span><span class="special">()!=</span><span class="identifier">new_value</span></code> and <code class="computeroutput"><span class="keyword">this</span><span class="special">-></span><span class="identifier">get</span><span class="special">()</span></code> is non-<code class="computeroutput"><span class="identifier">NULL</span></code>, invoke <code class="computeroutput"><span class="keyword">delete</span> <span class="keyword">this</span><span class="special">-></span><span class="identifier">get</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">cleanup_function</span><span class="special">(</span><span class="keyword">this</span><span class="special">-></span><span class="identifier">get</span><span class="special">())</span></code> as appropriate. Store <code class="computeroutput"><span class="identifier">new_value</span></code> as the pointer associated with the current thread. </p></dd> <dt><span class="term">Postcondition:</span></dt> <dd><p> <code class="computeroutput"><span class="keyword">this</span><span class="special">-></span><span class="identifier">get</span><span class="special">()==</span><span class="identifier">new_value</span></code> </p></dd> <dt><span class="term">Throws:</span></dt> <dd><p> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">thread_resource_error</span></code> if an error occurs. </p></dd> </dl> </div> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="thread.thread_local_storage.thread_specific_ptr.release"></a><a class="link" href="thread_local_storage.html#thread.thread_local_storage.thread_specific_ptr.release" title="T* release();"> <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span> <span class="identifier">release</span><span class="special">();</span></code></a> </h4></div></div></div> <div class="variablelist"> <p class="title"><b></b></p> <dl> <dt><span class="term">Effects:</span></dt> <dd><p> Return <code class="computeroutput"><span class="keyword">this</span><span class="special">-></span><span class="identifier">get</span><span class="special">()</span></code> and store <code class="computeroutput"><span class="identifier">NULL</span></code> as the pointer associated with the current thread without invoking the cleanup function. </p></dd> <dt><span class="term">Postcondition:</span></dt> <dd><p> <code class="computeroutput"><span class="keyword">this</span><span class="special">-></span><span class="identifier">get</span><span class="special">()==</span><span class="number">0</span></code> </p></dd> <dt><span class="term">Throws:</span></dt> <dd><p> Nothing. </p></dd> </dl> </div> </div> </div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2007 -8 Anthony Williams<p> Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="synchronization.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../thread.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="time.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>