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<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"><head><title>Chapter 30. Concurrency</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><meta name="keywords" content=" ISO C++ , library "/><meta name="keywords" content=" ISO C++ , library "/><meta name="keywords" content=" ISO C++ , runtime , library "/><link rel="home" href="../index.html" title="The GNU C++ Library"/><link rel="up" href="extensions.html" title="Part III. Extensions"/><link rel="prev" href="ext_demangling.html" title="Chapter 29. Demangling"/><link rel="next" href="bk01pt03ch30s02.html" title="Implementation"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 30. Concurrency</th></tr><tr><td align="left"><a accesskey="p" href="ext_demangling.html">Prev</a> </td><th width="60%" align="center">Part III. Extensions </th><td align="right"> <a accesskey="n" href="bk01pt03ch30s02.html">Next</a></td></tr></table><hr/></div><div class="chapter" title="Chapter 30. Concurrency"><div class="titlepage"><div><div><h2 class="title"><a id="manual.ext.concurrency"/>Chapter 30. Concurrency</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl><dt><span class="section"><a href="ext_concurrency.html#manual.ext.concurrency.design">Design</a></span></dt><dd><dl><dt><span class="section"><a href="ext_concurrency.html#manual.ext.concurrency.design.threads">Interface to Locks and Mutexes</a></span></dt><dt><span class="section"><a href="ext_concurrency.html#manual.ext.concurrency.design.atomics">Interface to Atomic Functions</a></span></dt></dl></dd><dt><span class="section"><a href="bk01pt03ch30s02.html">Implementation</a></span></dt><dd><dl><dt><span class="section"><a href="bk01pt03ch30s02.html#manual.ext.concurrency.impl.atomic_fallbacks">Using Builtin Atomic Functions</a></span></dt><dt><span class="section"><a href="bk01pt03ch30s02.html#manual.ext.concurrency.impl.thread">Thread Abstraction</a></span></dt></dl></dd><dt><span class="section"><a href="bk01pt03ch30s03.html">Use</a></span></dt></dl></div><div class="section" title="Design"><div class="titlepage"><div><div><h2 class="title"><a id="manual.ext.concurrency.design"/>Design</h2></div></div></div><div class="section" title="Interface to Locks and Mutexes"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.concurrency.design.threads"/>Interface to Locks and Mutexes</h3></div></div></div><p>The file <code class="filename"><ext/concurrence.h></code> contains all the higher-level constructs for playing with threads. In contrast to the atomics layer, the concurrence layer consists largely of types. All types are defined within <code class="code">namespace __gnu_cxx</code>. </p><p> These types can be used in a portable manner, regardless of the specific environment. They are carefully designed to provide optimum efficiency and speed, abstracting out underlying thread calls and accesses when compiling for single-threaded situations (even on hosts that support multiple threads.) </p><p>The enumerated type <code class="code">_Lock_policy</code> details the set of available locking policies: <code class="code">_S_single</code>, <code class="code">_S_mutex</code>, and <code class="code">_S_atomic</code>. </p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p><code class="code">_S_single</code></p><p>Indicates single-threaded code that does not need locking. </p></li><li class="listitem"><p><code class="code">_S_mutex</code></p><p>Indicates multi-threaded code using thread-layer abstractions. </p></li><li class="listitem"><p><code class="code">_S_atomic</code></p><p>Indicates multi-threaded code using atomic operations. </p></li></ul></div><p>The compile-time constant <code class="code">__default_lock_policy</code> is set to one of the three values above, depending on characteristics of the host environment and the current compilation flags. </p><p>Two more datatypes make up the rest of the interface: <code class="code">__mutex</code>, and <code class="code">__scoped_lock</code>. </p><p>The scoped lock idiom is well-discussed within the C++ community. This version takes a <code class="code">__mutex</code> reference, and locks it during construction of <code class="code">__scoped_lock</code> and unlocks it during destruction. This is an efficient way of locking critical sections, while retaining exception-safety. These types have been superseded in the ISO C++ 2011 standard by the mutex and lock types defined in the header <code class="filename"><mutex></code>. </p></div><div class="section" title="Interface to Atomic Functions"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.concurrency.design.atomics"/>Interface to Atomic Functions</h3></div></div></div><p> Two functions and one type form the base of atomic support. </p><p>The type <code class="code">_Atomic_word</code> is a signed integral type supporting atomic operations. </p><p> The two functions functions are: </p><pre class="programlisting"> _Atomic_word __exchange_and_add_dispatch(volatile _Atomic_word*, int); void __atomic_add_dispatch(volatile _Atomic_word*, int); </pre><p>Both of these functions are declared in the header file <ext/atomicity.h>, and are in <code class="code">namespace __gnu_cxx</code>. </p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p> <code class="code"> __exchange_and_add_dispatch </code> </p><p>Adds the second argument's value to the first argument. Returns the old value. </p></li><li class="listitem"><p> <code class="code"> __atomic_add_dispatch </code> </p><p>Adds the second argument's value to the first argument. Has no return value. </p></li></ul></div><p> These functions forward to one of several specialized helper functions, depending on the circumstances. For instance, </p><p> <code class="code"> __exchange_and_add_dispatch </code> </p><p> Calls through to either of: </p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p><code class="code">__exchange_and_add</code> </p><p>Multi-thread version. Inlined if compiler-generated builtin atomics can be used, otherwise resolved at link time to a non-builtin code sequence. </p></li><li class="listitem"><p><code class="code">__exchange_and_add_single</code> </p><p>Single threaded version. Inlined.</p></li></ul></div><p>However, only <code class="code">__exchange_and_add_dispatch</code> and <code class="code">__atomic_add_dispatch</code> should be used. These functions can be used in a portable manner, regardless of the specific environment. They are carefully designed to provide optimum efficiency and speed, abstracting out atomic accesses when they are not required (even on hosts that support compiler intrinsics for atomic operations.) </p><p> In addition, there are two macros </p><p> <code class="code"> _GLIBCXX_READ_MEM_BARRIER </code> </p><p> <code class="code"> _GLIBCXX_WRITE_MEM_BARRIER </code> </p><p> Which expand to the appropriate write and read barrier required by the host hardware and operating system. </p></div></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="ext_demangling.html">Prev</a> </td><td align="center"><a accesskey="u" href="extensions.html">Up</a></td><td align="right"> <a accesskey="n" href="bk01pt03ch30s02.html">Next</a></td></tr><tr><td align="left" valign="top">Chapter 29. Demangling </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Implementation</td></tr></table></div></body></html>
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