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| <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 29. Concurrency</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><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 28. Demangling" /><link rel="next" href="ext_concurrency_impl.html" title="Implementation" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 29. Concurrency</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ext_demangling.html">Prev</a> </td><th width="60%" align="center">Part III. |
| Extensions |
| |
| </th><td width="20%" align="right"> <a accesskey="n" href="ext_concurrency_impl.html">Next</a></td></tr></table><hr /></div><div class="chapter"><div class="titlepage"><div><div><h2 class="title"><a id="manual.ext.concurrency"></a>Chapter 29. Concurrency</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><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="ext_concurrency_impl.html">Implementation</a></span></dt><dd><dl><dt><span class="section"><a href="ext_concurrency_impl.html#manual.ext.concurrency.impl.atomic_fallbacks">Using Built-in Atomic Functions</a></span></dt><dt><span class="section"><a href="ext_concurrency_impl.html#manual.ext.concurrency.impl.thread">Thread Abstraction</a></span></dt></dl></dd><dt><span class="section"><a href="ext_concurrency_use.html">Use</a></span></dt></dl></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.ext.concurrency.design"></a>Design</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.concurrency.design.threads"></a>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" style="list-style-type: disc; "><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"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.concurrency.design.atomics"></a>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" style="list-style-type: disc; "><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" style="list-style-type: disc; "><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 width="40%" align="left"><a accesskey="p" href="ext_demangling.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="extensions.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ext_concurrency_impl.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 28. Demangling </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Implementation</td></tr></table></div></body></html> |
