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   3   Extensions
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   5 </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 30. 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 Builtin 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">&lt;ext/concurrence.h&gt;</code>
   6 contains all the higher-level
   7 constructs for playing with threads. In contrast to the atomics layer,
   8 the concurrence layer consists largely of types. All types are defined within <code class="code">namespace __gnu_cxx</code>.
   9 </p><p>
  10 These types can be used in a portable manner, regardless of the
  11 specific environment. They are carefully designed to provide optimum
  12 efficiency and speed, abstracting out underlying thread calls and
  13 accesses when compiling for single-threaded situations (even on hosts
  14 that support multiple threads.)
  15 </p><p>The enumerated type <code class="code">_Lock_policy</code> details the set of
  16 available locking
  17 policies: <code class="code">_S_single</code>, <code class="code">_S_mutex</code>,
  18 and <code class="code">_S_atomic</code>.
  19 </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.
  20 </p></li><li class="listitem"><p><code class="code">_S_mutex</code></p><p>Indicates multi-threaded code using thread-layer abstractions.
  21 </p></li><li class="listitem"><p><code class="code">_S_atomic</code></p><p>Indicates multi-threaded code using atomic operations.
  22 </p></li></ul></div><p>The compile-time constant <code class="code">__default_lock_policy</code> is set
  23 to one of the three values above, depending on characteristics of the
  24 host environment and the current compilation flags.
  25 </p><p>Two more datatypes make up the rest of the
  26 interface: <code class="code">__mutex</code>, and <code class="code">__scoped_lock</code>.
  27 </p><p>The scoped lock idiom is well-discussed within the C++
  28 community. This version takes a <code class="code">__mutex</code> reference, and
  29 locks it during construction of <code class="code">__scoped_lock</code> and
  30 unlocks it during destruction. This is an efficient way of locking
  31 critical sections, while retaining exception-safety.
  32 These types have been superseded in the ISO C++ 2011 standard by the
  33 mutex and lock types defined in the header
  34 <code class="filename">&lt;mutex&gt;</code>.
  35 </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>
  36 Two functions and one type form the base of atomic support.
  37 </p><p>The type <code class="code">_Atomic_word</code> is a signed integral type
  38 supporting atomic operations.
  39 </p><p>
  40 The two functions functions are:
  41 </p><pre class="programlisting">
  42 _Atomic_word
  43 __exchange_and_add_dispatch(volatile _Atomic_word*, int);
  44
  45 void
  46 __atomic_add_dispatch(volatile _Atomic_word*, int);
  47 </pre><p>Both of these functions are declared in the header file
  48 &lt;ext/atomicity.h&gt;, and are in <code class="code">namespace __gnu_cxx</code>.
  49 </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
  50 <code class="code">
  51 __exchange_and_add_dispatch
  52 </code>
  53 </p><p>Adds the second argument's value to the first argument. Returns the old value.
  54 </p></li><li class="listitem"><p>
  55 <code class="code">
  56 __atomic_add_dispatch
  57 </code>
  58 </p><p>Adds the second argument's value to the first argument. Has no return value.
  59 </p></li></ul></div><p>
  60 These functions forward to one of several specialized helper
  61 functions, depending on the circumstances. For instance,
  62 </p><p>
  63 <code class="code">
  64 __exchange_and_add_dispatch
  65 </code>
  66 </p><p>
  67 Calls through to either of:
  68 </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p><code class="code">__exchange_and_add</code>
  69 </p><p>Multi-thread version. Inlined if compiler-generated builtin atomics
  70 can be used, otherwise resolved at link time to a non-builtin code
  71 sequence.
  72 </p></li><li class="listitem"><p><code class="code">__exchange_and_add_single</code>
  73 </p><p>Single threaded version. Inlined.</p></li></ul></div><p>However, only <code class="code">__exchange_and_add_dispatch</code>
  74 and <code class="code">__atomic_add_dispatch</code> should be used. These functions
  75 can be used in a portable manner, regardless of the specific
  76 environment. They are carefully designed to provide optimum efficiency
  77 and speed, abstracting out atomic accesses when they are not required
  78 (even on hosts that support compiler intrinsics for atomic
  79 operations.)
  80 </p><p>
  81 In addition, there are two macros
  82 </p><p>
  83 <code class="code">
  84 _GLIBCXX_READ_MEM_BARRIER
  85 </code>
  86 </p><p>
  87 <code class="code">
  88 _GLIBCXX_WRITE_MEM_BARRIER
  89 </code>
  90 </p><p>
  91 Which expand to the appropriate write and read barrier required by the
  92 host hardware and operating system.
  93 </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 29. 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>