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| 4 Extensions | |
| 5 | |
| 6 </th><td width="20%" align="right"> <a accesskey="n" href="bk01pt12ch40s02.html"
>Next</a></td></tr></table><hr /></div><div class="chapter" lang="en" xml:lang="
en"><div class="titlepage"><div><div><h2 class="title"><a id="manual.ext.concurr
ency"></a>Chapter 40. Concurrency</h2></div></div></div><div class="toc"><p><b>T
able of Contents</b></p><dl><dt><span class="sect1"><a href="ext_concurrency.htm
l#manual.ext.concurrency.design">Design</a></span></dt><dd><dl><dt><span class="
sect2"><a href="ext_concurrency.html#manual.ext.concurrency.design.threads">Inte
rface to Locks and Mutexes</a></span></dt><dt><span class="sect2"><a href="ext_c
oncurrency.html#manual.ext.concurrency.design.atomics">Interface to Atomic Funct
ions</a></span></dt></dl></dd><dt><span class="sect1"><a href="bk01pt12ch40s02.h
tml">Implementation</a></span></dt><dd><dl><dt><span class="sect2"><a href="bk01
pt12ch40s02.html#manual.ext.concurrency.impl.atomic_fallbacks">Using Builtin Ato
mic Functions</a></span></dt><dt><span class="sect2"><a href="bk01pt12ch40s02.ht
ml#manual.ext.concurrency.impl.thread">Thread Abstraction</a></span></dt></dl></
dd><dt><span class="sect1"><a href="bk01pt12ch40s03.html">Use</a></span></dt></d
l></div><div class="sect1" lang="en" xml:lang="en"><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="sect2" lang="en" xml:lang="en"><div
class="titlepage"><div><div><h3 class="title"><a id="manual.ext.concurrency.des
ign.threads"></a>Interface to Locks and Mutexes</h3></div></div></div><p>The fil
e <ext/concurrence.h> 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 <c
ode 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 se
t 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 type="disc"><li><p><code class="code">_S_singl
e</code></p><p>Indicates single-threaded code that does not need locking. | |
| 20 </p></li><li><p><code class="code">_S_mutex</code></p><p>Indicates multi-threade
d code using thread-layer abstractions. | |
| 21 </p></li><li><p><code class="code">_S_atomic</code></p><p>Indicates multi-thread
ed code using atomic operations. | |
| 22 </p></li></ul></div><p>The compile-time constant <code class="code">__default_lo
ck_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_lo
ck</code>. | |
| 27 </p><p> | |
| 28 </p><p>The scoped lock idiom is well-discussed within the C++ | |
| 29 community. This version takes a <code class="code">__mutex</code> reference, and | |
| 30 locks it during construction of <code class="code">__scoped_locke</code> and | |
| 31 unlocks it during destruction. This is an efficient way of locking | |
| 32 critical sections, while retaining exception-safety. | |
| 33 </p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div
><div><h3 class="title"><a id="manual.ext.concurrency.design.atomics"></a>Interf
ace to Atomic Functions</h3></div></div></div><p> | |
| 34 Two functions and one type form the base of atomic support. | |
| 35 </p><p>The type <code class="code">_Atomic_word</code> is a signed integral type | |
| 36 supporting atomic operations. | |
| 37 </p><p> | |
| 38 The two functions functions are: | |
| 39 </p><pre class="programlisting"> | |
| 40 _Atomic_word | |
| 41 __exchange_and_add_dispatch(volatile _Atomic_word*, int); | |
| 42 | |
| 43 void | |
| 44 __atomic_add_dispatch(volatile _Atomic_word*, int); | |
| 45 </pre><p>Both of these functions are declared in the header file | |
| 46 <ext/atomicity.h>, and are in <code class="code">namespace __gnu_cxx</code
>. | |
| 47 </p><div class="itemizedlist"><ul type="disc"><li><p> | |
| 48 <code class="code"> | |
| 49 __exchange_and_add_dispatch | |
| 50 </code> | |
| 51 </p><p>Adds the second argument's value to the first argument. Returns the old v
alue. | |
| 52 </p></li><li><p> | |
| 53 <code class="code"> | |
| 54 __atomic_add_dispatch | |
| 55 </code> | |
| 56 </p><p>Adds the second argument's value to the first argument. Has no return val
ue. | |
| 57 </p></li></ul></div><p> | |
| 58 These functions forward to one of several specialized helper | |
| 59 functions, depending on the circumstances. For instance, | |
| 60 </p><p> | |
| 61 <code class="code"> | |
| 62 __exchange_and_add_dispatch | |
| 63 </code> | |
| 64 </p><p> | |
| 65 Calls through to either of: | |
| 66 </p><div class="itemizedlist"><ul type="disc"><li><p><code class="code">__exchan
ge_and_add</code> | |
| 67 </p><p>Multi-thread version. Inlined if compiler-generated builtin atomics | |
| 68 can be used, otherwise resolved at link time to a non-builtin code | |
| 69 sequence. | |
| 70 </p></li><li><p><code class="code">__exchange_and_add_single</code> | |
| 71 </p><p>Single threaded version. Inlined.</p></li></ul></div><p>However, only <co
de class="code">__exchange_and_add_dispatch</code> | |
| 72 and <code class="code">__atomic_add_dispatch</code> should be used. These functi
ons | |
| 73 can be used in a portable manner, regardless of the specific | |
| 74 environment. They are carefully designed to provide optimum efficiency | |
| 75 and speed, abstracting out atomic accesses when they are not required | |
| 76 (even on hosts that support compiler intrinsics for atomic | |
| 77 operations.) | |
| 78 </p><p> | |
| 79 In addition, there are two macros | |
| 80 </p><p> | |
| 81 <code class="code"> | |
| 82 _GLIBCXX_READ_MEM_BARRIER | |
| 83 </code> | |
| 84 </p><p> | |
| 85 <code class="code"> | |
| 86 _GLIBCXX_WRITE_MEM_BARRIER | |
| 87 </code> | |
| 88 </p><p> | |
| 89 Which expand to the appropriate write and read barrier required by the | |
| 90 host hardware and operating system. | |
| 91 </p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="
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