📄 item_012.htm
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<div class=Section1>
<h3><span lang=EN-US>12. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>了解何时及如何为并发性编写代码。（</span><span lang=EN-US>Know
when and how to code for concurrency.</span><span style='font-family:宋体;
mso-ascii-font-family:Arial;mso-hansi-font-family:Arial'>）</span></h3>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><b style='mso-bidi-font-weight:normal'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>摘要</span><span lang=EN-US><o:p></o:p></span></b></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><sup><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>线</span></sup><span class=docemphasis><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>安</span></span><sup><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>程</span></sup><span
class=docemphasis><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>全：如果你的应用程序使用多线程或多进程，那么应该了解如何尽可能地最小化共享的对象（参见第</span><span
lang=EN-US>10</span></span><span class=docemphasis><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>条），并安全地共享相应的对象。</span></span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><b style='mso-bidi-font-weight:normal'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>讨论</span></b></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>线程是一个非常大的领域。本条存在的原因就是因为该领域非常重要，而且需要被清楚地认知，但是一个条目是不够的，因此我们仅概括了一些基本的问题；有关更多的细节及技术请参见参考资料。最重要的问题包括死锁（</span><span
lang=EN-US>deadlock</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>），活锁（</span><span
class=SpellE><span lang=EN-US>livelock</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>），以及竞争条件（</span><span
lang=EN-US>race condition</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>）（包括因锁定不足而造成数据被破坏）。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US>C++</span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>标准对线程只字未提。尽管如此，</span><span
lang=EN-US>C++</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>仍然被广泛地用来编写坚固的多线程代码。如果你的应用程序在线程之间共享数据，那么应该保证操作的安全性：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<ul style='margin-top:0cm' type=disc>
<li class=MsoNormal style='mso-list:l1 level1 lfo1;tab-stops:list 36.0pt'><i
style='mso-bidi-font-style:normal'><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>参阅目标平台的文档，找到基本的本地同步机制（</span><span
lang=EN-US>local synchronization primitive</span></i><i style='mso-bidi-font-style:
normal'><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>）：</span></i><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>一般来说这些机制涵盖了从轻量级的原子整数操作（</span><span lang=EN-US>atomic
integer operation</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>），到内存屏障（</span><span
lang=EN-US>memory barrier</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>），到进程内及进程间的互斥器（</span><span
class=SpellE><span lang=EN-US>mutex</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>）。</span><i
style='mso-bidi-font-style:normal'><span lang=EN-US><o:p></o:p></span></i></li>
</ul>
<p class=MsoNormal style='margin-left:18.0pt'><span lang=EN-US><o:p> </o:p></span></p>
<ul style='margin-top:0cm' type=disc>
<li class=MsoNormal style='mso-list:l1 level1 lfo1;tab-stops:list 36.0pt'><i
style='mso-bidi-font-style:normal'><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>最好是用自己的抽象把与平台相关的基本同步机制封装起来：</span></i><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>这是一个好主意，尤其是在你需要跨平台的可移植性时更是如此。作为另一种选择，你还可以使用一个帮你做这些事情的程序库（例如：</span><span
class=SpellE><span lang=EN-US>pthreads</span></span><span lang=EN-US>
[Butenhof97]</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>）。</span><i style='mso-bidi-font-style:
normal'><span lang=EN-US><o:p></o:p></span></i></li>
</ul>
<p class=MsoNormal style='margin-left:18.0pt'><span lang=EN-US><o:p> </o:p></span></p>
<ul style='margin-top:0cm' type=disc>
<li class=MsoNormal style='mso-list:l1 level1 lfo1;tab-stops:list 36.0pt'><i
style='mso-bidi-font-style:normal'><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>确保你正在使用的类型可以在多线程程序中安全地使用：</span></i><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>具体地说，每个类型必须至少：</span><i style='mso-bidi-font-style:
normal'><span lang=EN-US><o:p></o:p></span></i></li>
</ul>
<p class=MsoNormal style='margin-left:18.0pt'><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:75.0pt;text-indent:-21.0pt;mso-list:l0 level2 lfo2;
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Wingdings;mso-fareast-font-family:Wingdings;mso-bidi-font-family:Wingdings'><span
style='mso-list:Ignore'>²<span style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span style='font-family:宋体;mso-ascii-font-family:
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