628 lines
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HTML
628 lines
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HTML
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<title>内存同步 - Go语言圣经</title>
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<nav id="sidebar" class="sidebar" aria-label="Table of contents">
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<div class="sidebar-scrollbox">
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<ol class="chapter">
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<li class="chapter-item expanded ">
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<a href="../index.html" >Go语言圣经</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../preface-zh.html" >译者序</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../preface.html" >前言</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1.html" ><strong aria-hidden="true">1.</strong> 入门</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-01.html" ><strong aria-hidden="true">1.1.</strong> Hello, World</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-02.html" ><strong aria-hidden="true">1.2.</strong> 命令行参数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-03.html" ><strong aria-hidden="true">1.3.</strong> 查找重复的行</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-04.html" ><strong aria-hidden="true">1.4.</strong> GIF动画</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-05.html" ><strong aria-hidden="true">1.5.</strong> 获取URL</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-06.html" ><strong aria-hidden="true">1.6.</strong> 并发获取多个URL</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-07.html" ><strong aria-hidden="true">1.7.</strong> Web服务</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-08.html" ><strong aria-hidden="true">1.8.</strong> 本章要点</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2.html" ><strong aria-hidden="true">2.</strong> 程序结构</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-01.html" ><strong aria-hidden="true">2.1.</strong> 命名</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-02.html" ><strong aria-hidden="true">2.2.</strong> 声明</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-03.html" ><strong aria-hidden="true">2.3.</strong> 变量</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-04.html" ><strong aria-hidden="true">2.4.</strong> 赋值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-05.html" ><strong aria-hidden="true">2.5.</strong> 类型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-06.html" ><strong aria-hidden="true">2.6.</strong> 包和文件</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-07.html" ><strong aria-hidden="true">2.7.</strong> 作用域</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3.html" ><strong aria-hidden="true">3.</strong> 基础数据类型</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-01.html" ><strong aria-hidden="true">3.1.</strong> 整型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-02.html" ><strong aria-hidden="true">3.2.</strong> 浮点数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-03.html" ><strong aria-hidden="true">3.3.</strong> 复数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-04.html" ><strong aria-hidden="true">3.4.</strong> 布尔型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-05.html" ><strong aria-hidden="true">3.5.</strong> 字符串</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-06.html" ><strong aria-hidden="true">3.6.</strong> 常量</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4.html" ><strong aria-hidden="true">4.</strong> 复合数据类型</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-01.html" ><strong aria-hidden="true">4.1.</strong> 数组</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-02.html" ><strong aria-hidden="true">4.2.</strong> Slice</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-03.html" ><strong aria-hidden="true">4.3.</strong> Map</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-04.html" ><strong aria-hidden="true">4.4.</strong> 结构体</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-05.html" ><strong aria-hidden="true">4.5.</strong> JSON</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-06.html" ><strong aria-hidden="true">4.6.</strong> 文本和HTML模板</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5.html" ><strong aria-hidden="true">5.</strong> 函数</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-01.html" ><strong aria-hidden="true">5.1.</strong> 函数声明</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-02.html" ><strong aria-hidden="true">5.2.</strong> 递归</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-03.html" ><strong aria-hidden="true">5.3.</strong> 多返回值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-04.html" ><strong aria-hidden="true">5.4.</strong> 错误</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-05.html" ><strong aria-hidden="true">5.5.</strong> 函数值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-06.html" ><strong aria-hidden="true">5.6.</strong> 匿名函数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-07.html" ><strong aria-hidden="true">5.7.</strong> 可变参数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-08.html" ><strong aria-hidden="true">5.8.</strong> Deferred函数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-09.html" ><strong aria-hidden="true">5.9.</strong> Panic异常</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-10.html" ><strong aria-hidden="true">5.10.</strong> Recover捕获异常</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6.html" ><strong aria-hidden="true">6.</strong> 方法</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-01.html" ><strong aria-hidden="true">6.1.</strong> 方法声明</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-02.html" ><strong aria-hidden="true">6.2.</strong> 基于指针对象的方法</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-03.html" ><strong aria-hidden="true">6.3.</strong> 通过嵌入结构体来扩展类型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-04.html" ><strong aria-hidden="true">6.4.</strong> 方法值和方法表达式</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-05.html" ><strong aria-hidden="true">6.5.</strong> 示例: Bit数组</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-06.html" ><strong aria-hidden="true">6.6.</strong> 封装</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7.html" ><strong aria-hidden="true">7.</strong> 接口</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-01.html" ><strong aria-hidden="true">7.1.</strong> 接口是合约</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-02.html" ><strong aria-hidden="true">7.2.</strong> 接口类型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-03.html" ><strong aria-hidden="true">7.3.</strong> 实现接口的条件</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-04.html" ><strong aria-hidden="true">7.4.</strong> flag.Value接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-05.html" ><strong aria-hidden="true">7.5.</strong> 接口值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-06.html" ><strong aria-hidden="true">7.6.</strong> sort.Interface接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-07.html" ><strong aria-hidden="true">7.7.</strong> http.Handler接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-08.html" ><strong aria-hidden="true">7.8.</strong> error接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-09.html" ><strong aria-hidden="true">7.9.</strong> 示例: 表达式求值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-10.html" ><strong aria-hidden="true">7.10.</strong> 类型断言</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-11.html" ><strong aria-hidden="true">7.11.</strong> 基于类型断言识别错误类型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-12.html" ><strong aria-hidden="true">7.12.</strong> 通过类型断言查询接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-13.html" ><strong aria-hidden="true">7.13.</strong> 类型分支</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-14.html" ><strong aria-hidden="true">7.14.</strong> 示例: 基于标记的XML解码</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-15.html" ><strong aria-hidden="true">7.15.</strong> 补充几点</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8.html" ><strong aria-hidden="true">8.</strong> Goroutines和Channels</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-01.html" ><strong aria-hidden="true">8.1.</strong> Goroutines</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-02.html" ><strong aria-hidden="true">8.2.</strong> 示例: 并发的Clock服务</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-03.html" ><strong aria-hidden="true">8.3.</strong> 示例: 并发的Echo服务</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-04.html" ><strong aria-hidden="true">8.4.</strong> Channels</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-05.html" ><strong aria-hidden="true">8.5.</strong> 并发的循环</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-06.html" ><strong aria-hidden="true">8.6.</strong> 示例: 并发的Web爬虫</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-07.html" ><strong aria-hidden="true">8.7.</strong> 基于select的多路复用</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-08.html" ><strong aria-hidden="true">8.8.</strong> 示例: 并发的目录遍历</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-09.html" ><strong aria-hidden="true">8.9.</strong> 并发的退出</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-10.html" ><strong aria-hidden="true">8.10.</strong> 示例: 聊天服务</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9.html" ><strong aria-hidden="true">9.</strong> 基于共享变量的并发</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-01.html" ><strong aria-hidden="true">9.1.</strong> 竞争条件</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-02.html" ><strong aria-hidden="true">9.2.</strong> sync.Mutex互斥锁</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-03.html" ><strong aria-hidden="true">9.3.</strong> sync.RWMutex读写锁</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-04.html" class="active"><strong aria-hidden="true">9.4.</strong> 内存同步</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-05.html" ><strong aria-hidden="true">9.5.</strong> sync.Once惰性初始化</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-06.html" ><strong aria-hidden="true">9.6.</strong> 竞争条件检测</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-07.html" ><strong aria-hidden="true">9.7.</strong> 示例: 并发的非阻塞缓存</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-08.html" ><strong aria-hidden="true">9.8.</strong> Goroutines和线程</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch10/ch10.html" ><strong aria-hidden="true">10.</strong> 包和工具</a>
|
||
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|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-01.html" ><strong aria-hidden="true">10.1.</strong> 包简介</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-02.html" ><strong aria-hidden="true">10.2.</strong> 导入路径</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-03.html" ><strong aria-hidden="true">10.3.</strong> 包声明</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-04.html" ><strong aria-hidden="true">10.4.</strong> 导入声明</a>
|
||
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|
||
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|
||
<a href="../ch10/ch10-05.html" ><strong aria-hidden="true">10.5.</strong> 包的匿名导入</a>
|
||
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|
||
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|
||
<a href="../ch10/ch10-06.html" ><strong aria-hidden="true">10.6.</strong> 包和命名</a>
|
||
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|
||
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|
||
<a href="../ch10/ch10-07.html" ><strong aria-hidden="true">10.7.</strong> 工具</a>
|
||
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|
||
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|
||
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|
||
<a href="../ch11/ch11.html" ><strong aria-hidden="true">11.</strong> 测试</a>
|
||
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|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11-01.html" ><strong aria-hidden="true">11.1.</strong> go test</a>
|
||
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|
||
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|
||
<a href="../ch11/ch11-02.html" ><strong aria-hidden="true">11.2.</strong> 测试函数</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
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<a href="../ch11/ch11-03.html" ><strong aria-hidden="true">11.3.</strong> 测试覆盖率</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11-04.html" ><strong aria-hidden="true">11.4.</strong> 基准测试</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
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<a href="../ch11/ch11-05.html" ><strong aria-hidden="true">11.5.</strong> 剖析</a>
|
||
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<a href="../ch11/ch11-06.html" ><strong aria-hidden="true">11.6.</strong> 示例函数</a>
|
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<a href="../ch12/ch12.html" ><strong aria-hidden="true">12.</strong> 反射</a>
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<a href="../ch12/ch12-01.html" ><strong aria-hidden="true">12.1.</strong> 为何需要反射?</a>
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||
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<a href="../ch12/ch12-02.html" ><strong aria-hidden="true">12.2.</strong> reflect.Type和reflect.Value</a>
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||
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<a href="../ch12/ch12-03.html" ><strong aria-hidden="true">12.3.</strong> Display递归打印</a>
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||
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|
||
<li class="chapter-item expanded ">
|
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<a href="../ch12/ch12-04.html" ><strong aria-hidden="true">12.4.</strong> 示例: 编码S表达式</a>
|
||
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|
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<li class="chapter-item expanded ">
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<a href="../ch12/ch12-05.html" ><strong aria-hidden="true">12.5.</strong> 通过reflect.Value修改值</a>
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<a href="../ch12/ch12-06.html" ><strong aria-hidden="true">12.6.</strong> 示例: 解码S表达式</a>
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<a href="../ch12/ch12-07.html" ><strong aria-hidden="true">12.7.</strong> 获取结构体字段标签</a>
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||
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|
||
<li class="chapter-item expanded ">
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<a href="../ch12/ch12-08.html" ><strong aria-hidden="true">12.8.</strong> 显示一个类型的方法集</a>
|
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<a href="../ch12/ch12-09.html" ><strong aria-hidden="true">12.9.</strong> 几点忠告</a>
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<a href="../ch13/ch13.html" ><strong aria-hidden="true">13.</strong> 底层编程</a>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch13/ch13-01.html" ><strong aria-hidden="true">13.1.</strong> unsafe.Sizeof, Alignof 和 Offsetof</a>
|
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<a href="../ch13/ch13-02.html" ><strong aria-hidden="true">13.2.</strong> unsafe.Pointer</a>
|
||
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|
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<li class="chapter-item expanded ">
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<a href="../ch13/ch13-03.html" ><strong aria-hidden="true">13.3.</strong> 示例: 深度相等判断</a>
|
||
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|
||
<li class="chapter-item expanded ">
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<a href="../ch13/ch13-04.html" ><strong aria-hidden="true">13.4.</strong> 通过cgo调用C代码</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13-05.html" ><strong aria-hidden="true">13.5.</strong> 几点忠告</a>
|
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|
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<a href="../appendix/appendix.html" ><strong aria-hidden="true">14.</strong> 附录</a>
|
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||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
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<a href="../appendix/appendix-a-errata.html" ><strong aria-hidden="true">14.1.</strong> 附录A：原文勘误</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
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<a href="../appendix/appendix-b-author.html" ><strong aria-hidden="true">14.2.</strong> 附录B：作者译者</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
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<a href="../appendix/appendix-c-cpoyright.html" ><strong aria-hidden="true">14.3.</strong> 附录C：译文授权</a>
|
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|
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|
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<a href="../appendix/appendix-d-translations.html" ><strong aria-hidden="true">14.4.</strong> 附录D：其它语言</a>
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<ul dir="auto"><li><em>凹语言(Go实现, 面向WASM设计): <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li><li><em>MnBook(Go语言实现的MD电子书构建工具): <a href="https://github.com/wa-lang/mnbook">https://github.com/wa-lang/mnbook</a></em></li></ul><hr>
|
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|
||
<h2>9.4. 内存同步</h2>
|
||
<p>你可能比较纠结为什么Balance方法需要用到互斥条件，无论是基于channel还是基于互斥量。毕竟和存款不一样，它只由一个简单的操作组成，所以不会碰到其它goroutine在其执行“期间”执行其它逻辑的风险。这里使用mutex有两方面考虑。第一Balance不会在其它操作比如Withdraw“中间”执行。第二（更重要的）是“同步”不仅仅是一堆goroutine执行顺序的问题，同样也会涉及到内存的问题。</p>
|
||
<p>在现代计算机中可能会有一堆处理器，每一个都会有其本地缓存（local cache）。为了效率，对内存的写入一般会在每一个处理器中缓冲，并在必要时一起flush到主存。这种情况下这些数据可能会以与当初goroutine写入顺序不同的顺序被提交到主存。像channel通信或者互斥量操作这样的原语会使处理器将其聚集的写入flush并commit，这样goroutine在某个时间点上的执行结果才能被其它处理器上运行的goroutine得到。</p>
|
||
<p>考虑一下下面代码片段的可能输出：</p>
|
||
<pre><code class="language-go">var x, y int
|
||
go func() {
|
||
x = 1 // A1
|
||
fmt.Print("y:", y, " ") // A2
|
||
}()
|
||
go func() {
|
||
y = 1 // B1
|
||
fmt.Print("x:", x, " ") // B2
|
||
}()
|
||
</code></pre>
|
||
<p>因为两个goroutine是并发执行，并且访问共享变量时也没有互斥，会有数据竞争，所以程序的运行结果没法预测的话也请不要惊讶。我们可能希望它能够打印出下面这四种结果中的一种，相当于几种不同的交错执行时的情况：</p>
|
||
<pre><code>y:0 x:1
|
||
x:0 y:1
|
||
x:1 y:1
|
||
y:1 x:1
|
||
</code></pre>
|
||
<p>第四行可以被解释为执行顺序A1,B1,A2,B2或者B1,A1,A2,B2的执行结果。然而实际运行时还是有些情况让我们有点惊讶：</p>
|
||
<pre><code>x:0 y:0
|
||
y:0 x:0
|
||
</code></pre>
|
||
<p>根据所使用的编译器，CPU，或者其它很多影响因子，这两种情况也是有可能发生的。那么这两种情况要怎么解释呢？</p>
|
||
<p>在一个独立的goroutine中，每一个语句的执行顺序是可以被保证的，也就是说goroutine内顺序是连贯的。但是在不使用channel且不使用mutex这样的显式同步操作时，我们就没法保证事件在不同的goroutine中看到的执行顺序是一致的了。尽管goroutine A中一定需要观察到x=1执行成功之后才会去读取y，但它没法确保自己观察得到goroutine B中对y的写入，所以A还可能会打印出y的一个旧版的值。</p>
|
||
<p>尽管去理解并发的一种尝试是去将其运行理解为不同goroutine语句的交错执行，但看看上面的例子，这已经不是现代的编译器和cpu的工作方式了。因为赋值和打印指向不同的变量，编译器可能会断定两条语句的顺序不会影响执行结果，并且会交换两个语句的执行顺序。如果两个goroutine在不同的CPU上执行，每一个核心有自己的缓存，这样一个goroutine的写入对于其它goroutine的Print，在主存同步之前就是不可见的了。</p>
|
||
<p>所有并发的问题都可以用一致的、简单的既定的模式来规避。所以可能的话，将变量限定在goroutine内部；如果是多个goroutine都需要访问的变量，使用互斥条件来访问。</p>
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