这是因为实际的源代码和变量绑定可以更改，这是建立在环境或者构建宏预处理程序指令的时间定义的基础上的。

This is because the actual source code and the variable binding can change, based on the environment or the build time definitions of macros and preprocessor commands.

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第4章介绍函数和程序结构—外部变量、作用域规则和多个源文件等，并涉及到预处理器。

Chapter 4 covers functions and program structure - external variables, scope rules, multiple source files, and so on - and also touches on the preprocessor.

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