As with any preprocessor variable ( Section 2.9.2, p. 71) we should not use the name NULL for our own variables.
正如其他的预处理器变量一样（第2.9.2 节），不可以使用NULL这个标识符给自定义的变量命名。
The preprocessor variable defined to prevent a header from being included more than once in a single source file.
为防止头文件被同一源文件多次包含而定义的预处理器变量。
The preprocessor variable defined to prevent a header from being included more than once in a single source file.
为防止头文件被同一源文件多次包含而定义的预处理器变量。
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