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。
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.
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。
That is, a variable name should not be passed to a preprocessor directive and a symbol can only be evaluated by a preprocessor directive.
即，不应将变量名传递到预处理器指令，并且只能用预处理器指令计算符号。
That is, a variable name should not be passed to a preprocessor directive and a symbol can only be evaluated by a preprocessor directive.
即，不应将变量名传递到预处理器指令，并且只能用预处理器指令计算符号。
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