C包括后卫[复制](C include guard [duplicate])

C包括后卫[复制](C include guard [duplicate])

这个问题在这里已经有了答案：

头文件只在整个程序中包含一次？ 4个答案

当file1.c inc.h包含inc.h （包含include guard #ifndef INC_H ）时，执行#define INC_H 。 但是现在，当另一个file2.c包含相同的inc.h ，是否已经定义了宏INC_H ，它们都是相同的故事，并且以前的定义不会在这里传播？

This question already has an answer here:

Header file included only once in entire program? 4 answers

When file1.c includes inc.h (containing the include guard #ifndef INC_H) for the first time, the #define INC_H is performed. But now, when another file2.c includes the same inc.h, is the macro INC_H already defined, all it's the same story and previous definition is not propagated here?

最满意答案

但是现在，当另一个file2.c包含相同的inc.h时，已经定义了宏INC_H，

是和不，这取决于。

如果file2.c包含一些包含inc.h头文件，那么是的 ， INC_H已经为file2.c定义了。 这对于任何包含水平都是如此。

否则， 不，它尚未定义。

守卫防止头被间接或直接包含在文件中，不止一次！

But now, when another file2.c includes the same inc.h, is the macro INC_H already defined,

Yes and No. It depends.

If file2.c includes some header which includes inc.h, then yes, INC_H is already defined for file2.c. This is true for any level of inclusion.

Else, no it is not already defined.

Guards prevent header from being included in a file, indirectly or directly, more than once!