confusion on the preprocessor instrcution

[feverzsj]

as far as i know a header guard is used to avoid reduplicatively
including same file

// file xxx.h
#ifndef XXX_HEADER
#define XXX_HEADER

//.....
#endif

so, is the defined macro "XXX_HEADER" only valid in current Compiling
Units?
While the compiler moves to other units, these macros defined in other
units will be all abandoned? If not, files that need to include the
same header in other unit will have problem.

I've seached a lot ,but found few about such thing.

 

[Ian Collins]

as far as i know a header guard is used to avoid reduplicatively
including same file

// file xxx.h
#ifndef XXX_HEADER
#define XXX_HEADER

//.....
#endif

so, is the defined macro "XXX_HEADER" only valid in current Compiling
Units?
Yes.

While the compiler moves to other units, these macros defined in other
units will be all abandoned?

Yes.