multiple definition of inline function

Question

I have gone through some posts related to this topic but was not able to sort out my doubt completely. This might be a very naive question.

I have a header file

Answer 1

Your code is invalid from the C++ point of view, since it blatantly violates the One Definition Rule. The only reason you managed to compile it by C++ compiler is the loose error checking in your C++ compiler (it happens to be one of those parts of ODR where "no diagnostic is required").

Your code is not valid C, because it provides duplicate external definition of function func1. Note that it is func1, not func3 that is problematic from the C point of view. There's nothing formally wrong with your func3. Your func2 is also OK, as long as the two definitions never "meet" each other in the same translation unit.

One possible reason you might be getting a different diagnostic report from your compiler is that your C compiler might be supporting inline functions in some non-standard compiler-specific way (either a pre-C99 compiler or a modern compiler run in non-standard "legacy" mode).

Frankly, I find it hard to believe you are getting an error report about func3 from any compiler, assuming the code you posted accurately represents what you are trying to compile. Most likely what you posted is not the real code.