I'm working on a project that has a lot of legacy C code. We've started writing in C++, with the intent to eventually convert the legacy code, as well. I'm a little confused about how the C and C++ interact. I understand that by wrapping the C code with extern "C" the C++ compiler will not mangle the C code's names, but I'm not entirely sure how to implement this.

So, at the top of each C header file (after the include guards), we have

#ifdef __cplusplus
extern "C" {
#endif

and at the bottom, we write

#ifdef __cplusplus
}
#endif

In between the two, we have all of our includes, typedefs, and function prototypes. I have a few questions, to see if I'm understanding this correctly:

  1. If I have a C++ file A.hh which includes a C header file B.h, includes another C header file C.h, how does this work? I think that when the compiler steps into B.h, __cplusplus will be defined, so it will wrap the code with extern "C" (and __cplusplus will not be defined inside this block). So, when it steps into C.h, __cplusplus will not be defined and the code will not be wrapped in extern "C". Is this correct?

  2. Is there anything wrong with wrapping a piece of code with extern "C" { extern "C" { .. } }? What will the second extern "C" do?

  3. We don't put this wrapper around the .c files, just the .h files. So, what happens if a function doesn't have a prototype? Does the compiler think that it's a C++ function?

  4. We are also using some third-party code which is written in C, and does not have this sort of wrapper around it. Any time I include a header from that library, I've been putting an extern "C"

    c++ c c-preprocessor extern-c
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