Combining C++ and C - how does #ifdef __cplusplus work?

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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" around the #include. Is this the right way to deal with that?

  5. Finally, is this set up a good idea? Is there anything else we should do? We're going to be mixing C and C++ for the foreseeable future, and I want to make sure we're covering all our bases.

extern-c c-preprocessor c c++
4 Answers

It's about the ABI, in order to let both C and C++ application use C interfaces without any issue.

Since C language is very easy, code generation was stable for many years for different compilers, such as GCC, Borland C\C++, MSVC etc.

While C++ becomes more and more popular, a lot things must be added into the new C++ domain (for example finally the Cfront was abandoned at AT&T because C could not cover all the features it needs). Such as template feature, and compilation-time code generation, from the past, the different compiler vendors actually did the actual implementation of C++ compiler and linker separately, the actual ABIs are not compatible at all to the C++ program at different platforms.

People might still like to implement the actual program in C++ but still keep the old C interface and ABI as usual, the header file has to declare extern "C" {}, it tells the compiler generate compatible/old/simple/easy C ABI for the interface functions if the compiler is C compiler not C++ compiler.

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