C code compiled with C++: undefined reference

Question

I have a small program that I can compile with GCC and ICC without any difficulties, but I would also like the code to work with G++ and ICPC. I tried to add this:

#ifdef __cplusplus
  extern "C" {
#endif

at the beginning and this:

#ifdef __cplusplus
  }
#endif

at the end of all the header files, but I still get several `undefined reference to "..."' errors.

Answer 1

I think you're getting it wrong... The extern C is for disabling the function mangling; so if you do it just for the header files, when you try to link your mangled object code, the declared function names won't match with the function names in the object file.

Anyway, the extern C won't add any portability if the whole application is being compiled and linked with the same C++ compiler, it's intended for mixing C libraries with C++ code.

If your code is in the common subset of C and C++, you should be already able to compile it with either compiler, but I cannot see the reason to do that (besides working on the principle of least surprise, as C++ is more strict with some things).

Answer 2

You get undefined references because the declaration and the definition are not matching if you put extern "C", which prevents name mangling from happening: but in this case this is happening only in your header files.