Question

Say I have the following C++ code:

int x;
some_class y;

extern "C" {
  void foo () {
    // do something with x
    // do something with y
  }
}

Do x and/or y need to be declared with extern "C"?

Answer 1

No. There is no restriction on extern "C" functions accessing functions and variables with C++ language linkage.

C++ extern "C" functions are often used to provide a C interface into code with C++ language linkage so it would be more than a little restrictive if this were the case.