Possible Duplicate:
Why do we need extern “C”{ #include } in C++?

I have often seen programs coded like:

extern "C" bool doSomeWork() {
  //
  return true;
}

Why do we use an extern "C" block? Can we replace this with something in C++? Is there an advantage to using extern "C"?

I do see a link explaining this, but why do we need to compile something in C when we already have C++?