How does an extern “C” declaration work?

Question

I\'m taking a programming languages course and we\'re talking about the extern \"C\" declaration.

How does this declaration work at a deeper level othe

Answer 1

Let's look at a typical function that can compile in both C and C++:

int Add (int a, int b)
{
    return a+b;
}

Now in C the function is called "_Add" internally. Whereas the C++ function is called something completely different internally using a system called name-mangling. Its basically a way to name a function so that the same function with different parameters has a different internal name.

So if Add() is defined in add.c, and you have the prototype in add.h you will get a problem if you try to include add.h in a C++ file. Because the C++ code is looking for a function with a name different to the one in add.c you will get a linker error. To get around that problem you must include add.c by this method:

extern "C"
{
#include "add.h"
}

Now the C++ code will link with _Add instead of the C++ name mangled version.

That's one of the uses of the expression. Bottom line, if you need to compile code that is strictly C in a C++ program (via an include statement or some other means) you need to wrap it with a extern "C" { ... } declaration.