better understanding of extern "C" functions

Question

I am just trying to further understand extern C functions.

According to my knowledge, an extern C function is always a function you are trying call from an application that has already been compiled. Either a executable, static or dynamic library.

extern "C" 
{
   HRESULT CreateDevice();
   typedef HRESULT (*CREATEDEVICE)();

   HRESULT ReleaseDevice();
   typedef HRESULT (*RELEASEDEVICE)();
}

So my question is...

Is my understanding correct ??

Does it always have to be a C function pointer ??'

Why must you use a typedef for each function ??

I presume that when you use the GetProcAddress(). You are allocating memory on that particulars applications HEAP and not the one you are calling it from. Therefore you must release it from that heap as well ??

Answer 1

extern "C" has 2 implications. First, it declares that the symbolic names of the functions are not "name mangled" to support C++. Second, it tells the compiler that the function is called using the C calling convention rather than the PASCAL calling convention. The difference has to do with when the return address is pushed on the stack. Using the wrong calling convention will crash your app.

This declaration is for the compiler, not the linker. So the extern C function could exist in your own modules or in a binary library: the source of actual bytes for the function implementation are resolved by the linker. If the function signature is declared as a regular C++ function and not extern C, the compiler will mangle the symbolic name to encode type information from the function signature. This will make it incompatible with object code generated by other C++ compilers. Therefore creating an extern C function allows you to share code between compilers in binary form. Note that you can't expose member functions this way, only old-style C functions.

Answer 2

It doesn't have to be a function pointer. You can specify the function declaration normally and prefix it with extern "C", as shown in some Microsoft examples.

If you use GetProcAddress() you are not allocating any memory. You simply get the memory address of the function inside the DLL that has already been loaded into memory (with LoadLibrary(), presumably).

Even when using function pointers (such as those returned by GetProcAddress) you don't have to use typedef, it's just that the code looks pretty ugly without it. It's always hard to figure out what to write as well. I think it would be something like:

void (*pReleaseDevice)() = (void (__cdecl *)(void))GetProcAddress(hInstance, "ReleaseDevice");