The & operator turns a vartype into a vartype *. Your function already has a vartype *, using & on that turns it into a vartype **varname -- which actually ends up pointing to the local pointer variable inside your function. C++ notices you're trying to put a pointer of the wrong type into it and refuses to let you, which is good, since that'd probably crash. In C, this is only a warning. When you do a function declaration, int calc_struct( STRUCT1 *myStruct1 ) that "STRUCT *myStruct1" declares a local variable named myStruct1, of the the type "pointer". That's just a numeric variable with enough bits to hold a memory address on your architecture. You already have a pointer and really couldn't have anything else but a pointer if you gave it one -- pointer operations are always done explicitly, the compiler will never decide to make a pointer a not-pointer for you (or vice-versa).

In other words, pointers never do anything weird behind your back, don't use their contents for you, don't change into different types for you. They're just numbers, they just sit there. All that's different is their *, &, and [] operators. I think so.

What extern "C" does is it forces the function to have that exact same in the compiled program. Without it, C++ will give it some crazy hashed name, which lets you do operator overloading -- different functions with the same text name won't have the literal same name in the program, and so won't interfere with each other, as long as they take different parameters. This is unrelated to extern "C". What it means is you put the actual function inside the header file, so it got compiled separately in every source file that used it. Or declared a variable in the header without declaring it as extern. Yes, exactly. Don't define the contents of functions or variables or objects or members in headers -- all headers should do is state that they exist.

The same goes for variables if you want to share them between different files.

Code:

// In the header
extern int some_var; // Variables need extern here.
void function_name(void); // Functions don't, if you don't give them a code block.

// In ONE AND ONLY ONE source file
#include "var_header.h"
int some_var=32; // This actually creates the variable.

void function_name(void)
{
        printf("OMGWTFBBQ\n");
}


// Variable and function are available in every other source file
// when you link them all together.
#include "var_header.h"
printf("Var is %d\n", some_var);
function_name();