Using .c and .cpp files in Visual Studio at the same time - c++

Question

Trying to figure out how to get an application to compile that uses both C and C++ files. Not the entire code, but enough to get the idea:
main.cpp:
#include <windows.h>
#include <stdio.h>
#include <string.h>
#include "one.h"
#include "two.h"
int __stdcall WinMain(HINSTANCE hInst, HINSTANCE hInst2, LPSTR lpCmdLine, int nShowCmd) {
FunctionOne();
FunctionTwo();
}
one.cpp:
#include <stdio.h>
#include <stdlib.h>
#include <windows.h>
#include <gdiplus.h>
#include <gdiplusflat.h>
using namespace Gdiplus;
using namespace Gdiplus::DllExports;
int FunctionOne() {
}
two.c
#include <stdio.h>
#include <stdlib.h>
#include <windows.h>
int FunctionTwo() {
}
The header files contain only definitions for those functions.
Now, if I compile this with a main.cpp, I get an "unresolved external symbol" for FunctionTwo. If I compile this with a main.c, I get the same thing for FunctionOne. Is this even possible, and if so, how would I set up the project to compile properly (Visual Studio 2010)?
It compiles fine if I comment out the alternate function depending on the extension for main.
Thanks!

Answer 1

The problem is two.h, it almost certainly wasn't written to allow a C++ compiler to properly compile the C function prototype. You'll want to take advantage of the predefined __cplusplus macro, like this:
two.h:
#ifdef __cplusplus
extern "C" {
#endif
int FunctionTwo();
// etc...
#ifdef __cplusplus
}
#endif
Lovely macro soup ;) If the header is pre-baked and never saw a C++ compiler before then do this in your .cpp source code file:
#include <windows.h>
#include <stdio.h>
#include <string.h>
#include "one.h"
extern "C" {
#include "two.h"
}
Some programmers name their header files .hpp if they contain C++ declarations and .h if they contain C declarations. That's a pretty good practice I personally favor. So does the Boost team. It didn't otherwise set the world on fire.

Answer 2

C++ does name-mangling to support function overloading while C does not. You will have to mark your function as extern "C" to prevent name mangling.
// main.cpp
extern "C" int FunctionTwo();
.. the rest ..
// two.c
extern "C" int FunctionTwo() {
// stuff
}
See http://www.parashift.com/c++-faq/mixing-c-and-cpp.html for more information on mixing C and C++.