writing a dll in Visual Studio in C (not C++) causing unresolved symbols

Question

I've been going through the dll walkthrough on MSDN and it works fine. I then removed all the C++ style code in the dll and replaced it with the C equivalent, and it still works.

BUT, when I rename the file from X.cpp to X.c (which I guess causes compilation in C-mode), I get error LNK2019 (unresolved external symbol) for every function in the dll. For my purposes it's essential that the dll be in C not C++ because that's what Java Native Access supports.

Here's the header of the dll:

__declspec(dllexport) double Add(double a, double b);
__declspec(dllexport) double Subtract(double a, double b);
__declspec(dllexport) double Multiply(double a, double b);
__declspec(dllexport) double Divide(double a, double b);

Here's the body of the (C++) testing program that uses the dll:

#include <iostream>
#include "MyMathFuncs.h"
using namespace std;
int main()
{
    double a = 7.4;
    int b = 99;

    cout << "a + b = " <<
        Add(a, b) << endl;
    cout << "a - b = " <<
        Subtract(a, b) << endl;
    cout << "a * b = " <<
        Multiply(a, b) << endl;
    cout << "a / b = " <<
        Divide(a, b) << endl;

    return 0;
}

(Just to clarify it's fine that the testing program is in C++; it's only the dll I'm trying to compile in C).

Answer 1

Add

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

Answer 2

After you changed the extension, you are now using the wrong names in the client code. Those names are no longer decorated as they were when you compiled it as C++ code. The proper way to export names like this, so that those decorations are never used and you don't depend on the language:

extern "C" __declspec(dllexport)
double Add(double a, double b);

To see the exported names, use Dumpbin.exe /exports on your DLL.