Issue: "error LNK2019: unresolved external symbol"

Question

I have relatively simple setup.

I have a new Console C++ project.

But I was playing with global vars in C, and added two new .c files like this.

// Fruit.h
extern int global_variable;

Now the source:

// Fruit.c
#include "Fruit.h"
#include <stdio.h>

int global_variable = 37;

Also,

// Orange.h
void use_it(void);

and

// Orange.c
#include "Orange.h"
#include "Fruit.h"
#include <stdio.h>

void use_it(void)
{
    printf("Global variable: %d\n", global_variable++);
}

Finally, this is my main:

#include "stdafx.h"
#include "Orange.h"
#include "Fruit.h"

int _tmain(int argc, _TCHAR* argv[])
{
    use_it();

    return 0;
}

But this is the error I get: "error LNK2019: unresolved external symbol "void __cdecl use_it(void)" (?use_it@@YAXXZ) referenced in function _wmain"

Any help?

followed this advice on global vars: here

Answer 1

Your main file is a C++ file and the external file is C, if you want to reference a C function from C++ in the header should surround the declarations with

#ifdef __cplusplus
extern "C"{
#endif

// declarations here

#ifdef __cplusplus
}
#endif

or

preface them with extern "C" if those declarations will never be seen from a C file, as C has no idea what extern "C" means.

The issue is that the compiler is looking for a C++ name mangled function not a C function, which uses a different mangling (usually none).