How do I use C Headers in a C++ Program?

Question

I am working on a project in Visual Studio 2010 which is to produce a win 32 dll file. The examples I have are C files and the compile and work well. I would like to incorporate some functionality from C++ function I've written but I have hit a bit of a wall.

If I attempt to link the C++ functions to the C program, it knows nothing about strings, etc and just doesn't work at all (of course).

So I am left with changing the example into a C++ program, then I can use my other files with impunity. When I attempt to do this I get a link error that I don't understand and am uncertain about how to resolve.

The examples use vendor provided headers, which include statements such as

 typedef void ( __cdecl *BINDING_PROC_BEVNT)(WORD_T choice, INT_T * pStatus, 
            I_EVNT_T  * pIn, O_EVNT_T  * pOut);

In the body of the main code, following the examples:

extern BINDING_PROC_BEVNT       b_evnt;

Which then allows you to write

b_evnt(choice, &status, &inpEvent, &outpEvent);

In a vendor provided C file, these are again referenced as:

BINDING_PROC_BEVNT      b_evnt; 
b_evnt = (BINDING_PROC_BEVNT)GetProcAddress(hCNCMod, "bevnt");

The linker error I am seeing is:

error LNK2001: unresolved external symbol "void (__cdecl* b_evnt)(unsigned short,short *,union I_EVNT_T *,union O_EVNT_T *)" (?b_evnt@@3P6AXGPAFPATI_EVNT_T@@PATO_EVNT_T@@@ZA)

If I rename my main file and recompile as a C program, and omit my C++ functions, everything compiles perfectly. Even when the main file is processed as a C++ file, Intellisense seems to recognize the definitions (hovering over shows the correct definitions).

Additionally I attempted to add extern "C" to a few different locations but it didn't seem to make a difference in the C++ files, and generated a compile error in the C files (about not knowing about strings).

Any insight would be appreciated, I may have simply stared at this too long today to be picking up on something obvious, or it may be something I'm completely unaware of.

Thanks for the help!

Answer 1

If you are compiling against a library that has C-language bindings, you have to tell C++ explicitly that the header files for the library reference C-objects, not C++ objects, or C++ name mangling will prevent correct linking. Often you can do this like so:

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

This will tell the C++ compiler that the symbols between the braces are C symbols, and should not have name mangling applied.