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

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 开发者_JAVA技巧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!

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.

To include a C header file from C++, do something like this:

test.cpp

extern "C" {

#include "c_header_file.h"

}

It sounds like the above is what you might need to do to include the vendor header file in your C++ code.

Relatedly, to make a header file automatically work for both C and C++:

c_header_file.h

#ifdef __cplusplus
extern "C" {
#endif

void f(int);
// all declarations go here

#ifdef __cplusplus
}
#endif

Not all vendor-provided header files will contain the above __cplusplus detection, so you will have to wrap them manually in extern "C" as in the first example.

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)

That means, the C++ mangled variable b_evnt can't be found. That's true, because it should've been C mangled (just an _ prefix). To fix that, tell it to the compiler in the header when compiling for C++:

#ifdef __cplusplus
extern "C" BINDING_PROC_BEVNT       b_evnt;
#else
extern BINDING_PROC_BEVNT       b_evnt;
#endif

If that's all, you're done. If there are more symbols you need, you might want to use Greg's solution instead - but be aware that that is also not a fixall.

Your vendor provided headers are being included in a C++ compilation unit, but they aren't prepared for C++. So the declarations of the functions are being compiled with name mangling the C++ compielr requires to support overloading.

The headers need to be wrapped in an extern "C" {} block to let the C++ compiler know that these declarations use a C linkage.

Probably the easiest way to do this it to use your own wrapper headers that do something like:

#ifndef FOO_WRAPPER_H
#define FOO_WRAPPER_H

#if __cplusplus
extern "C" {
#endif

#include "foo.h"

#if __cplusplus
}
#endif

#endif

And include yiour wrapper header instead of the vendor's header - the wrapper will work for wither C or C++ compiles.

Also, contact your library vendor and let them know they should make these changes - users of the library shouldn't have to do this workaround to use the library from C++.