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

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.

Answer 2

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.

Answer 3

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.

Answer 4

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++.