How to properly create a DLL from C code and use it in a C++ project - c++

Question

I have a set of functions written in C that I need to be able to call from another project written in C++. The C code is essentially some functions that do some calculations on a large data set. I didn't write them - all I want to do is allow my C++ project to be able to call those functions. My solution was to create a DLL for the C code and link it to my C++ project.
In order to make the DLL, I structured myCproj.h (the header in the C project, not C++ project) like so:
#ifdef __cplusplus
extern "C" {
#endif
struct __declspec(dllexport) neededStruct {
int a;
//I need to be able to initialize this struct in my C++ project.
}
__declspec(dllexport) void neededFunc( struct neededStruct *input ) {}
//I need to be able to call this function from my C++ project and feed
//it my local instance of neededStruct.
#ifdef __cplusplus
}
#endif
The src file, myCproj.c, was not changed at all. The function definitions do not have __declspec(dllexport)in front of them, nor is extern "C" inserted anywhere. The code compiles without error and produces myCproj.dll and myCproj.lib.
I then tell my C++ project in VS where to find myCproj.lib and myCproj.h accordingly and copy the DLL over to the directory where my C++ executable lives. To use the DLL, I gave myCPPproj.cpp the following addition:
#define DLLImport __declspec(dllimport)
struct DLLImport neededStruct input;
input.a = 0;
extern "C" DLLImport void neededFunc( &input );
However, I get error EO335 'linkage specification is not allowed' on that last line. What am I doing wrong?

Answer 1

It is preferable to use the same header for both the library and using code.
As mentioned, it is usually done by a conditional define, like the following:
MyLibrary.h:
#if defined(MYLIBRARY_API)
#define MYLIBRARY_EXPORTS __declspec(dllexport)
#else
#define MYLIBRARY_EXPORTS __declspec(dllimport)
#endif
#if defined(__cplusplus)
extern "C" {
#endif
MYLIBRARY_API bool MyLibFunc();
#if defined(__cplusplus)
#endif
MyLibrary.c:
#include "MyLibrary.h"
void MyLibFunc()
{
....
}
App.cpp:
#include <MyLibrary.h>
int main()
{
MyLibFunc();
}
The symbol MYLIBRARY_API will be defined for the library project (usually as a /D on the compiler command line). And if you use visual studio that is pretty much exactly what you get when creating a dll project with exports.