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If you have functions in a DLL written in C, you can use a preprocessor macro to make them easy to access from both C language and C++ language code. The __cplusplus preprocessor macro indicates which language is being compiled. You may use it to declare the functions with C linkage when called from C++ language code. If you use this technique and provide header files for your DLL, these functions can be used by C and C++ users with no change.
The following code shows a header file that both C and C++ client applications can use:
// MyCFuncs.h
#ifdef __cplusplus
extern "C" { // only need to export C interface if
// used by C++ source code
#endif
__declspec( dllimport ) void MyCFunc();
__declspec( dllimport ) void AnotherCFunc();
#ifdef __cplusplus
}
#endif
Sometimes you may need to link C functions to your C++ executable, but the function declaration header files haven't used the above technique. You can still call the functions from C++. In the C++ source file, wrap the #include directive to prevent the compiler from decorating the C function names:
extern "C" {
#include "MyCHeader.h"
}