Cpp return curiosity

Question

A lot o functions in OpenNI return like this:

XN_C_API XnInt32 XN_C_DECL  xnVersionCompare (const XnVersion *pVersion1, const XnVersion *pVersion2)

I'd like to now what is XN_C_API and XN_C_DECL. Just curiosity!

Answer 1

It's not anything to do with the return value. Without looking at the source, I suspect that when compiling for Windows:

• XN_C_API gets defined to either __declspec(dllexport) or __declspec(dllimport) depending on whether you're compiling the DLL or importing functions from it.

• XN_C_DECL gets defined to the desired calling convention for the function (probably __stdcall or __cdecl)

Answer 2

Without having looked at the definitions, I'd guess:

• XN_C_API is effectively extern "C" for a C++ compiler so that a C function can be called from C++.

• XN_C_DECL deals with the calling conventions and export/import issues imposed by Windows. That might be __declspec(ddlexport) or __declspec(dllimport), and it might include __stdcall etc, and might also worry about FAR etc (but probably doesn't need to any more).