A warning with building 64bit dll

Question

dll export header

extern "C"
void _declspec(dllexport) __stdcall foo();

.def file

EXPORTS
foo         @1

When I build the dll by 64bit build config, I meet this warning.

warning LNK4197: export 'foo' specified multiple times; using first specification

But If I build the dll by 32bit build config, the warning never occurs.
What is the problem? What is the difference.

In dll header for interface, we usually use this technic,

#ifdef EXPORT_DLL
#define BASICAPI _declspec(dllexport)
#else
#define BASICAPI _declspec(dllimport)
#endif //_EXPORT_DLL

But if def file also exists, we always will be meet the warning when we are building 64bit dll.
So, should we write the codes like this?

#ifdef EXPORT_DLL
#define BASICAPI
#else
#define BASICAPI _declspec(dllimport)
#endif //_EXPORT_DLL

It works well. But it's not familiar to me.
Give me any your opinions.

Answer 1

It's generally not good practise to specify exports twice for the same function. If you already have __declspec(dllexport) then you do not need to specify the export in a .def file as well. Conversely, if you have the export listed in a .def file, then there's no need for a __declspec(dllexport).

I believe the reason for the warning is that in x86 builds, the __declspec(dllexport) is exporting the decorated name with a leading underscore, but the 64-bit compiler does not decorate names with a leading underscore, leading to the duplicate. To verify this, you could look at the 32-bit DLL in Dependency Walker and you should see two exported functions, "foo" and "_foo".

Answer 2

__declspec(dllexport) and .def files are two different ways to export symbols from a dll. You don't need both and should omit ono of them. The __declspec method is far more versatile for c++ programs as it exports names with c++ mangling, allowing overloaded functions to be exported, but conversely that does make the names harder to import via GetProcAddress.

Also, using a generic macro like EXPORT_DLL is dangerous as it means that you can't build a dll, that uses another dll, without the one one dll trying to export all the symbols of both dlls.

DevStudio automatically creates a symbol on dll projects: <PROJECT>_EXPORTS making it easy and safe to create a EXPORT macro:

#ifdef EXPORT
#undef EXPORT
#endif
#ifdef PROJECTNAMEHERE_EXPORTS
#define EXPORT __declspec(dllexport)
#else
#define EXPORT __declspec(dllimport)
#endif

EXTERN_C EXPORT void __stdcall Function1(void);
EXTERN_C EXPORT void __cdecl Function2(...);
         EXPORT void Function3(void);

Functions 1 & 2 can be gotten with GetProcAddress as _Function1@0 and Function2 respectively. Function3 is going to be exported via a compiler specific mangled name that will look something like: @Function3@@UAG_DB@Z. This name is different for each overload of the function, which is how it allows overloading to work.

Its important to know the name mangling of __declspec as .def files don't care and would just export Function1, Function2 and Function3.