Re: [Mingw-users] MingW linker warning about auto-importing

[Edward D. <eld...@tr...>]

On 9/27/2010 1:01 PM, Martin Mocko wrote:
> On Mon, Sep 27, 2010 at 4:10 PM, Edward Diener
> <eld...@tr...
> <mailto:eld...@tr...>> wrote:
>
>     My experience with __declspec(dllimport) with VC++ is that a header file
>     automatically dllimports whatever is necessary when using a dll. Thta is
>     why I am not used to doing it manually.
>
>
> I am not sure I understand correctly what you were meaning, but all that
> the __declspec(dllimport) does in VC++ is that it adds external symbol
> "__imp__<decorated name>" to resulting object file. Nothing more. You
> still have to specify .lib file which defines this symbol to linker
> command line (manually, or using #pragma comment(lib, "file"), or even
> #pragma comment(linker, "file.lib")  )

I understand this. But once the correct lib is being imported I don't 
have to manually declare __declspec(dllimport) on some exported symbol 
because a header file which I am using which representatives some 
entities for that lib will automatically do it for me. The header file 
code will look something like this:

#if defined(BUILD_DLL_XXX)
#define EXPIMP_XXX_SYMBOLS __declspec(dllexport)
#else
#if defined(USING_DLL_XXX) /* this can usually be determined 
automatically by whether the end-user is linking with the library as a 
static library or dll */
#define EXPIMP_XXX_SYMBOLS __declspec(dllimport)
#else
#define EXPIMP_XXX_SYMBOLS
#endif
#endif

and then:

EXPIMP_XXX_SYMBOLS class SomeClass etc.

This technique of having a library automatically export or import 
symbols is well known. That is why the warning message I encountered 
seemed strange to me, and not because I do not know exactly what 
exporting and importing symbols from a shared library means.