Re: [Mingw-users] Re: Final Q on libjpeg dll build

[Tor L. <tm...@ik...>]

 > It makes the header file "jmorecfg.h" read
 > 
 > ---------------------------------------------------
 > #if defined(_WIN32) || defined(__declspec) || !defined(STATIC)
 > #ifdef BUILD
 > #define GLOBAL(type)		__declspec(dllexport) type
 > #define EXTERN(type)		extern __declspec(dllexport) type
 > #else
 > #define GLOBAL(type)		__declspec(dllimport) type
 > #define EXTERN(type)		extern __declspec(dllimport) type
 > #endif
 > #else
 > /* a function referenced thru EXTERNs: */
 > #define GLOBAL(type)		type
 > /* a reference to a GLOBAL function: */
 > #define EXTERN(type)		extern type
 > #endif
 > ---------------------------------------------------

I would like to raise an important point about this code: It is not
good to use a generic identifier like "BUILD" (which is quite likely
to clash with some end-user source code identifier) to switch between
dllimport and dllexport in a header file. What if libjpeg is used by
some *other* library, which also is built into a DLL. Then when
compiling *that* library we want the relevant declarations in *that*
library's header files to read as dllexport, but still the
declarations in libjpeg's header files to read as dllimport.

One should use library-specific identifiers, that are unlikely to
clash, like "NOW_COMPILING_LIBJPEG" or whatever.

--tml