在define宏中使用typedef定义函数

Question

I have seen typedef and #define used in seperate scenarios. However, I have never come across code with one inside the other. Namely, the following

#define DECL_STDXL_FUNC(apiname, apitype, args)    \
  typedef XLstatus (_XL_EXPORT_API *apitype) args

This is later used to declare a function prototype (apparently) like this

DECL_STDXL_FUNC ( xlGetChannelTime,   XLGETCHANNELTIME, (
              XLportHandle        portHandle, 
              XLaccess            accessMask,
              XLuint64            *pChannelTime )
);

Basically, the header file is supposed to define functions that I only have the dll for. This suggests to me that I need to use _declspec(dllimport) and the correspond export of that. But the above just does not make any sense to me. Can anybody help make sense of this definition? If you need more clarification on the code, I can absolutely provide it. This is by far the most frustrating and convoluted way I've ever seen a function prototype written.

Answer 1

Run it through the preprocessor, at least mentally (this is what I'm doing):

#define DECL_STDXL_FUNC(apiname, apitype, args)    \
  typedef XLstatus (_XL_EXPORT_API *apitype) args

Interesting: the apiname argument to the macro is unused.

DECL_STDXL_FUNC ( xlGetChannelTime,   XLGETCHANNELTIME, (
              XLportHandle        portHandle, 
              XLaccess            accessMask,
              XLuint64            *pChannelTime )
);

When translated, this becomes:

typedef XLstatus (_XL_EXPORT_API *XLGETCHANNELTIME)(XLportHandle portHandle, 
              XLaccess accessMask, XLuint64 *pChannelTime);

That is, XLGETCHANNELTIME becomes a name for a specific pointer to function type. The _XL_EXPORT_API is presumably related to importing from or exporting to shared library (DLL) symbol lists. And the various XLlowercase names are types defined by the code.

You'll be able to define variables of this type:

XLGETCHANNELTIME get_channel_time = …;

which will presumably need to be initialized by some sort of symbol lookup function ( dlopen() in the most common Unix-like system).