What's with "#ifdef _MAC" in Windows header files?

At one time Microsoft was the largest developer of Macintosh software in the world. Excel and Word dominated their respective markets on the Macintosh, and later so did Office. So it's not that surprising that the applications division at MS would want a subset of the Windows header files that worked on the MAC - to make their cross platform software easier to maintain.

But there was never any version of the Windows OS that ran on the Macintosh.

In any case, this fragment is from objidl.h, seems to indicate that _MAC does indeed mean Macintosh in the header files though...

//FSSpec is Macintosh only, defined in macos\files.h
#ifdef _MAC
    typedef struct tagSTATSTG
    {                      
        LPOLESTR pwcsName;
            FSSpec *pspec;
        DWORD type;
        ULARGE_INTEGER cbSize;
...
    } STATSTG;
#else //_MAC

As others have noted, Microsoft apps were ported to the Mac, and they probably found it easier to turn the underlying APIs/frameworks into a portable abstraction layer, rather than rewrite the apps themselves... just as the QuickTime team reportedly did when porting in the opposite direction (from Mac to Windows -- there are similar #if WIN32 conditionals in the Mac's Carbon headers). E.g., rather than riffle though your entire Win32 app's code base looking for CreateFile() and replacing or conditionalizing each reference with #ifdefs, just create a Mac version of CreateFile() and be done with it. Repeat for each Win32 API call.

The surprising bit of historical trivia is this: the end result of Microsoft's porting effort was available to third parties in the form of "Microsoft Visual C++ Cross-Development Edition for Macintosh". So anyone could take their Win32 app and port it to Mac using this abstraction layer.

Quote from my trusty MSDN October 1996 CD:

"Microsoft Visual C++ version 4.0 Cross-Development Edition for Macintosh facilitates the transfer of programs for the Microsoft Windows operating environment to the Apple 680x0 Macintosh or Power Macintosh environment. Designed to provide a complete program development environment, Visual C++ for Macintosh supports C, the standard C run-time library, C++, most of the Microsoft Win32 API including OLE and ODBC, and the Microsoft Foundation Class Library."

So it even included MFC. My guess is that any "#ifdef _MAC" is an artifact of the MSVC++ Cross-Development Edition for Macintosh (R.I.P.).

I assume it is/was used for compiling Microsoft software (Office, IE, Windows Media Player) for MacOS. I know that IE and WMP for Solaris included a subset of Windows itself (e.g. libwinnt.so, libkernel32.so) as a direct replacement for the corresponding Windows DLLs.