__stdcall - WINAPI vs STDMETHODCALLTYPE vs APIENTRY

Question

Im reading some of the game fix codes which deal with memory manipulation to fix a game's "issue". I found out that the code uses 2 macros that are WINAPI and STDMETHODCALLTYPE. These macros all get evaluated into __stdcall which specifies the calling convention for a function. I also found out that APIENTRY is also another macro alias for WINAPI. So are there any differences between these macros ? It seems to me that they are just aliases. Why are there so many of them ?

Answer 1

All data types and calling conventions in the Windows API are defined as aliases (preprocessor macros or typedefs). This allows for a stable ABI, irrespective of the compiler or toolchain. It also allows to change the ABI without breaking existing code (e.g. when 64-bit Windows was introduced).

Both WINAPI and STDMETHODCALLTYPE expand to the same thing: __stdcall for x86 and nothing for everything else. So why have 2 aliases for the same thing then? Because they control the calling convention for different subsets:

• WINAPI specifies the calling convention for the flat C-based Windows API.
• STDMETHODCALLTYPE, on the other hand, controls the calling convention for COM (Component Object Model).

COM and the Windows API are independent. Having 2 aliases to control the calling convention for either makes perfect sense. You wouldn't want to break all COM, just because you're moving to a new ABI for Win128.