What is the syntax of this method header?

[DarrenH]

Hello,

I am working on Win32 stuff and I came across a method header like this

ATOM WINAPI RegisterClassEx(
_In_ const WNDCLASSEX *lpwcx
);

ATOM is the return type.
RegisterClassEx is the name,
but what is the WINAPI part?

[xismn]

WINAPI in this case is a calling convention:
http://www.codeproject.com/Articles/1388/Calling-Conventions-Demystified

[DarrenH]

I didn't know such a thing existed in C++. Thank you.