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I'm learning about Win32 programming, and the WinMain prototype looks like:
int WINAPI WinMain ( HINSTANCE instance, HINSTANCE prev_instance, PSTR cmd_line, int cmd_show )
I was confused as to what this WINAPI identifier was for and found:
#define WINAPI __stdcall
What does this do? I'm confused by this having something at all after a return type. What is __stdcall for? What does it mean when there is something between the return type and function name?
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__stdcall is a calling convention: a way of determining how parameters are passed to a function (on the stack or in registers) and who is responsible for cleaning up after the function returns (the caller or the callee).
The stdcall calling convention is a variation on the Pascal calling convention in which the callee is responsible for cleaning up the stack, but the parameters are pushed onto the stack in right-to-left order, as in the _cdecl calling convention. Registers EAX, ECX, and EDX are designated for use within the function.
In CDECL arguments are pushed onto the stack in revers order, the caller clears the stack and result is returned via processor registry (later I will call it "register A"). In STDCALL there is one difference, the caller doeasn't clear the stack, the calle do. You are asking which one is faster.
__cdecl is the default calling convention for C and C++ programs. Because the stack is cleaned up by the caller, it can do vararg functions. The __cdecl calling convention creates larger executables than __stdcall, because it requires each function call to include stack cleanup code.
__stdcall is the calling convention used for the function. This tells the compiler the rules that apply for setting up the stack, pushing arguments and getting a return value.
There are a number of other calling conventions, __cdecl, __thiscall, __fastcall and the wonderfully named __declspec(naked). __stdcall is the standard calling convention for Win32 system calls.
Wikipedia covers the details.
It primarily matters when you are calling a function outside of your code (e.g. an OS API) or the OS is calling you (as is the case here with WinMain). If the compiler doesn't know the correct calling convention then you will likely get very strange crashes as the stack will not be managed correctly.
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