It still does not make it an OS API, it is the C runtime library which is quite different even if bundled with the OS.

I know some folks who think of the NT syscall interface as the 'true' OS API and see kernel32.dll as an add-on. I'll ask, but I don't think these folks make a distinction between user32.dll and the msvcrt.dll that ships with the OS.

> I know some folks who think of the NT syscall interface as the 'true' OS API and see kernel32.dll as an add-on.

This is 100% correct, many of the real syscalls are located in Ntoskrnl, while kernel32.dll is actually part of the Win32 subsystem, given the personalities feature from Windows.

So you could be using another subsystem.

However, given that the POSIX and OS/2 subsystems are no longer supported, the only available subsystem is Win32.

Metro is another matter as it uses another architecture not based on the subsystems mechanism.

But in the end these are all OS APIs, not language runtimes.

OK, asking around, the word I get is that the OS team owns the msvcrt.dll in %systemroot%\system32. Although unofficially it might be the case that x86 apps built with VC6 depend on this dll to implement a stable interface, it's not officially recommended to take dependencies on it.