This is the description in the Microsoft API Documentation:

Return value
If the function succeeds, the return value is a handle to the newly created file mapping object.

If the object exists before the function call, the function returns a handle to the existing object (with its current size, not the specified size), and GetLastError returns ERROR_ALREADY_EXISTS.

If the function fails, the return value is NULL. To get extended error information, call GetLastError.

https://docs.microsoft.com/zh-cn/windows/win32/api/memoryapi/nf-memoryapi-createfilemappingw

I think we should write it this way to handle errors, for example.

func CreateFileMapping(...) {
	...
	handle = Handle(r0)
	err = errnoErr(e1)
	return
}