it is equivalent to

mbtowc((wchar_t *)0, (const char *)0, 0); // ADDED
mbtowc((wchar_t *)0, s, n);

ADD to description:

If multibyte characters have a state-dependent encoding, the function stores the initial conversion state in an internal static-duration object at program startup. It updates this object on each call to reflect the current conversion state. If s is a null pointer, the resulting conversion state is the initial conversion state.

ADD after "(including any shift sequences)" in Description:

using the conversion state stored at *ps,

ADD after "(including any shift sequences)" in Returns:

and stores the resulting conversion state at *ps