If s is a NULL pointer, mbrtowc is equivalent to the call:

        mbrtowc(NULL, "", 1, ps)

    In this case, the values of pwc and n are ignored.

    If s is not a NULL pointer, mbrtowc inspects at most n bytes
    beginning with the byte pointed to by s to determine the
    number of bytes needed to complete the next multibyte character
    (including any shift sequences).

    If the function determines that the next multibyte character
    is completed, it determines the value of the corresponding wide
    character and then, if pwc is not a NULL pointer, stores that
    value in the object pointed to by pwc. If the corresponding
    wide character is the null wide character, the resulting state
    described is the initial conversion state.

    If mbrtowc is called as a counting function, which means that pwc
    is a NULL pointer and s is neither a NULL pointer nor a pointer
    to a null byte, the value of the internal mbstate_t object will
    remain unchanged.