Effects: If
s is a null pointer,
equivalent to
mbrtoc8(nullptr, "", 1, ps). Otherwise, the function 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 complete and valid,
it determines the values of the corresponding UTF-8 code units and then,
if
pc8 is not a null pointer,
stores the value of the first (or only) such code unit
in the object pointed to by
pc8. Subsequent calls will store successive UTF-8 code units
without consuming any additional input
until all the code units have been stored
. If the corresponding Unicode character is
U+0000 null,
the resulting state described is the initial conversion state
.