The main case for this function is when s is not NULL and wc is
not L'\0'. In this case, the wcrtomb function converts the wide
character wc to its multibyte representation and stores it at the
beginning of the character array pointed to by s. It updates the shift
state *ps, and returns the length of said multibyte representation,
i.e. the number of bytes written at s.
A different case is when s is not NULL but wc is L'\0'. In this
case the wcrtomb function stores at the character array pointed to by
s the shift sequence needed to bring *ps back to the initial
state, followed by a '\0' byte. It updates the shift state *ps (i.e.
brings it into the initial state), and returns the length of the shift
sequence plus one, i.e. the number of bytes written at s.
A third case is when s is NULL. In this case wc is ignored, and
the function effectively returns wcrtomb(buf,L'\0', ps) where buf is an
internal anonymous buffer.
In all of the above cases, if ps is a NULL pointer, a static anonymous
state only known to the wcrtomb function is used instead.
The wcrtomb function returns the number of bytes that have been or would
have been written to the byte array at s. If wc can not be
represented as a multibyte sequence (according to the current locale),
(size_t)(-1) is returned, and errno set to EILSEQ.