If s is not NULL, the wctomb 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, which is stored in a static anonymous variable only known to the wctomb function, and returns the length of said multibyte representation, i.e. the number of bytes written at s.
The programmer must ensure that there is room for at least MB_CUR_MAX bytes at s.
If s is not NULL, the wctomb function returns the number of bytes that have been written to the byte array at s. If wc can not be represented as a multibyte sequence (according to the current locale), -1 is returned.
The behaviour of wctomb depends on the LC_CTYPE category of the current locale.