WCRTOMB(3) Linux Programmer's Manual WCRTOMB(3)
wcrtomb - convert a wide character to a multibyte sequence
size_t wcrtomb(char *s, wchar_t wc, mbstate_t *ps);
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.
ISO/ANSI C, UNIX98
The behaviour of wcrtomb depends on the LC_CTYPE category of the current locale.
Passing NULL as ps is not multi-thread safe.
GNU 1999-07-25 WCRTOMB(3)