The main case for this function is when s is not NULL and pwc is not NULL. In this case, the mbtowc function inspects at most n bytes of the multibyte string starting at s, extracts the next complete multibyte character, converts it to a wide character and stores it at *pwc. It updates an internal shift state only known to the mbtowc function. If s does not point to a '0' byte, it returns the number of bytes that were consumed from s, otherwise it returns 0.
If the n bytes starting at s do not contain a complete multibyte character, or if they contain an invalid multibyte sequence, mbtowc returns -1. This can happen even if n MB_CUR_MAX, if the multibyte string contains redundant shift sequences.
A different case is when s is not NULL but pwc is NULL. In this case the mbtowc function behaves as above, excepts that it does not store the converted wide character in memory.
If s is not NULL, the mbtowc function returns the number of consumed bytes starting at s, or 0 if s points to a null byte, or -1 upon failure.
The behaviour of mbtowc depends on the LC_CTYPE category of the current locale.