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.
A third case is when s is NULL. In this case, pwc and n are
ignored. The mbtowc function resets the shift state, only known to this
function, to the initial state, and returns non-zero if the encoding has
non-trivial shift state, or zero if the encoding is stateless.
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.
If s is NULL, the mbtowc function returns non-zero if the encoding
has non-trivial shift state, or zero if the encoding is stateless.