mbtowc_left
int mbtowc_left(wchar_t *, const char *, size_t);
if ((len = mbtowc_left(&ch, linebuf + curr, curr)) <= 0)
if ((len = mbtowc_left(&prev_ch, linebuf + curr, curr)) <= 0)
if (mbtowc_left(&prev_ch, linebuf + curr, curr) <= 0)