Counting multibyte characters
A call such as mbsrlen(start, 0) counts the characters represented by a multibyte string, rather than treating its bytes as individual characters. The zero state argument indicates that scanning needs no prior context. section 14.3
TAKEAWAY: Parsing a multibyte string may require a conversion state. The C standard type mbstate_t holds that state; it is provided by wchar.h and also by uchar.h. When a scan starts partway through a string, its state must be consistent with the multibyte character at that position. section 14.3
30 size_t mbsrlen(char const*s, mbstate_t const*restrict state) {
31 if (!state) state = MBSTATE;
32 mbstate_t st = *state;
33 size_t mblen = mbsrtowcs(0, &s, 0, &st);
34 if (mblen == -1) errno = 0;
35 return mblen;
36 }
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In this mbsrlen implementation, a local mbstate_t copy is passed to mbsrtowcs, so counting does not modify the caller’s state. section 14.3
