Count characters without converting into an array
A character-counting helper calls `mbsrtowcs(0, &s, 0, &st)`, where `st` is a copy of the conversion state. Unlike `strlen`, this counts wide characters rather than bytes in the multibyte string. section 14.3
`mbsrtowcs` means “multibyte string, restartable, to wide character string.” Its `src` argument identifies the multibyte string; `dst` receives wide characters of type `wchar_t`. A wide-character string is a null-terminated array, much like an ordinary `char` string. section 14.3
TAKEAWAY: Use a null destination to obtain the length before allocating storage. A conversion helper then allocates room for that many wide characters plus the terminator and calls `mbsrtowcs` again with the allocated destination. The counting helper checks for a return value of `-1` before using the length. 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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Inspect the counting call and its copied conversion state. section 14.3
