source: azure_iot_hub/trunk/musl-1.1.18/src/multibyte/mbsnrtowcs.c@ 388

#include <wchar.h>

size_t mbsnrtowcs(wchar_t *restrict wcs, const char **restrict src, size_t n, size_t wn, mbstate_t *restrict st)
{
        size_t l, cnt=0, n2;
        wchar_t *ws, wbuf[256];
        const char *s = *src;
        const char *tmp_s;

        if (!wcs) ws = wbuf, wn = sizeof wbuf / sizeof *wbuf;
        else ws = wcs;

        /* making sure output buffer size is at most n/4 will ensure
         * that mbsrtowcs never reads more than n input bytes. thus
         * we can use mbsrtowcs as long as it's practical.. */

        while ( s && wn && ( (n2=n/4)>=wn || n2>32 ) ) {
                if (n2>=wn) n2=wn;
                tmp_s = s;
                l = mbsrtowcs(ws, &s, n2, st);
                if (!(l+1)) {
                        cnt = l;
                        wn = 0;
                        break;
                }
                if (ws != wbuf) {
                        ws += l;
                        wn -= l;
                }
                n = s ? n - (s - tmp_s) : 0;
                cnt += l;
        }
        if (s) while (wn && n) {
                l = mbrtowc(ws, s, n, st);
                if (l+2<=2) {
                        if (!(l+1)) {
                                cnt = l;
                                break;
                        }
                        if (!l) {
                                s = 0;
                                break;
                        }
                        /* have to roll back partial character */
                        *(unsigned *)st = 0;
                        break;
                }
                s += l; n -= l;
                /* safe - this loop runs fewer than sizeof(wbuf)/8 times */
                ws++; wn--;
                cnt++;
        }
        if (wcs) *src = s;
        return cnt;
}