source: azure_iot_hub/trunk/musl-1.1.18/src/multibyte/mbrtoc16.c@ 389

#include <uchar.h>
#include <wchar.h>

size_t mbrtoc16(char16_t *restrict pc16, const char *restrict s, size_t n, mbstate_t *restrict ps)
{
        static unsigned internal_state;
        if (!ps) ps = (void *)&internal_state;
        unsigned *pending = (unsigned *)ps;

        if (!s) return mbrtoc16(0, "", 1, ps);

        /* mbrtowc states for partial UTF-8 characters have the high bit set;
         * we use nonzero states without high bit for pending surrogates. */
        if ((int)*pending > 0) {
                if (pc16) *pc16 = *pending;
                *pending = 0;
                return -3;
        }

        wchar_t wc;
        size_t ret = mbrtowc(&wc, s, n, ps);
        if (ret <= 4) {
                if (wc >= 0x10000) {
                        *pending = (wc & 0x3ff) + 0xdc00;
                        wc = 0xd7c0 + (wc >> 10);
                }
                if (pc16) *pc16 = wc;
        }
        return ret;
}