source: azure_iot_hub/trunk/musl-1.1.18/src/multibyte/c16rtomb.c@ 389

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

size_t c16rtomb(char *restrict s, char16_t c16, mbstate_t *restrict ps)
{
        static unsigned internal_state;
        if (!ps) ps = (void *)&internal_state;
        unsigned *x = (unsigned *)ps;
        wchar_t wc;

        if (!s) {
                if (*x) goto ilseq;
                return 1;
        }

        if (!*x && c16 - 0xd800u < 0x400) {
                *x = c16 - 0xd7c0 << 10;
                return 0;
        }

        if (*x) {
                if (c16 - 0xdc00u >= 0x400) goto ilseq;
                else wc = *x + c16 - 0xdc00;
                *x = 0;
        } else {
                wc = c16;
        }
        return wcrtomb(s, wc, 0);

ilseq:
        *x = 0;
        errno = EILSEQ;
        return -1;
}