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

#include <stdlib.h>
#include <wchar.h>
#include <errno.h>
#include "internal.h"

size_t wcrtomb(char *restrict s, wchar_t wc, mbstate_t *restrict st)
{
        if (!s) return 1;
        if ((unsigned)wc < 0x80) {
                *s = wc;
                return 1;
        } else if (MB_CUR_MAX == 1) {
                if (!IS_CODEUNIT(wc)) {
                        errno = EILSEQ;
                        return -1;
                }
                *s = wc;
                return 1;
        } else if ((unsigned)wc < 0x800) {
                *s++ = 0xc0 | (wc>>6);
                *s = 0x80 | (wc&0x3f);
                return 2;
        } else if ((unsigned)wc < 0xd800 || (unsigned)wc-0xe000 < 0x2000) {
                *s++ = 0xe0 | (wc>>12);
                *s++ = 0x80 | ((wc>>6)&0x3f);
                *s = 0x80 | (wc&0x3f);
                return 3;
        } else if ((unsigned)wc-0x10000 < 0x100000) {
                *s++ = 0xf0 | (wc>>18);
                *s++ = 0x80 | ((wc>>12)&0x3f);
                *s++ = 0x80 | ((wc>>6)&0x3f);
                *s = 0x80 | (wc&0x3f);
                return 4;
        }
        errno = EILSEQ;
        return -1;
}