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

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

int mbtowc(wchar_t *restrict wc, const char *restrict src, size_t n)
{
        unsigned c;
        const unsigned char *s = (const void *)src;
        wchar_t dummy;

        if (!s) return 0;
        if (!n) goto ilseq;
        if (!wc) wc = &dummy;

        if (*s < 0x80) return !!(*wc = *s);
        if (MB_CUR_MAX==1) return (*wc = CODEUNIT(*s)), 1;
        if (*s-SA > SB-SA) goto ilseq;
        c = bittab[*s++-SA];

        /* Avoid excessive checks against n: If shifting the state n-1
         * times does not clear the high bit, then the value of n is
         * insufficient to read a character */
        if (n<4 && ((c<<(6*n-6)) & (1U<<31))) goto ilseq;

        if (OOB(c,*s)) goto ilseq;
        c = c<<6 | *s++-0x80;
        if (!(c&(1U<<31))) {
                *wc = c;
                return 2;
        }

        if (*s-0x80u >= 0x40) goto ilseq;
        c = c<<6 | *s++-0x80;
        if (!(c&(1U<<31))) {
                *wc = c;
                return 3;
        }

        if (*s-0x80u >= 0x40) goto ilseq;
        *wc = c<<6 | *s++-0x80;
        return 4;

ilseq:
        errno = EILSEQ;
        return -1;
}