source: azure_iot_hub/trunk/musl-1.1.18/src/stdlib/wcstol.c@ 388

#include "stdio_impl.h"
#include "intscan.h"
#include "shgetc.h"
#include <inttypes.h>
#include <limits.h>
#include <wctype.h>
#include <wchar.h>

/* This read function heavily cheats. It knows:
 *  (1) len will always be 1
 *  (2) non-ascii characters don't matter */

static size_t do_read(FILE *f, unsigned char *buf, size_t len)
{
        size_t i;
        const wchar_t *wcs = f->cookie;

        if (!wcs[0]) wcs=L"@";
        for (i=0; i<f->buf_size && wcs[i]; i++)
                f->buf[i] = wcs[i] < 128 ? wcs[i] : '@';
        f->rpos = f->buf;
        f->rend = f->buf + i;
        f->cookie = (void *)(wcs+i);

        if (i && len) {
                *buf = *f->rpos++;
                return 1;
        }
        return 0;
}

static unsigned long long wcstox(const wchar_t *s, wchar_t **p, int base, unsigned long long lim)
{
        wchar_t *t = (wchar_t *)s;
        unsigned char buf[64];
        FILE f = {0};
        f.flags = 0;
        f.rpos = f.rend = 0;
        f.buf = buf + 4;
        f.buf_size = sizeof buf - 4;
        f.lock = -1;
        f.read = do_read;
        while (iswspace(*t)) t++;
        f.cookie = (void *)t;
        shlim(&f, 0);
        unsigned long long y = __intscan(&f, base, 1, lim);
        if (p) {
                size_t cnt = shcnt(&f);
                *p = cnt ? t + cnt : (wchar_t *)s;
        }
        return y;
}

unsigned long long wcstoull(const wchar_t *restrict s, wchar_t **restrict p, int base)
{
        return wcstox(s, p, base, ULLONG_MAX);
}

long long wcstoll(const wchar_t *restrict s, wchar_t **restrict p, int base)
{
        return wcstox(s, p, base, LLONG_MIN);
}

unsigned long wcstoul(const wchar_t *restrict s, wchar_t **restrict p, int base)
{
        return wcstox(s, p, base, ULONG_MAX);
}

long wcstol(const wchar_t *restrict s, wchar_t **restrict p, int base)
{
        return wcstox(s, p, base, 0UL+LONG_MIN);
}

intmax_t wcstoimax(const wchar_t *restrict s, wchar_t **restrict p, int base)
{
        return wcstoll(s, p, base);
}

uintmax_t wcstoumax(const wchar_t *restrict s, wchar_t **restrict p, int base)
{
        return wcstoull(s, p, base);
}