source: azure_iot_hub/trunk/musl-1.1.18/src/network/inet_pton.c@ 389

#include <sys/socket.h>
#include <arpa/inet.h>
#include <ctype.h>
#include <errno.h>
#include <string.h>

static int hexval(unsigned c)
{
        if (c-'0'<10) return c-'0';
        c |= 32;
        if (c-'a'<6) return c-'a'+10;
        return -1;
}

int inet_pton(int af, const char *restrict s, void *restrict a0)
{
        uint16_t ip[8];
        unsigned char *a = a0;
        int i, j, v, d, brk=-1, need_v4=0;

        if (af==AF_INET) {
                for (i=0; i<4; i++) {
                        for (v=j=0; j<3 && isdigit(s[j]); j++)
                                v = 10*v + s[j]-'0';
                        if (j==0 || (j>1 && s[0]=='0') || v>255) return 0;
                        a[i] = v;
                        if (s[j]==0 && i==3) return 1;
                        if (s[j]!='.') return 0;
                        s += j+1;
                }
                return 0;
        } else if (af!=AF_INET6) {
                errno = EAFNOSUPPORT;
                return -1;
        }

        if (*s==':' && *++s!=':') return 0;

        for (i=0; ; i++) {
                if (s[0]==':' && brk<0) {
                        brk=i;
                        ip[i&7]=0;
                        if (!*++s) break;
                        if (i==7) return 0;
                        continue;
                }
                for (v=j=0; j<4 && (d=hexval(s[j]))>=0; j++)
                        v=16*v+d;
                if (j==0) return 0;
                ip[i&7] = v;
                if (!s[j] && (brk>=0 || i==7)) break;
                if (i==7) return 0;
                if (s[j]!=':') {
                        if (s[j]!='.' || (i<6 && brk<0)) return 0;
                        need_v4=1;
                        i++;
                        break;
                }
                s += j+1;
        }
        if (brk>=0) {
                memmove(ip+brk+7-i, ip+brk, 2*(i+1-brk));
                for (j=0; j<7-i; j++) ip[brk+j] = 0;
        }
        for (j=0; j<8; j++) {
                *a++ = ip[j]>>8;
                *a++ = ip[j];
        }
        if (need_v4 && inet_pton(AF_INET, (void *)s, a-4) <= 0) return 0;
        return 1;
}