inet_aton.c

/*
**
**  Replacement for a missing inet_aton.
**
**  Written by Russ Allbery <rra@stanford.edu>
**  This work is hereby placed in the public domain by its author.
**
**  Provides the same functionality as the standard library routine
**  inet_aton for those platforms that don't have it.  inet_aton is
**  thread-safe.
*/

#include "config.h"
#ifdef HAVE_NETINET_IN_H
#include <netinet/in.h>
#endif

/* If we're running the test suite, rename inet_ntoa to avoid conflicts with
   the system version. */
#if TESTING
# define inet_aton test_inet_aton
int test_inet_aton(const char *, struct in_addr *);
#endif

int
inet_aton(const char *s, struct in_addr *addr)
{
    unsigned long octet[4], address;
    const char *p;
    int base, i;
    int part = 0;

    if (s == NULL) return 0;

    /* Step through each period-separated part of the address.  If we see
       more than four parts, the address is invalid. */
    for (p = s; *p != 0; part++) {
        if (part > 3) return 0;

        /* Determine the base of the section we're looking at.  Numbers are
           represented the same as in C; octal starts with 0, hex starts
           with 0x, and anything else is decimal. */
        if (*p == '0') {
            p++;
            if (*p == 'x') {
                p++;
                base = 16;
            } else {
                base = 8;
            }
        } else {
            base = 10;
        }

        /* Make sure there's actually a number.  (A section of just "0"
           would set base to 8 and leave us pointing at a period; allow
           that.) */
        if (*p == '.' && base != 8) return 0;
        octet[part] = 0;

        /* Now, parse this segment of the address.  For each digit, multiply
           the result so far by the base and then add the value of the
           digit.  Be careful of arithmetic overflow in cases where an
           unsigned long is 32 bits; we need to detect it *before* we
           multiply by the base since otherwise we could overflow and wrap
           and then not detect the error. */
        for (; *p != 0 && *p != '.'; p++) {
            if (octet[part] > 0xffffffffUL / base) return 0;

            /* Use a switch statement to parse each digit rather than
               assuming ASCII.  Probably pointless portability.... */
            switch (*p) {
                case '0':           i = 0;  break;
                case '1':           i = 1;  break;
                case '2':           i = 2;  break;
                case '3':           i = 3;  break;
                case '4':           i = 4;  break;
                case '5':           i = 5;  break;
                case '6':           i = 6;  break;
                case '7':           i = 7;  break;
                case '8':           i = 8;  break;
                case '9':           i = 9;  break;
                case 'A': case 'a': i = 10; break;
                case 'B': case 'b': i = 11; break;
                case 'C': case 'c': i = 12; break;
                case 'D': case 'd': i = 13; break;
                case 'E': case 'e': i = 14; break;
                case 'F': case 'f': i = 15; break;
                default:            return 0;
            }
            if (i >= base) return 0;
            octet[part] = (octet[part] * base) + i;
        }

        /* Advance over periods; the top of the loop will increment the
           count of parts we've seen.  We need a check here to detect an
           illegal trailing period. */
        if (*p == '.') {
            p++;
            if (*p == 0) return 0;
        }
    }
    if (part == 0) return 0;

    /* IPv4 allows three types of address specification:

           a.b
           a.b.c
           a.b.c.d

       If there are fewer than four segments, the final segment accounts for
       all of the remaining portion of the address.  For example, in the a.b
       form, b is the final 24 bits of the address.  We also allow a simple
       number, which is interpreted as the 32-bit number corresponding to
       the full IPv4 address.

       The first for loop below ensures that any initial segments represent
       only 8 bits of the address and builds the upper portion of the IPv4
       address.  Then, the remaining segment is checked to make sure it's no
       bigger than the remaining space in the address and then is added into
       the result. */
    address = 0;
    for (i = 0; i < part - 1; i++) {
        if (octet[i] > 0xff) return 0;
        address |= octet[i] << (8 * (3 - i));
    }
    if (octet[i] > (0xffffffffUL >> (i * 8))) return 0;
    address |= octet[i];
    if (addr != NULL) addr->s_addr = htonl(address);
    return 1;
}