This is xnu-8019. See this file in:
/*
 * Copyright (c) 2004 by Internet Systems Consortium, Inc. ("ISC")
 * Copyright (c) 1996-1999 by Internet Software Consortium.
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
 * OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#if defined(LIBC_SCCS) && !defined(lint)
static const char rcsid[] = "$Id: inet_ntop.c,v 1.3.18.2 2005/11/03 23:02:22 marka Exp $";
#endif /* LIBC_SCCS and not lint */

#include <sys/param.h>
#include <sys/socket.h>
#include <sys/systm.h>

#include <netinet/in.h>

/*%
 * WARNING: Don't even consider trying to compile this on a system where
 * sizeof(int) < 4.  sizeof(int) > 4 is fine; all the world's not a VAX.
 */

static char     *inet_ntop4(const u_char *src, char *dst, socklen_t size);
static char     *inet_ntop6(const u_char *src, char *dst, socklen_t size);

/* char *
 * inet_ntop(af, src, dst, size)
 *	convert a network format address to presentation format.
 * return:
 *	pointer to presentation format address (`dst'), or NULL (see errno).
 * author:
 *	Paul Vixie, 1996.
 */
const char *
inet_ntop(int af, const void *src, char *dst, socklen_t size)
{
	switch (af) {
	case AF_INET:
		return inet_ntop4(src, dst, size);
	case AF_INET6:
		return inet_ntop6(src, dst, size);
	default:
		return NULL;
	}
	/* NOTREACHED */
}

/* const char *
 * inet_ntop4(src, dst, size)
 *	format an IPv4 address
 * return:
 *	`dst' (as a const)
 * notes:
 *	(1) uses no statics
 *	(2) takes a u_char* not an in_addr as input
 * author:
 *	Paul Vixie, 1996.
 */
static char *
inet_ntop4(const u_char *src, char *dst, socklen_t size)
{
	static const char fmt[] = "%u.%u.%u.%u";
	char tmp[sizeof "255.255.255.255"];
	int l;

	l = snprintf(tmp, sizeof(tmp), fmt, src[0], src[1], src[2], src[3]);
	if (l <= 0 || (socklen_t) l >= size) {
		return NULL;
	}
	strlcpy(dst, tmp, size);
	return dst;
}

/* const char *
 * inet_ntop6(src, dst, size)
 *	convert IPv6 binary address into presentation (printable) format
 * author:
 *	Paul Vixie, 1996.
 */
static char *
inet_ntop6(const u_char *src, char *dst, socklen_t size)
{
	/*
	 * Note that int32_t and int16_t need only be "at least" large enough
	 * to contain a value of the specified size.  On some systems, like
	 * Crays, there is no such thing as an integer variable with 16 bits.
	 * Keep this in mind if you think this function should have been coded
	 * to use pointer overlays.  All the world's not a VAX.
	 */
	char tmp[sizeof "ffff:ffff:ffff:ffff:ffff:ffff:255.255.255.255"], *tp;
	struct { int base, len; } best, cur;
#define NS_IN6ADDRSZ    16
#define NS_INT16SZ      2
	u_int words[NS_IN6ADDRSZ / NS_INT16SZ];
	int i;

	/*
	 * Preprocess:
	 *	Copy the input (bytewise) array into a wordwise array.
	 *	Find the longest run of 0x00's in src[] for :: shorthanding.
	 */
	memset(words, '\0', sizeof words);
	for (i = 0; i < NS_IN6ADDRSZ; i++) {
		words[i / 2] |= (src[i] << ((1 - (i % 2)) << 3));
	}
	best.base = -1;
	best.len = 0;
	cur.base = -1;
	cur.len = 0;
	for (i = 0; i < (NS_IN6ADDRSZ / NS_INT16SZ); i++) {
		if (words[i] == 0) {
			if (cur.base == -1) {
				cur.base = i;
				cur.len = 1;
			} else {
				cur.len++;
			}
		} else {
			if (cur.base != -1) {
				if (best.base == -1 || cur.len > best.len) {
					best = cur;
				}
				cur.base = -1;
			}
		}
	}
	if (cur.base != -1) {
		if (best.base == -1 || cur.len > best.len) {
			best = cur;
		}
	}
	if (best.base != -1 && best.len < 2) {
		best.base = -1;
	}

	/*
	 * Format the result.
	 */
	tp = tmp;
	for (i = 0; i < (NS_IN6ADDRSZ / NS_INT16SZ); i++) {
		/* Are we inside the best run of 0x00's? */
		if (best.base != -1 && i >= best.base &&
		    i < (best.base + best.len)) {
			if (i == best.base) {
				*tp++ = ':';
			}
			continue;
		}
		/* Are we following an initial run of 0x00s or any real hex? */
		if (i != 0) {
			*tp++ = ':';
		}
		/* Is this address an encapsulated IPv4? */
		if (i == 6 && best.base == 0 && (best.len == 6 ||
		    (best.len == 7 && words[7] != 0x0001) ||
		    (best.len == 5 && words[5] == 0xffff))) {
			if (!inet_ntop4(src + 12, tp,
			    (socklen_t)(sizeof tmp - (tp - tmp)))) {
				return NULL;
			}
			tp += strlen(tp);
			break;
		}
		tp += scnprintf(tp, sizeof(tmp), "%x", words[i]);
	}
	/* Was it a trailing run of 0x00's? */
	if (best.base != -1 && (best.base + best.len) ==
	    (NS_IN6ADDRSZ / NS_INT16SZ)) {
		*tp++ = ':';
	}
	*tp++ = '\0';

	/*
	 * Check for overflow, copy, and we're done.
	 */
	if ((socklen_t)(tp - tmp) > size) {
		return NULL;
	}
	strlcpy(dst, tmp, size);
	return dst;
}

/*! \file */