2 * minor utilities for subnet-mask manipulation
3 * Copyright (C) 1998, 1999 Henry Spencer.
5 * This library is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU Library General Public License as published by
7 * the Free Software Foundation; either version 2 of the License, or (at your
8 * option) any later version. See <http://www.fsf.org/copyleft/lgpl.txt>.
10 * This library is distributed in the hope that it will be useful, but
11 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
12 * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public
13 * License for more details.
15 * RCSID $Id: goodmask.c,v 1.7 2000/02/16 18:54:24 henry Exp $
21 - goodmask - is this a good (^1*0*$) subnet mask?
22 * You are not expected to understand this. See Henry S. Warren Jr,
23 * "Functions realizable with word-parallel logical and two's-complement
24 * addition instructions", CACM 20.6 (June 1977), p.439.
30 unsigned long x = ntohl(mask.s_addr);
31 /* clear rightmost contiguous string of 1-bits */
32 # define CRCS1B(x) (((x|(x-1))+1)&x)
33 # define TOPBIT (1UL << 31)
35 /* either zero, or has one string of 1-bits which is left-justified */
36 if (x == 0 || (CRCS1B(x) == 0 && (x&TOPBIT)))
42 - masktobits - how many bits in this mask?
43 * The algorithm is essentially a binary search, but highly optimized
44 * for this particular task.
46 int /* -1 means !goodmask() */
50 unsigned long m = ntohl(mask.s_addr);
59 if (m&(0x0000ffffUL<<1)) { /* <<1 for 1-origin numbering */
63 if (m&(0x00ff0000UL<<1)) {
67 if (m&(0x0f000000UL<<1)) {
71 if (m&(0x30000000UL<<1)) {
75 if (m&(0x40000000UL<<1))
82 - bitstomask - return a mask with this many high bits on
88 struct in_addr result;
90 if (n > 0 && n <= ABITS)
91 result.s_addr = htonl(~((1UL << (ABITS - n)) - 1));
95 result.s_addr = 0; /* best error report we can do */