1 /* help detect directory cycles efficiently
3 Copyright (C) 2003, 2004, 2005, 2006, 2009 Free Software
6 This program is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 3 of the License, or
9 (at your option) any later version.
11 This program is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>. */
19 /* Written by Jim Meyering */
23 #include <sys/types.h>
31 #include "cycle-check.h"
33 #define CC_MAGIC 9827862
35 /* Return true if I is a power of 2, or is zero. */
38 is_zero_or_power_of_two (uintmax_t i)
40 return (i & (i - 1)) == 0;
44 cycle_check_init (struct cycle_check_state *state)
46 state->chdir_counter = 0;
47 state->magic = CC_MAGIC;
50 /* In traversing a directory hierarchy, call this function once for each
51 descending chdir call, with SB corresponding to the chdir operand.
52 If SB corresponds to a directory that has already been seen,
53 return true to indicate that there is a directory cycle.
54 Note that this is done `lazily', which means that some of
55 the directories in the cycle may be processed twice before
56 the cycle is detected. */
59 cycle_check (struct cycle_check_state *state, struct stat const *sb)
61 assert (state->magic == CC_MAGIC);
63 /* If the current directory ever happens to be the same
64 as the one we last recorded for the cycle detection,
65 then it's obviously part of a cycle. */
66 if (state->chdir_counter && SAME_INODE (*sb, state->dev_ino) == 1)
69 /* If the number of `descending' chdir calls is a power of two,
70 record the dev/ino of the current directory. */
71 if (is_zero_or_power_of_two (++(state->chdir_counter)))
73 /* On all architectures that we know about, if the counter
74 overflows then there is a directory cycle here somewhere,
75 even if we haven't detected it yet. Typically this happens
76 only after the counter is incremented 2**64 times, so it's a
77 fairly theoretical point. */
78 if (state->chdir_counter == 0)
81 state->dev_ino.st_dev = sb->st_dev;
82 state->dev_ino.st_ino = sb->st_ino;