1 /* help detect directory cycles efficiently
3 Copyright (C) 2003, 2004 Free Software Foundation, Inc.
5 This program is free software; you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation; either version 2, or (at your option)
10 This program is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with this program; see the file COPYING.
17 If not, write to the Free Software Foundation,
18 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */
20 /* Written by Jim Meyering */
26 #include <sys/types.h>
34 #include "cycle-check.h"
37 #define SAME_INODE(Stat_buf_1, Stat_buf_2) \
38 ((Stat_buf_1).st_ino == (Stat_buf_2).st_ino \
39 && (Stat_buf_1).st_dev == (Stat_buf_2).st_dev)
41 #define CC_MAGIC 9827862
43 /* Return true if I is a power of 2, or is zero. */
46 is_zero_or_power_of_two (uintmax_t i)
48 return (i & (i - 1)) == 0;
52 cycle_check_init (struct cycle_check_state *state)
54 state->chdir_counter = 0;
55 state->magic = CC_MAGIC;
58 /* In traversing a directory hierarchy, call this function once for each
59 descending chdir call, with SB corresponding to the chdir operand.
60 If SB corresponds to a directory that has already been seen,
61 return true to indicate that there is a directory cycle.
62 Note that this is done `lazily', which means that some of
63 the directories in the cycle may be processed twice before
64 the cycle is detected. */
67 cycle_check (struct cycle_check_state *state, struct stat const *sb)
69 assert (state->magic == CC_MAGIC);
71 /* If the current directory ever happens to be the same
72 as the one we last recorded for the cycle detection,
73 then it's obviously part of a cycle. */
74 if (state->chdir_counter && SAME_INODE (*sb, state->dev_ino))
77 /* If the number of `descending' chdir calls is a power of two,
78 record the dev/ino of the current directory. */
79 if (is_zero_or_power_of_two (++(state->chdir_counter)))
81 /* On all architectures that we know about, if the counter
82 overflows then there is a directory cycle here somewhere,
83 even if we haven't detected it yet. Typically this happens
84 only after the counter is incremented 2**64 times, so it's a
85 fairly theoretical point. */
86 if (state->chdir_counter == 0)
89 state->dev_ino.st_dev = sb->st_dev;
90 state->dev_ino.st_ino = sb->st_ino;