1 /* help detect directory cycles efficiently
2
3 Copyright (C) 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
4
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)
8 any later version.
9
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.
14
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 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
19
20 /* Written by Jim Meyering */
21
22 #include <config.h>
23
24 #include <sys/types.h>
25 #include <sys/stat.h>
26 #include <stdio.h>
27 #include <assert.h>
28 #include <stdlib.h>
29
30 #include <stdbool.h>
31
32 #include "cycle-check.h"
33
34 #define CC_MAGIC 9827862
35
36 /* Return true if I is a power of 2, or is zero. */
37
38 static inline bool
is_zero_or_power_of_two(uintmax_t i)39 is_zero_or_power_of_two (uintmax_t i)
40 {
41 return (i & (i - 1)) == 0;
42 }
43
44 void
cycle_check_init(struct cycle_check_state * state)45 cycle_check_init (struct cycle_check_state *state)
46 {
47 state->chdir_counter = 0;
48 state->magic = CC_MAGIC;
49 }
50
51 /* In traversing a directory hierarchy, call this function once for each
52 descending chdir call, with SB corresponding to the chdir operand.
53 If SB corresponds to a directory that has already been seen,
54 return true to indicate that there is a directory cycle.
55 Note that this is done `lazily', which means that some of
56 the directories in the cycle may be processed twice before
57 the cycle is detected. */
58
59 bool
cycle_check(struct cycle_check_state * state,struct stat const * sb)60 cycle_check (struct cycle_check_state *state, struct stat const *sb)
61 {
62 assert (state->magic == CC_MAGIC);
63
64 /* If the current directory ever happens to be the same
65 as the one we last recorded for the cycle detection,
66 then it's obviously part of a cycle. */
67 if (state->chdir_counter && SAME_INODE (*sb, state->dev_ino))
68 return true;
69
70 /* If the number of `descending' chdir calls is a power of two,
71 record the dev/ino of the current directory. */
72 if (is_zero_or_power_of_two (++(state->chdir_counter)))
73 {
74 /* On all architectures that we know about, if the counter
75 overflows then there is a directory cycle here somewhere,
76 even if we haven't detected it yet. Typically this happens
77 only after the counter is incremented 2**64 times, so it's a
78 fairly theoretical point. */
79 if (state->chdir_counter == 0)
80 return true;
81
82 state->dev_ino.st_dev = sb->st_dev;
83 state->dev_ino.st_ino = sb->st_ino;
84 }
85
86 return false;
87 }
88