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Diffstat (limited to 'lib/cycle-check.c')
-rw-r--r-- | lib/cycle-check.c | 87 |
1 files changed, 87 insertions, 0 deletions
diff --git a/lib/cycle-check.c b/lib/cycle-check.c new file mode 100644 index 0000000..5359d71 --- /dev/null +++ b/lib/cycle-check.c @@ -0,0 +1,87 @@ +/* help detect directory cycles efficiently + + Copyright (C) 2003, 2004, 2005, 2006 Free Software Foundation, Inc. + + This program is free software; you can redistribute it and/or modify + it under the terms of the GNU General Public License as published by + the Free Software Foundation; either version 2, or (at your option) + any later version. + + This program is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + GNU General Public License for more details. + + You should have received a copy of the GNU General Public License + along with this program; see the file COPYING. + If not, write to the Free Software Foundation, + 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ + +/* Written by Jim Meyering */ + +#include <config.h> + +#include <sys/types.h> +#include <sys/stat.h> +#include <stdio.h> +#include <assert.h> +#include <stdlib.h> + +#include <stdbool.h> + +#include "cycle-check.h" + +#define CC_MAGIC 9827862 + +/* Return true if I is a power of 2, or is zero. */ + +static inline bool +is_zero_or_power_of_two (uintmax_t i) +{ + return (i & (i - 1)) == 0; +} + +void +cycle_check_init (struct cycle_check_state *state) +{ + state->chdir_counter = 0; + state->magic = CC_MAGIC; +} + +/* In traversing a directory hierarchy, call this function once for each + descending chdir call, with SB corresponding to the chdir operand. + If SB corresponds to a directory that has already been seen, + return true to indicate that there is a directory cycle. + Note that this is done `lazily', which means that some of + the directories in the cycle may be processed twice before + the cycle is detected. */ + +bool +cycle_check (struct cycle_check_state *state, struct stat const *sb) +{ + assert (state->magic == CC_MAGIC); + + /* If the current directory ever happens to be the same + as the one we last recorded for the cycle detection, + then it's obviously part of a cycle. */ + if (state->chdir_counter && SAME_INODE (*sb, state->dev_ino)) + return true; + + /* If the number of `descending' chdir calls is a power of two, + record the dev/ino of the current directory. */ + if (is_zero_or_power_of_two (++(state->chdir_counter))) + { + /* On all architectures that we know about, if the counter + overflows then there is a directory cycle here somewhere, + even if we haven't detected it yet. Typically this happens + only after the counter is incremented 2**64 times, so it's a + fairly theoretical point. */ + if (state->chdir_counter == 0) + return true; + + state->dev_ino.st_dev = sb->st_dev; + state->dev_ino.st_ino = sb->st_ino; + } + + return false; +} |