lib/cycle-check.c

   1 /* help detect directory cycles efficiently
   2    Copyright 2003 Free Software Foundation, Inc.
   3
   4    This program is free software; you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation; either version 2, or (at your option)
   7    any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program; see the file COPYING.
  16    If not, write to the Free Software Foundation,
  17    59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.  */
  18
  19 /* Written by Jim Meyering */
  20
  21 #if HAVE_CONFIG_H
  22 # include <config.h>
  23 #endif
  24
  25 #include <sys/types.h>
  26 #include <sys/stat.h>
  27 #include <stdio.h>
  28 #include <assert.h>
  29 #include <stdlib.h>
  30
  31 #if HAVE_STDBOOL_H
  32 # include <stdbool.h>
  33 #else
  34 typedef enum {false = 0, true = 1} bool;
  35 #endif
  36
  37 #include "cycle-check.h"
  38 #include "xalloc.h"
  39
  40 #define SAME_INODE(Stat_buf_1, Stat_buf_2) \
  41   ((Stat_buf_1).st_ino == (Stat_buf_2).st_ino \
  42    && (Stat_buf_1).st_dev == (Stat_buf_2).st_dev)
  43
  44 #define CC_MAGIC 9827862
  45
  46 static inline bool
  47 is_power_of_two (unsigned int i)
  48 {
  49   return (i & (i - 1)) == 0;
  50 }
  51
  52 void
  53 cycle_check_init (struct cycle_check_state *state)
  54 {
  55   state->chdir_counter = 0;
  56   state->magic = CC_MAGIC;
  57 }
  58
  59 /* In traversing a directory hierarchy, call this function once for each
  60    descending chdir call, with SB corresponding to the chdir operand.
  61    If SB corresponds to a directory that has already been seen,
  62    return true to indicate that there is a directory cycle.
  63    Note that this is done `lazily', which means that some of
  64    the directories in the cycle may be processed twice before
  65    the cycle is detected.  */
  66
  67 bool
  68 cycle_check (struct cycle_check_state *state, struct stat const *sb)
  69 {
  70   assert (state->magic == CC_MAGIC);
  71
  72   /* If the current directory ever happens to be the same
  73      as the one we last recorded for the cycle detection,
  74      then it's obviously part of a cycle.  */
  75   if (state->chdir_counter && SAME_INODE (*sb, state->dev_ino))
  76     return true;
  77
  78   /* If the number of `descending' chdir calls is a power of two,
  79      record the dev/ino of the current directory.  */
  80   if (is_power_of_two (++(state->chdir_counter)))
  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 }