libfat/fatchain.c

   1 /* ----------------------------------------------------------------------- *
   2  *
   3  *   Copyright 2004-2008 H. Peter Anvin - All Rights Reserved
   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, Inc., 53 Temple Place Ste 330,
   8  *   Boston MA 02111-1307, USA; either version 2 of the License, or
   9  *   (at your option) any later version; incorporated herein by reference.
  10  *
  11  * ----------------------------------------------------------------------- */
  12
  13 /*
  14  * fatchain.c
  15  *
  16  * Follow a FAT chain
  17  */
  18
  19 #include "libfatint.h"
  20 #include "ulint.h"
  21
  22 /*
  23  * Convert a cluster number (or 0 for the root directory) to a
  24  * sector number.  Return -1 on failure.
  25  */
  26 libfat_sector_t libfat_clustertosector(const struct libfat_filesystem *fs,
  27                                        int32_t cluster)
  28 {
  29   if ( cluster == 0 )
  30     cluster = fs->rootcluster;
  31
  32   if ( cluster == 0 )
  33     return fs->rootdir;
  34   else if ( cluster < 2 || cluster >= fs->endcluster )
  35     return -1;
  36   else
  37     return fs->data + ((libfat_sector_t)(cluster-2) << fs->clustshift);
  38 }
  39
  40 /*
  41  * Get the next sector of either the root directory or a FAT chain.
  42  * Returns 0 on end of file and -1 on error.
  43  */
  44
  45 libfat_sector_t libfat_nextsector(struct libfat_filesystem *fs,
  46                                   libfat_sector_t s)
  47 {
  48   int32_t cluster, nextcluster;
  49   uint32_t fatoffset;
  50   libfat_sector_t fatsect;
  51   uint8_t *fsdata;
  52   uint32_t clustmask = fs->clustsize - 1;
  53   libfat_sector_t rs;
  54
  55   if ( s < fs->data ) {
  56     if ( s < fs->rootdir )
  57       return -1;
  58
  59     /* Root directory */
  60     s++;
  61     return ( s < fs->data ) ? s : 0;
  62   }
  63
  64   rs = s - fs->data;
  65
  66   if ( ~rs & clustmask )
  67     return s+1;                 /* Next sector in cluster */
  68
  69   cluster = 2 + (rs >> fs->clustshift);
  70
  71   if ( cluster >= fs->endcluster )
  72     return -1;
  73
  74   switch ( fs->fat_type ) {
  75   case FAT12:
  76     /* Get first byte */
  77     fatoffset = cluster + (cluster >> 1);
  78     fatsect = fs->fat + (fatoffset >> LIBFAT_SECTOR_SHIFT);
  79     fsdata = libfat_get_sector(fs, fatsect);
  80     if ( !fsdata )
  81       return -1;
  82     nextcluster = fsdata[fatoffset & LIBFAT_SECTOR_MASK];
  83
  84     /* Get second byte */
  85     fatoffset++;
  86     fatsect = fs->fat + (fatoffset >> LIBFAT_SECTOR_SHIFT);
  87     fsdata = libfat_get_sector(fs, fatsect);
  88     if ( !fsdata )
  89       return -1;
  90     nextcluster |= fsdata[fatoffset & LIBFAT_SECTOR_MASK] << 8;
  91
  92     /* Extract the FAT entry */
  93     if ( cluster & 1 )
  94       nextcluster >>= 4;
  95     else
  96       nextcluster &= 0x0FFF;
  97
  98     if ( nextcluster >= 0x0FF8 )
  99       return 0;
 100     break;
 101
 102   case FAT16:
 103     fatoffset = cluster << 1;
 104     fatsect = fs->fat + (fatoffset >> LIBFAT_SECTOR_SHIFT);
 105     fsdata = libfat_get_sector(fs, fatsect);
 106     if ( !fsdata )
 107       return -1;
 108     nextcluster = read16((le16_t *)&fsdata[fatoffset & LIBFAT_SECTOR_MASK]);
 109
 110     if ( nextcluster >= 0x0FFF8 )
 111       return 0;
 112     break;
 113
 114   case FAT28:
 115     fatoffset = cluster << 2;
 116     fatsect = fs->fat + (fatoffset >> LIBFAT_SECTOR_SHIFT);
 117     fsdata = libfat_get_sector(fs, fatsect);
 118     if ( !fsdata )
 119       return -1;
 120     nextcluster = read32((le32_t *)&fsdata[fatoffset & LIBFAT_SECTOR_MASK]);
 121     nextcluster &= 0x0FFFFFFF;
 122
 123     if ( nextcluster >= 0x0FFFFFF8 )
 124       return 0;
 125     break;
 126
 127   default:
 128     return -1;                  /* WTF? */
 129   }
 130
 131   return libfat_clustertosector(fs, nextcluster);
 132 }