Documentation/filesystems/vfat.txt

   1 USING VFAT
   2 ----------------------------------------------------------------------
   3 To use the vfat filesystem, use the filesystem type 'vfat'.  i.e.
   4   mount -t vfat /dev/fd0 /mnt
   5
   6 No special partition formatter is required.  mkdosfs will work fine
   7 if you want to format from within Linux.
   8
   9 VFAT MOUNT OPTIONS
  10 ----------------------------------------------------------------------
  11 umask=###     -- The permission mask (for files and directories, see umask(1)).
  12                  The default is the umask of current process.
  13
  14 dmask=###     -- The permission mask for the directory.
  15                  The default is the umask of current process.
  16
  17 fmask=###     -- The permission mask for files.
  18                  The default is the umask of current process.
  19
  20 allow_utime=### -- This option controls the permission check of mtime/atime.
  21
  22                   20 - If current process is in group of file's group ID,
  23                        you can change timestamp.
  24                    2 - Other users can change timestamp.
  25
  26                  The default is set from `dmask' option. (If the directory is
  27                  writable, utime(2) is also allowed. I.e. ~dmask & 022)
  28
  29                  Normally utime(2) checks current process is owner of
  30                  the file, or it has CAP_FOWNER capability.  But FAT
  31                  filesystem doesn't have uid/gid on disk, so normal
  32                  check is too unflexible. With this option you can
  33                  relax it.
  34
  35 codepage=###  -- Sets the codepage number for converting to shortname
  36                  characters on FAT filesystem.
  37                  By default, FAT_DEFAULT_CODEPAGE setting is used.
  38
  39 iocharset=name -- Character set to use for converting between the
  40                  encoding is used for user visible filename and 16 bit
  41                  Unicode characters. Long filenames are stored on disk
  42                  in Unicode format, but Unix for the most part doesn't
  43                  know how to deal with Unicode.
  44                  By default, FAT_DEFAULT_IOCHARSET setting is used.
  45
  46                  There is also an option of doing UTF-8 translations
  47                  with the utf8 option.
  48
  49                  NOTE: "iocharset=utf8" is not recommended. If unsure,
  50                  you should consider the following option instead.
  51
  52 utf8=<bool>   -- UTF-8 is the filesystem safe version of Unicode that
  53                  is used by the console.  It can be enabled for the
  54                  filesystem with this option. If 'uni_xlate' gets set,
  55                  UTF-8 gets disabled.
  56
  57 uni_xlate=<bool> -- Translate unhandled Unicode characters to special
  58                  escaped sequences.  This would let you backup and
  59                  restore filenames that are created with any Unicode
  60                  characters.  Until Linux supports Unicode for real,
  61                  this gives you an alternative.  Without this option,
  62                  a '?' is used when no translation is possible.  The
  63                  escape character is ':' because it is otherwise
  64                  illegal on the vfat filesystem.  The escape sequence
  65                  that gets used is ':' and the four digits of hexadecimal
  66                  unicode.
  67
  68 nonumtail=<bool> -- When creating 8.3 aliases, normally the alias will
  69                  end in '~1' or tilde followed by some number.  If this
  70                  option is set, then if the filename is
  71                  "longfilename.txt" and "longfile.txt" does not
  72                  currently exist in the directory, 'longfile.txt' will
  73                  be the short alias instead of 'longfi~1.txt'.
  74
  75 usefree       -- Use the "free clusters" value stored on FSINFO. It'll
  76                  be used to determine number of free clusters without
  77                  scanning disk. But it's not used by default, because
  78                  recent Windows don't update it correctly in some
  79                  case. If you are sure the "free clusters" on FSINFO is
  80                  correct, by this option you can avoid scanning disk.
  81
  82 quiet         -- Stops printing certain warning messages.
  83
  84 check=s|r|n   -- Case sensitivity checking setting.
  85                  s: strict, case sensitive
  86                  r: relaxed, case insensitive
  87                  n: normal, default setting, currently case insensitive
  88
  89 shortname=lower|win95|winnt|mixed
  90               -- Shortname display/create setting.
  91                  lower: convert to lowercase for display,
  92                         emulate the Windows 95 rule for create.
  93                  win95: emulate the Windows 95 rule for display/create.
  94                  winnt: emulate the Windows NT rule for display/create.
  95                  mixed: emulate the Windows NT rule for display,
  96                         emulate the Windows 95 rule for create.
  97                  Default setting is `lower'.
  98
  99 <bool>: 0,1,yes,no,true,false
 100
 101 TODO
 102 ----------------------------------------------------------------------
 103 * Need to get rid of the raw scanning stuff.  Instead, always use
 104   a get next directory entry approach.  The only thing left that uses
 105   raw scanning is the directory renaming code.
 106
 107
 108 POSSIBLE PROBLEMS
 109 ----------------------------------------------------------------------
 110 * vfat_valid_longname does not properly checked reserved names.
 111 * When a volume name is the same as a directory name in the root
 112   directory of the filesystem, the directory name sometimes shows
 113   up as an empty file.
 114 * autoconv option does not work correctly.
 115
 116 BUG REPORTS
 117 ----------------------------------------------------------------------
 118 If you have trouble with the VFAT filesystem, mail bug reports to
 119 chaffee@bmrc.cs.berkeley.edu.  Please specify the filename
 120 and the operation that gave you trouble.
 121
 122 TEST SUITE
 123 ----------------------------------------------------------------------
 124 If you plan to make any modifications to the vfat filesystem, please
 125 get the test suite that comes with the vfat distribution at
 126
 127   http://bmrc.berkeley.edu/people/chaffee/vfat.html
 128
 129 This tests quite a few parts of the vfat filesystem and additional
 130 tests for new features or untested features would be appreciated.
 131
 132 NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
 133 ----------------------------------------------------------------------
 134 (This documentation was provided by Galen C. Hunt <gchunt@cs.rochester.edu>
 135  and lightly annotated by Gordon Chaffee).
 136
 137 This document presents a very rough, technical overview of my
 138 knowledge of the extended FAT file system used in Windows NT 3.5 and
 139 Windows 95.  I don't guarantee that any of the following is correct,
 140 but it appears to be so.
 141
 142 The extended FAT file system is almost identical to the FAT
 143 file system used in DOS versions up to and including 6.223410239847
 144 :-).  The significant change has been the addition of long file names.
 145 These names support up to 255 characters including spaces and lower
 146 case characters as opposed to the traditional 8.3 short names.
 147
 148 Here is the description of the traditional FAT entry in the current
 149 Windows 95 filesystem:
 150
 151         struct directory { // Short 8.3 names
 152                 unsigned char name[8];          // file name
 153                 unsigned char ext[3];           // file extension
 154                 unsigned char attr;             // attribute byte
 155                 unsigned char lcase;            // Case for base and extension
 156                 unsigned char ctime_ms;         // Creation time, milliseconds
 157                 unsigned char ctime[2];         // Creation time
 158                 unsigned char cdate[2];         // Creation date
 159                 unsigned char adate[2];         // Last access date
 160                 unsigned char reserved[2];      // reserved values (ignored)
 161                 unsigned char time[2];          // time stamp
 162                 unsigned char date[2];          // date stamp
 163                 unsigned char start[2];         // starting cluster number
 164                 unsigned char size[4];          // size of the file
 165         };
 166
 167 The lcase field specifies if the base and/or the extension of an 8.3
 168 name should be capitalized.  This field does not seem to be used by
 169 Windows 95 but it is used by Windows NT.  The case of filenames is not
 170 completely compatible from Windows NT to Windows 95.  It is not completely
 171 compatible in the reverse direction, however.  Filenames that fit in
 172 the 8.3 namespace and are written on Windows NT to be lowercase will
 173 show up as uppercase on Windows 95.
 174
 175 Note that the "start" and "size" values are actually little
 176 endian integer values.  The descriptions of the fields in this
 177 structure are public knowledge and can be found elsewhere.
 178
 179 With the extended FAT system, Microsoft has inserted extra
 180 directory entries for any files with extended names.  (Any name which
 181 legally fits within the old 8.3 encoding scheme does not have extra
 182 entries.)  I call these extra entries slots.  Basically, a slot is a
 183 specially formatted directory entry which holds up to 13 characters of
 184 a file's extended name.  Think of slots as additional labeling for the
 185 directory entry of the file to which they correspond.  Microsoft
 186 prefers to refer to the 8.3 entry for a file as its alias and the
 187 extended slot directory entries as the file name.
 188
 189 The C structure for a slot directory entry follows:
 190
 191         struct slot { // Up to 13 characters of a long name
 192                 unsigned char id;               // sequence number for slot
 193                 unsigned char name0_4[10];      // first 5 characters in name
 194                 unsigned char attr;             // attribute byte
 195                 unsigned char reserved;         // always 0
 196                 unsigned char alias_checksum;   // checksum for 8.3 alias
 197                 unsigned char name5_10[12];     // 6 more characters in name
 198                 unsigned char start[2];         // starting cluster number
 199                 unsigned char name11_12[4];     // last 2 characters in name
 200         };
 201
 202 If the layout of the slots looks a little odd, it's only
 203 because of Microsoft's efforts to maintain compatibility with old
 204 software.  The slots must be disguised to prevent old software from
 205 panicking.  To this end, a number of measures are taken:
 206
 207         1) The attribute byte for a slot directory entry is always set
 208            to 0x0f.  This corresponds to an old directory entry with
 209            attributes of "hidden", "system", "read-only", and "volume
 210            label".  Most old software will ignore any directory
 211            entries with the "volume label" bit set.  Real volume label
 212            entries don't have the other three bits set.
 213
 214         2) The starting cluster is always set to 0, an impossible
 215            value for a DOS file.
 216
 217 Because the extended FAT system is backward compatible, it is
 218 possible for old software to modify directory entries.  Measures must
 219 be taken to ensure the validity of slots.  An extended FAT system can
 220 verify that a slot does in fact belong to an 8.3 directory entry by
 221 the following:
 222
 223         1) Positioning.  Slots for a file always immediately proceed
 224            their corresponding 8.3 directory entry.  In addition, each
 225            slot has an id which marks its order in the extended file
 226            name.  Here is a very abbreviated view of an 8.3 directory
 227            entry and its corresponding long name slots for the file
 228            "My Big File.Extension which is long":
 229
 230                 <proceeding files...>
 231                 <slot #3, id = 0x43, characters = "h is long">
 232                 <slot #2, id = 0x02, characters = "xtension whic">
 233                 <slot #1, id = 0x01, characters = "My Big File.E">
 234                 <directory entry, name = "MYBIGFIL.EXT">
 235
 236            Note that the slots are stored from last to first.  Slots
 237            are numbered from 1 to N.  The Nth slot is or'ed with 0x40
 238            to mark it as the last one.
 239
 240         2) Checksum.  Each slot has an "alias_checksum" value.  The
 241            checksum is calculated from the 8.3 name using the
 242            following algorithm:
 243
 244                 for (sum = i = 0; i < 11; i++) {
 245                         sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
 246                 }
 247
 248         3) If there is free space in the final slot, a Unicode NULL (0x0000)
 249            is stored after the final character.  After that, all unused
 250            characters in the final slot are set to Unicode 0xFFFF.
 251
 252 Finally, note that the extended name is stored in Unicode.  Each Unicode
 253 character takes two bytes.