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 uid=###       -- Set the owner of all files on this filesystem.
  12                  The default is the uid of current process.
  13
  14 gid=###       -- Set the group of all files on this filesystem.
  15                  The default is the gid of current process.
  16
  17 umask=###     -- The permission mask (for files and directories, see umask(1)).
  18                  The default is the umask of current process.
  19
  20 dmask=###     -- The permission mask for the directory.
  21                  The default is the umask of current process.
  22
  23 fmask=###     -- The permission mask for files.
  24                  The default is the umask of current process.
  25
  26 allow_utime=### -- This option controls the permission check of mtime/atime.
  27
  28                   20 - If current process is in group of file's group ID,
  29                        you can change timestamp.
  30                    2 - Other users can change timestamp.
  31
  32                  The default is set from `dmask' option. (If the directory is
  33                  writable, utime(2) is also allowed. I.e. ~dmask & 022)
  34
  35                  Normally utime(2) checks current process is owner of
  36                  the file, or it has CAP_FOWNER capability.  But FAT
  37                  filesystem doesn't have uid/gid on disk, so normal
  38                  check is too unflexible. With this option you can
  39                  relax it.
  40
  41 codepage=###  -- Sets the codepage number for converting to shortname
  42                  characters on FAT filesystem.
  43                  By default, FAT_DEFAULT_CODEPAGE setting is used.
  44
  45 iocharset=<name> -- Character set to use for converting between the
  46                  encoding is used for user visible filename and 16 bit
  47                  Unicode characters. Long filenames are stored on disk
  48                  in Unicode format, but Unix for the most part doesn't
  49                  know how to deal with Unicode.
  50                  By default, FAT_DEFAULT_IOCHARSET setting is used.
  51
  52                  There is also an option of doing UTF-8 translations
  53                  with the utf8 option.
  54
  55                  NOTE: "iocharset=utf8" is not recommended. If unsure,
  56                  you should consider the following option instead.
  57
  58 utf8=<bool>   -- UTF-8 is the filesystem safe version of Unicode that
  59                  is used by the console.  It can be enabled for the
  60                  filesystem with this option. If 'uni_xlate' gets set,
  61                  UTF-8 gets disabled.
  62
  63 uni_xlate=<bool> -- Translate unhandled Unicode characters to special
  64                  escaped sequences.  This would let you backup and
  65                  restore filenames that are created with any Unicode
  66                  characters.  Until Linux supports Unicode for real,
  67                  this gives you an alternative.  Without this option,
  68                  a '?' is used when no translation is possible.  The
  69                  escape character is ':' because it is otherwise
  70                  illegal on the vfat filesystem.  The escape sequence
  71                  that gets used is ':' and the four digits of hexadecimal
  72                  unicode.
  73
  74 nonumtail=<bool> -- When creating 8.3 aliases, normally the alias will
  75                  end in '~1' or tilde followed by some number.  If this
  76                  option is set, then if the filename is
  77                  "longfilename.txt" and "longfile.txt" does not
  78                  currently exist in the directory, 'longfile.txt' will
  79                  be the short alias instead of 'longfi~1.txt'.
  80
  81 usefree       -- Use the "free clusters" value stored on FSINFO. It'll
  82                  be used to determine number of free clusters without
  83                  scanning disk. But it's not used by default, because
  84                  recent Windows don't update it correctly in some
  85                  case. If you are sure the "free clusters" on FSINFO is
  86                  correct, by this option you can avoid scanning disk.
  87
  88 quiet         -- Stops printing certain warning messages.
  89
  90 check=s|r|n   -- Case sensitivity checking setting.
  91                  s: strict, case sensitive
  92                  r: relaxed, case insensitive
  93                  n: normal, default setting, currently case insensitive
  94
  95 nocase        -- This was deprecated for vfat. Use shortname=win95 instead.
  96
  97 shortname=lower|win95|winnt|mixed
  98               -- Shortname display/create setting.
  99                  lower: convert to lowercase for display,
 100                         emulate the Windows 95 rule for create.
 101                  win95: emulate the Windows 95 rule for display/create.
 102                  winnt: emulate the Windows NT rule for display/create.
 103                  mixed: emulate the Windows NT rule for display,
 104                         emulate the Windows 95 rule for create.
 105                  Default setting is `lower'.
 106
 107 tz=UTC        -- Interpret timestamps as UTC rather than local time.
 108                  This option disables the conversion of timestamps
 109                  between local time (as used by Windows on FAT) and UTC
 110                  (which Linux uses internally).  This is particularly
 111                  useful when mounting devices (like digital cameras)
 112                  that are set to UTC in order to avoid the pitfalls of
 113                  local time.
 114
 115 showexec      -- If set, the execute permission bits of the file will be
 116                  allowed only if the extension part of the name is .EXE,
 117                  .COM, or .BAT. Not set by default.
 118
 119 debug         -- Can be set, but unused by the current implementation.
 120
 121 sys_immutable -- If set, ATTR_SYS attribute on FAT is handled as
 122                  IMMUTABLE flag on Linux. Not set by default.
 123
 124 flush         -- If set, the filesystem will try to flush to disk more
 125                  early than normal. Not set by default.
 126
 127 rodir         -- FAT has the ATTR_RO (read-only) attribute. But on Windows,
 128                  the ATTR_RO of the directory will be just ignored actually,
 129                  and is used by only applications as flag. E.g. it's setted
 130                  for the customized folder.
 131
 132                  If you want to use ATTR_RO as read-only flag even for
 133                  the directory, set this option.
 134
 135 <bool>: 0,1,yes,no,true,false
 136
 137 TODO
 138 ----------------------------------------------------------------------
 139 * Need to get rid of the raw scanning stuff.  Instead, always use
 140   a get next directory entry approach.  The only thing left that uses
 141   raw scanning is the directory renaming code.
 142
 143
 144 POSSIBLE PROBLEMS
 145 ----------------------------------------------------------------------
 146 * vfat_valid_longname does not properly checked reserved names.
 147 * When a volume name is the same as a directory name in the root
 148   directory of the filesystem, the directory name sometimes shows
 149   up as an empty file.
 150 * autoconv option does not work correctly.
 151
 152 BUG REPORTS
 153 ----------------------------------------------------------------------
 154 If you have trouble with the VFAT filesystem, mail bug reports to
 155 chaffee@bmrc.cs.berkeley.edu.  Please specify the filename
 156 and the operation that gave you trouble.
 157
 158 TEST SUITE
 159 ----------------------------------------------------------------------
 160 If you plan to make any modifications to the vfat filesystem, please
 161 get the test suite that comes with the vfat distribution at
 162
 163   http://bmrc.berkeley.edu/people/chaffee/vfat.html
 164
 165 This tests quite a few parts of the vfat filesystem and additional
 166 tests for new features or untested features would be appreciated.
 167
 168 NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
 169 ----------------------------------------------------------------------
 170 (This documentation was provided by Galen C. Hunt <gchunt@cs.rochester.edu>
 171  and lightly annotated by Gordon Chaffee).
 172
 173 This document presents a very rough, technical overview of my
 174 knowledge of the extended FAT file system used in Windows NT 3.5 and
 175 Windows 95.  I don't guarantee that any of the following is correct,
 176 but it appears to be so.
 177
 178 The extended FAT file system is almost identical to the FAT
 179 file system used in DOS versions up to and including 6.223410239847
 180 :-).  The significant change has been the addition of long file names.
 181 These names support up to 255 characters including spaces and lower
 182 case characters as opposed to the traditional 8.3 short names.
 183
 184 Here is the description of the traditional FAT entry in the current
 185 Windows 95 filesystem:
 186
 187         struct directory { // Short 8.3 names
 188                 unsigned char name[8];          // file name
 189                 unsigned char ext[3];           // file extension
 190                 unsigned char attr;             // attribute byte
 191                 unsigned char lcase;            // Case for base and extension
 192                 unsigned char ctime_ms;         // Creation time, milliseconds
 193                 unsigned char ctime[2];         // Creation time
 194                 unsigned char cdate[2];         // Creation date
 195                 unsigned char adate[2];         // Last access date
 196                 unsigned char reserved[2];      // reserved values (ignored)
 197                 unsigned char time[2];          // time stamp
 198                 unsigned char date[2];          // date stamp
 199                 unsigned char start[2];         // starting cluster number
 200                 unsigned char size[4];          // size of the file
 201         };
 202
 203 The lcase field specifies if the base and/or the extension of an 8.3
 204 name should be capitalized.  This field does not seem to be used by
 205 Windows 95 but it is used by Windows NT.  The case of filenames is not
 206 completely compatible from Windows NT to Windows 95.  It is not completely
 207 compatible in the reverse direction, however.  Filenames that fit in
 208 the 8.3 namespace and are written on Windows NT to be lowercase will
 209 show up as uppercase on Windows 95.
 210
 211 Note that the "start" and "size" values are actually little
 212 endian integer values.  The descriptions of the fields in this
 213 structure are public knowledge and can be found elsewhere.
 214
 215 With the extended FAT system, Microsoft has inserted extra
 216 directory entries for any files with extended names.  (Any name which
 217 legally fits within the old 8.3 encoding scheme does not have extra
 218 entries.)  I call these extra entries slots.  Basically, a slot is a
 219 specially formatted directory entry which holds up to 13 characters of
 220 a file's extended name.  Think of slots as additional labeling for the
 221 directory entry of the file to which they correspond.  Microsoft
 222 prefers to refer to the 8.3 entry for a file as its alias and the
 223 extended slot directory entries as the file name.
 224
 225 The C structure for a slot directory entry follows:
 226
 227         struct slot { // Up to 13 characters of a long name
 228                 unsigned char id;               // sequence number for slot
 229                 unsigned char name0_4[10];      // first 5 characters in name
 230                 unsigned char attr;             // attribute byte
 231                 unsigned char reserved;         // always 0
 232                 unsigned char alias_checksum;   // checksum for 8.3 alias
 233                 unsigned char name5_10[12];     // 6 more characters in name
 234                 unsigned char start[2];         // starting cluster number
 235                 unsigned char name11_12[4];     // last 2 characters in name
 236         };
 237
 238 If the layout of the slots looks a little odd, it's only
 239 because of Microsoft's efforts to maintain compatibility with old
 240 software.  The slots must be disguised to prevent old software from
 241 panicking.  To this end, a number of measures are taken:
 242
 243         1) The attribute byte for a slot directory entry is always set
 244            to 0x0f.  This corresponds to an old directory entry with
 245            attributes of "hidden", "system", "read-only", and "volume
 246            label".  Most old software will ignore any directory
 247            entries with the "volume label" bit set.  Real volume label
 248            entries don't have the other three bits set.
 249
 250         2) The starting cluster is always set to 0, an impossible
 251            value for a DOS file.
 252
 253 Because the extended FAT system is backward compatible, it is
 254 possible for old software to modify directory entries.  Measures must
 255 be taken to ensure the validity of slots.  An extended FAT system can
 256 verify that a slot does in fact belong to an 8.3 directory entry by
 257 the following:
 258
 259         1) Positioning.  Slots for a file always immediately proceed
 260            their corresponding 8.3 directory entry.  In addition, each
 261            slot has an id which marks its order in the extended file
 262            name.  Here is a very abbreviated view of an 8.3 directory
 263            entry and its corresponding long name slots for the file
 264            "My Big File.Extension which is long":
 265
 266                 <proceeding files...>
 267                 <slot #3, id = 0x43, characters = "h is long">
 268                 <slot #2, id = 0x02, characters = "xtension whic">
 269                 <slot #1, id = 0x01, characters = "My Big File.E">
 270                 <directory entry, name = "MYBIGFIL.EXT">
 271
 272            Note that the slots are stored from last to first.  Slots
 273            are numbered from 1 to N.  The Nth slot is or'ed with 0x40
 274            to mark it as the last one.
 275
 276         2) Checksum.  Each slot has an "alias_checksum" value.  The
 277            checksum is calculated from the 8.3 name using the
 278            following algorithm:
 279
 280                 for (sum = i = 0; i < 11; i++) {
 281                         sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
 282                 }
 283
 284         3) If there is free space in the final slot, a Unicode NULL (0x0000)
 285            is stored after the final character.  After that, all unused
 286            characters in the final slot are set to Unicode 0xFFFF.
 287
 288 Finally, note that the extended name is stored in Unicode.  Each Unicode
 289 character takes two bytes.