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 `mixed'.
 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 time_offset=minutes
 115               -- Set offset for conversion of timestamps from local time
 116                  used by FAT to UTC. I.e. <minutes> minutes will be subtracted
 117                  from each timestamp to convert it to UTC used internally by
 118                  Linux. This is useful when time zone set in sys_tz is
 119                  not the time zone used by the filesystem. Note that this
 120                  option still does not provide correct time stamps in all
 121                  cases in presence of DST - time stamps in a different DST
 122                  setting will be off by one hour.
 123
 124 showexec      -- If set, the execute permission bits of the file will be
 125                  allowed only if the extension part of the name is .EXE,
 126                  .COM, or .BAT. Not set by default.
 127
 128 debug         -- Can be set, but unused by the current implementation.
 129
 130 sys_immutable -- If set, ATTR_SYS attribute on FAT is handled as
 131                  IMMUTABLE flag on Linux. Not set by default.
 132
 133 flush         -- If set, the filesystem will try to flush to disk more
 134                  early than normal. Not set by default.
 135
 136 rodir         -- FAT has the ATTR_RO (read-only) attribute. On Windows,
 137                  the ATTR_RO of the directory will just be ignored,
 138                  and is used only by applications as a flag (e.g. it's set
 139                  for the customized folder).
 140
 141                  If you want to use ATTR_RO as read-only flag even for
 142                  the directory, set this option.
 143
 144 errors=panic|continue|remount-ro
 145               -- specify FAT behavior on critical errors: panic, continue
 146                  without doing anything or remount the partition in
 147                  read-only mode (default behavior).
 148
 149 discard       -- If set, issues discard/TRIM commands to the block
 150                  device when blocks are freed. This is useful for SSD devices
 151                  and sparse/thinly-provisoned LUNs.
 152
 153 nfs           -- This option maintains an index (cache) of directory
 154                  inodes by i_logstart which is used by the nfs-related code to
 155                  improve look-ups.
 156
 157                  Enable this only if you want to export the FAT filesystem
 158                  over NFS
 159
 160 <bool>: 0,1,yes,no,true,false
 161
 162 TODO
 163 ----------------------------------------------------------------------
 164 * Need to get rid of the raw scanning stuff.  Instead, always use
 165   a get next directory entry approach.  The only thing left that uses
 166   raw scanning is the directory renaming code.
 167
 168
 169 POSSIBLE PROBLEMS
 170 ----------------------------------------------------------------------
 171 * vfat_valid_longname does not properly checked reserved names.
 172 * When a volume name is the same as a directory name in the root
 173   directory of the filesystem, the directory name sometimes shows
 174   up as an empty file.
 175 * autoconv option does not work correctly.
 176
 177 BUG REPORTS
 178 ----------------------------------------------------------------------
 179 If you have trouble with the VFAT filesystem, mail bug reports to
 180 chaffee@bmrc.cs.berkeley.edu.  Please specify the filename
 181 and the operation that gave you trouble.
 182
 183 TEST SUITE
 184 ----------------------------------------------------------------------
 185 If you plan to make any modifications to the vfat filesystem, please
 186 get the test suite that comes with the vfat distribution at
 187
 188   http://web.archive.org/web/*/http://bmrc.berkeley.edu/
 189   people/chaffee/vfat.html
 190
 191 This tests quite a few parts of the vfat filesystem and additional
 192 tests for new features or untested features would be appreciated.
 193
 194 NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
 195 ----------------------------------------------------------------------
 196 (This documentation was provided by Galen C. Hunt <gchunt@cs.rochester.edu>
 197  and lightly annotated by Gordon Chaffee).
 198
 199 This document presents a very rough, technical overview of my
 200 knowledge of the extended FAT file system used in Windows NT 3.5 and
 201 Windows 95.  I don't guarantee that any of the following is correct,
 202 but it appears to be so.
 203
 204 The extended FAT file system is almost identical to the FAT
 205 file system used in DOS versions up to and including 6.223410239847
 206 :-).  The significant change has been the addition of long file names.
 207 These names support up to 255 characters including spaces and lower
 208 case characters as opposed to the traditional 8.3 short names.
 209
 210 Here is the description of the traditional FAT entry in the current
 211 Windows 95 filesystem:
 212
 213         struct directory { // Short 8.3 names
 214                 unsigned char name[8];          // file name
 215                 unsigned char ext[3];           // file extension
 216                 unsigned char attr;             // attribute byte
 217                 unsigned char lcase;            // Case for base and extension
 218                 unsigned char ctime_ms;         // Creation time, milliseconds
 219                 unsigned char ctime[2];         // Creation time
 220                 unsigned char cdate[2];         // Creation date
 221                 unsigned char adate[2];         // Last access date
 222                 unsigned char reserved[2];      // reserved values (ignored)
 223                 unsigned char time[2];          // time stamp
 224                 unsigned char date[2];          // date stamp
 225                 unsigned char start[2];         // starting cluster number
 226                 unsigned char size[4];          // size of the file
 227         };
 228
 229 The lcase field specifies if the base and/or the extension of an 8.3
 230 name should be capitalized.  This field does not seem to be used by
 231 Windows 95 but it is used by Windows NT.  The case of filenames is not
 232 completely compatible from Windows NT to Windows 95.  It is not completely
 233 compatible in the reverse direction, however.  Filenames that fit in
 234 the 8.3 namespace and are written on Windows NT to be lowercase will
 235 show up as uppercase on Windows 95.
 236
 237 Note that the "start" and "size" values are actually little
 238 endian integer values.  The descriptions of the fields in this
 239 structure are public knowledge and can be found elsewhere.
 240
 241 With the extended FAT system, Microsoft has inserted extra
 242 directory entries for any files with extended names.  (Any name which
 243 legally fits within the old 8.3 encoding scheme does not have extra
 244 entries.)  I call these extra entries slots.  Basically, a slot is a
 245 specially formatted directory entry which holds up to 13 characters of
 246 a file's extended name.  Think of slots as additional labeling for the
 247 directory entry of the file to which they correspond.  Microsoft
 248 prefers to refer to the 8.3 entry for a file as its alias and the
 249 extended slot directory entries as the file name.
 250
 251 The C structure for a slot directory entry follows:
 252
 253         struct slot { // Up to 13 characters of a long name
 254                 unsigned char id;               // sequence number for slot
 255                 unsigned char name0_4[10];      // first 5 characters in name
 256                 unsigned char attr;             // attribute byte
 257                 unsigned char reserved;         // always 0
 258                 unsigned char alias_checksum;   // checksum for 8.3 alias
 259                 unsigned char name5_10[12];     // 6 more characters in name
 260                 unsigned char start[2];         // starting cluster number
 261                 unsigned char name11_12[4];     // last 2 characters in name
 262         };
 263
 264 If the layout of the slots looks a little odd, it's only
 265 because of Microsoft's efforts to maintain compatibility with old
 266 software.  The slots must be disguised to prevent old software from
 267 panicking.  To this end, a number of measures are taken:
 268
 269         1) The attribute byte for a slot directory entry is always set
 270            to 0x0f.  This corresponds to an old directory entry with
 271            attributes of "hidden", "system", "read-only", and "volume
 272            label".  Most old software will ignore any directory
 273            entries with the "volume label" bit set.  Real volume label
 274            entries don't have the other three bits set.
 275
 276         2) The starting cluster is always set to 0, an impossible
 277            value for a DOS file.
 278
 279 Because the extended FAT system is backward compatible, it is
 280 possible for old software to modify directory entries.  Measures must
 281 be taken to ensure the validity of slots.  An extended FAT system can
 282 verify that a slot does in fact belong to an 8.3 directory entry by
 283 the following:
 284
 285         1) Positioning.  Slots for a file always immediately proceed
 286            their corresponding 8.3 directory entry.  In addition, each
 287            slot has an id which marks its order in the extended file
 288            name.  Here is a very abbreviated view of an 8.3 directory
 289            entry and its corresponding long name slots for the file
 290            "My Big File.Extension which is long":
 291
 292                 <proceeding files...>
 293                 <slot #3, id = 0x43, characters = "h is long">
 294                 <slot #2, id = 0x02, characters = "xtension whic">
 295                 <slot #1, id = 0x01, characters = "My Big File.E">
 296                 <directory entry, name = "MYBIGFIL.EXT">
 297
 298            Note that the slots are stored from last to first.  Slots
 299            are numbered from 1 to N.  The Nth slot is or'ed with 0x40
 300            to mark it as the last one.
 301
 302         2) Checksum.  Each slot has an "alias_checksum" value.  The
 303            checksum is calculated from the 8.3 name using the
 304            following algorithm:
 305
 306                 for (sum = i = 0; i < 11; i++) {
 307                         sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
 308                 }
 309
 310         3) If there is free space in the final slot, a Unicode NULL (0x0000)
 311            is stored after the final character.  After that, all unused
 312            characters in the final slot are set to Unicode 0xFFFF.
 313
 314 Finally, note that the extended name is stored in Unicode.  Each Unicode
 315 character takes two bytes.