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