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 tz=UTC        -- Interpret timestamps as UTC rather than local time.
 100                  This option disables the conversion of timestamps
 101                  between local time (as used by Windows on FAT) and UTC
 102                  (which Linux uses internally).  This is particuluarly
 103                  useful when mounting devices (like digital cameras)
 104                  that are set to UTC in order to avoid the pitfalls of
 105                  local time.
 106
 107 <bool>: 0,1,yes,no,true,false
 108
 109 TODO
 110 ----------------------------------------------------------------------
 111 * Need to get rid of the raw scanning stuff.  Instead, always use
 112   a get next directory entry approach.  The only thing left that uses
 113   raw scanning is the directory renaming code.
 114
 115
 116 POSSIBLE PROBLEMS
 117 ----------------------------------------------------------------------
 118 * vfat_valid_longname does not properly checked reserved names.
 119 * When a volume name is the same as a directory name in the root
 120   directory of the filesystem, the directory name sometimes shows
 121   up as an empty file.
 122 * autoconv option does not work correctly.
 123
 124 BUG REPORTS
 125 ----------------------------------------------------------------------
 126 If you have trouble with the VFAT filesystem, mail bug reports to
 127 chaffee@bmrc.cs.berkeley.edu.  Please specify the filename
 128 and the operation that gave you trouble.
 129
 130 TEST SUITE
 131 ----------------------------------------------------------------------
 132 If you plan to make any modifications to the vfat filesystem, please
 133 get the test suite that comes with the vfat distribution at
 134
 135   http://bmrc.berkeley.edu/people/chaffee/vfat.html
 136
 137 This tests quite a few parts of the vfat filesystem and additional
 138 tests for new features or untested features would be appreciated.
 139
 140 NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
 141 ----------------------------------------------------------------------
 142 (This documentation was provided by Galen C. Hunt <gchunt@cs.rochester.edu>
 143  and lightly annotated by Gordon Chaffee).
 144
 145 This document presents a very rough, technical overview of my
 146 knowledge of the extended FAT file system used in Windows NT 3.5 and
 147 Windows 95.  I don't guarantee that any of the following is correct,
 148 but it appears to be so.
 149
 150 The extended FAT file system is almost identical to the FAT
 151 file system used in DOS versions up to and including 6.223410239847
 152 :-).  The significant change has been the addition of long file names.
 153 These names support up to 255 characters including spaces and lower
 154 case characters as opposed to the traditional 8.3 short names.
 155
 156 Here is the description of the traditional FAT entry in the current
 157 Windows 95 filesystem:
 158
 159         struct directory { // Short 8.3 names
 160                 unsigned char name[8];          // file name
 161                 unsigned char ext[3];           // file extension
 162                 unsigned char attr;             // attribute byte
 163                 unsigned char lcase;            // Case for base and extension
 164                 unsigned char ctime_ms;         // Creation time, milliseconds
 165                 unsigned char ctime[2];         // Creation time
 166                 unsigned char cdate[2];         // Creation date
 167                 unsigned char adate[2];         // Last access date
 168                 unsigned char reserved[2];      // reserved values (ignored)
 169                 unsigned char time[2];          // time stamp
 170                 unsigned char date[2];          // date stamp
 171                 unsigned char start[2];         // starting cluster number
 172                 unsigned char size[4];          // size of the file
 173         };
 174
 175 The lcase field specifies if the base and/or the extension of an 8.3
 176 name should be capitalized.  This field does not seem to be used by
 177 Windows 95 but it is used by Windows NT.  The case of filenames is not
 178 completely compatible from Windows NT to Windows 95.  It is not completely
 179 compatible in the reverse direction, however.  Filenames that fit in
 180 the 8.3 namespace and are written on Windows NT to be lowercase will
 181 show up as uppercase on Windows 95.
 182
 183 Note that the "start" and "size" values are actually little
 184 endian integer values.  The descriptions of the fields in this
 185 structure are public knowledge and can be found elsewhere.
 186
 187 With the extended FAT system, Microsoft has inserted extra
 188 directory entries for any files with extended names.  (Any name which
 189 legally fits within the old 8.3 encoding scheme does not have extra
 190 entries.)  I call these extra entries slots.  Basically, a slot is a
 191 specially formatted directory entry which holds up to 13 characters of
 192 a file's extended name.  Think of slots as additional labeling for the
 193 directory entry of the file to which they correspond.  Microsoft
 194 prefers to refer to the 8.3 entry for a file as its alias and the
 195 extended slot directory entries as the file name.
 196
 197 The C structure for a slot directory entry follows:
 198
 199         struct slot { // Up to 13 characters of a long name
 200                 unsigned char id;               // sequence number for slot
 201                 unsigned char name0_4[10];      // first 5 characters in name
 202                 unsigned char attr;             // attribute byte
 203                 unsigned char reserved;         // always 0
 204                 unsigned char alias_checksum;   // checksum for 8.3 alias
 205                 unsigned char name5_10[12];     // 6 more characters in name
 206                 unsigned char start[2];         // starting cluster number
 207                 unsigned char name11_12[4];     // last 2 characters in name
 208         };
 209
 210 If the layout of the slots looks a little odd, it's only
 211 because of Microsoft's efforts to maintain compatibility with old
 212 software.  The slots must be disguised to prevent old software from
 213 panicking.  To this end, a number of measures are taken:
 214
 215         1) The attribute byte for a slot directory entry is always set
 216            to 0x0f.  This corresponds to an old directory entry with
 217            attributes of "hidden", "system", "read-only", and "volume
 218            label".  Most old software will ignore any directory
 219            entries with the "volume label" bit set.  Real volume label
 220            entries don't have the other three bits set.
 221
 222         2) The starting cluster is always set to 0, an impossible
 223            value for a DOS file.
 224
 225 Because the extended FAT system is backward compatible, it is
 226 possible for old software to modify directory entries.  Measures must
 227 be taken to ensure the validity of slots.  An extended FAT system can
 228 verify that a slot does in fact belong to an 8.3 directory entry by
 229 the following:
 230
 231         1) Positioning.  Slots for a file always immediately proceed
 232            their corresponding 8.3 directory entry.  In addition, each
 233            slot has an id which marks its order in the extended file
 234            name.  Here is a very abbreviated view of an 8.3 directory
 235            entry and its corresponding long name slots for the file
 236            "My Big File.Extension which is long":
 237
 238                 <proceeding files...>
 239                 <slot #3, id = 0x43, characters = "h is long">
 240                 <slot #2, id = 0x02, characters = "xtension whic">
 241                 <slot #1, id = 0x01, characters = "My Big File.E">
 242                 <directory entry, name = "MYBIGFIL.EXT">
 243
 244            Note that the slots are stored from last to first.  Slots
 245            are numbered from 1 to N.  The Nth slot is or'ed with 0x40
 246            to mark it as the last one.
 247
 248         2) Checksum.  Each slot has an "alias_checksum" value.  The
 249            checksum is calculated from the 8.3 name using the
 250            following algorithm:
 251
 252                 for (sum = i = 0; i < 11; i++) {
 253                         sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
 254                 }
 255
 256         3) If there is free space in the final slot, a Unicode NULL (0x0000)
 257            is stored after the final character.  After that, all unused
 258            characters in the final slot are set to Unicode 0xFFFF.
 259
 260 Finally, note that the extended name is stored in Unicode.  Each Unicode
 261 character takes two bytes.