linux/Documentation/filesystems/vfat.txt
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   1USING VFAT
   2----------------------------------------------------------------------
   3To use the vfat filesystem, use the filesystem type 'vfat'.  i.e.
   4  mount -t vfat /dev/fd0 /mnt
   5
   6No special partition formatter is required.  mkdosfs will work fine
   7if you want to format from within Linux.
   8
   9VFAT MOUNT OPTIONS
  10----------------------------------------------------------------------
  11uid=###       -- Set the owner of all files on this filesystem.
  12                 The default is the uid of current process.
  13
  14gid=###       -- Set the group of all files on this filesystem.
  15                 The default is the gid of current process.
  16
  17umask=###     -- The permission mask (for files and directories, see umask(1)).
  18                 The default is the umask of current process.
  19
  20dmask=###     -- The permission mask for the directory.
  21                 The default is the umask of current process.
  22
  23fmask=###     -- The permission mask for files.
  24                 The default is the umask of current process.
  25
  26allow_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
  41codepage=###  -- Sets the codepage number for converting to shortname
  42                 characters on FAT filesystem.
  43                 By default, FAT_DEFAULT_CODEPAGE setting is used.
  44
  45iocharset=<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
  58utf8=<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
  63uni_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
  74nonumtail=<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                  
  81usefree       -- 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
  88quiet         -- Stops printing certain warning messages.
  89
  90check=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
  95nocase        -- This was deprecated for vfat. Use shortname=win95 instead.
  96
  97shortname=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
 107tz=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.
 114time_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
 124showexec      -- 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
 128debug         -- Can be set, but unused by the current implementation.
 129
 130sys_immutable -- If set, ATTR_SYS attribute on FAT is handled as
 131                 IMMUTABLE flag on Linux. Not set by default.
 132
 133flush         -- If set, the filesystem will try to flush to disk more
 134                 early than normal. Not set by default.
 135
 136rodir         -- 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
 144errors=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
 149discard       -- 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
 153nfs=stale_rw|nostale_ro
 154                Enable this only if you want to export the FAT filesystem
 155                over NFS.
 156
 157                stale_rw: This option maintains an index (cache) of directory
 158                inodes by i_logstart which is used by the nfs-related code to
 159                improve look-ups. Full file operations (read/write) over NFS is
 160                supported but with cache eviction at NFS server, this could
 161                result in ESTALE issues.
 162
 163                nostale_ro: This option bases the inode number and filehandle
 164                on the on-disk location of a file in the MS-DOS directory entry.
 165                This ensures that ESTALE will not be returned after a file is
 166                evicted from the inode cache. However, it means that operations
 167                such as rename, create and unlink could cause filehandles that
 168                previously pointed at one file to point at a different file,
 169                potentially causing data corruption. For this reason, this
 170                option also mounts the filesystem readonly.
 171
 172                To maintain backward compatibility, '-o nfs' is also accepted,
 173                defaulting to stale_rw
 174
 175
 176<bool>: 0,1,yes,no,true,false
 177
 178TODO
 179----------------------------------------------------------------------
 180* Need to get rid of the raw scanning stuff.  Instead, always use
 181  a get next directory entry approach.  The only thing left that uses
 182  raw scanning is the directory renaming code.
 183
 184
 185POSSIBLE PROBLEMS
 186----------------------------------------------------------------------
 187* vfat_valid_longname does not properly checked reserved names.
 188* When a volume name is the same as a directory name in the root
 189  directory of the filesystem, the directory name sometimes shows
 190  up as an empty file.
 191* autoconv option does not work correctly.
 192
 193BUG REPORTS
 194----------------------------------------------------------------------
 195If you have trouble with the VFAT filesystem, mail bug reports to
 196chaffee@bmrc.cs.berkeley.edu.  Please specify the filename
 197and the operation that gave you trouble.
 198
 199TEST SUITE
 200----------------------------------------------------------------------
 201If you plan to make any modifications to the vfat filesystem, please
 202get the test suite that comes with the vfat distribution at
 203
 204  http://web.archive.org/web/*/http://bmrc.berkeley.edu/
 205  people/chaffee/vfat.html
 206
 207This tests quite a few parts of the vfat filesystem and additional
 208tests for new features or untested features would be appreciated.
 209
 210NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
 211----------------------------------------------------------------------
 212(This documentation was provided by Galen C. Hunt <gchunt@cs.rochester.edu>
 213 and lightly annotated by Gordon Chaffee).
 214
 215This document presents a very rough, technical overview of my
 216knowledge of the extended FAT file system used in Windows NT 3.5 and
 217Windows 95.  I don't guarantee that any of the following is correct,
 218but it appears to be so.
 219
 220The extended FAT file system is almost identical to the FAT
 221file system used in DOS versions up to and including 6.223410239847
 222:-).  The significant change has been the addition of long file names.
 223These names support up to 255 characters including spaces and lower
 224case characters as opposed to the traditional 8.3 short names.
 225
 226Here is the description of the traditional FAT entry in the current
 227Windows 95 filesystem:
 228
 229        struct directory { // Short 8.3 names 
 230                unsigned char name[8];          // file name 
 231                unsigned char ext[3];           // file extension 
 232                unsigned char attr;             // attribute byte 
 233                unsigned char lcase;            // Case for base and extension
 234                unsigned char ctime_ms;         // Creation time, milliseconds
 235                unsigned char ctime[2];         // Creation time
 236                unsigned char cdate[2];         // Creation date
 237                unsigned char adate[2];         // Last access date
 238                unsigned char reserved[2];      // reserved values (ignored) 
 239                unsigned char time[2];          // time stamp 
 240                unsigned char date[2];          // date stamp 
 241                unsigned char start[2];         // starting cluster number 
 242                unsigned char size[4];          // size of the file 
 243        };
 244
 245The lcase field specifies if the base and/or the extension of an 8.3
 246name should be capitalized.  This field does not seem to be used by
 247Windows 95 but it is used by Windows NT.  The case of filenames is not
 248completely compatible from Windows NT to Windows 95.  It is not completely
 249compatible in the reverse direction, however.  Filenames that fit in
 250the 8.3 namespace and are written on Windows NT to be lowercase will
 251show up as uppercase on Windows 95.
 252
 253Note that the "start" and "size" values are actually little
 254endian integer values.  The descriptions of the fields in this
 255structure are public knowledge and can be found elsewhere.
 256
 257With the extended FAT system, Microsoft has inserted extra
 258directory entries for any files with extended names.  (Any name which
 259legally fits within the old 8.3 encoding scheme does not have extra
 260entries.)  I call these extra entries slots.  Basically, a slot is a
 261specially formatted directory entry which holds up to 13 characters of
 262a file's extended name.  Think of slots as additional labeling for the
 263directory entry of the file to which they correspond.  Microsoft
 264prefers to refer to the 8.3 entry for a file as its alias and the
 265extended slot directory entries as the file name. 
 266
 267The C structure for a slot directory entry follows:
 268
 269        struct slot { // Up to 13 characters of a long name 
 270                unsigned char id;               // sequence number for slot 
 271                unsigned char name0_4[10];      // first 5 characters in name 
 272                unsigned char attr;             // attribute byte
 273                unsigned char reserved;         // always 0 
 274                unsigned char alias_checksum;   // checksum for 8.3 alias 
 275                unsigned char name5_10[12];     // 6 more characters in name
 276                unsigned char start[2];         // starting cluster number
 277                unsigned char name11_12[4];     // last 2 characters in name
 278        };
 279
 280If the layout of the slots looks a little odd, it's only
 281because of Microsoft's efforts to maintain compatibility with old
 282software.  The slots must be disguised to prevent old software from
 283panicking.  To this end, a number of measures are taken:
 284
 285        1) The attribute byte for a slot directory entry is always set
 286           to 0x0f.  This corresponds to an old directory entry with
 287           attributes of "hidden", "system", "read-only", and "volume
 288           label".  Most old software will ignore any directory
 289           entries with the "volume label" bit set.  Real volume label
 290           entries don't have the other three bits set.
 291
 292        2) The starting cluster is always set to 0, an impossible
 293           value for a DOS file.
 294
 295Because the extended FAT system is backward compatible, it is
 296possible for old software to modify directory entries.  Measures must
 297be taken to ensure the validity of slots.  An extended FAT system can
 298verify that a slot does in fact belong to an 8.3 directory entry by
 299the following:
 300
 301        1) Positioning.  Slots for a file always immediately proceed
 302           their corresponding 8.3 directory entry.  In addition, each
 303           slot has an id which marks its order in the extended file
 304           name.  Here is a very abbreviated view of an 8.3 directory
 305           entry and its corresponding long name slots for the file
 306           "My Big File.Extension which is long":
 307
 308                <proceeding files...>
 309                <slot #3, id = 0x43, characters = "h is long">
 310                <slot #2, id = 0x02, characters = "xtension which">
 311                <slot #1, id = 0x01, characters = "My Big File.E">
 312                <directory entry, name = "MYBIGFIL.EXT">
 313
 314           Note that the slots are stored from last to first.  Slots
 315           are numbered from 1 to N.  The Nth slot is or'ed with 0x40
 316           to mark it as the last one.
 317
 318        2) Checksum.  Each slot has an "alias_checksum" value.  The
 319           checksum is calculated from the 8.3 name using the
 320           following algorithm:
 321
 322                for (sum = i = 0; i < 11; i++) {
 323                        sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
 324                }
 325
 326        3) If there is free space in the final slot, a Unicode NULL (0x0000) 
 327           is stored after the final character.  After that, all unused 
 328           characters in the final slot are set to Unicode 0xFFFF.
 329
 330Finally, note that the extended name is stored in Unicode.  Each Unicode
 331character takes two bytes.
 332
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