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.
 114
 115showexec      -- 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
 119debug         -- Can be set, but unused by the current implementation.
 120
 121sys_immutable -- If set, ATTR_SYS attribute on FAT is handled as
 122                 IMMUTABLE flag on Linux. Not set by default.
 123
 124flush         -- If set, the filesystem will try to flush to disk more
 125                 early than normal. Not set by default.
 126
 127rodir         -- 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
 135errors=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
 140discard       -- If set, issues discard/TRIM commands to the block
 141                 device when blocks are freed. This is useful for SSD devices
 142                 and sparse/thinly-provisoned LUNs.
 143
 144nfs           -- This option maintains an index (cache) of directory
 145                 inodes by i_logstart which is used by the nfs-related code to
 146                 improve look-ups.
 147
 148                 Enable this only if you want to export the FAT filesystem
 149                 over NFS
 150
 151<bool>: 0,1,yes,no,true,false
 152
 153TODO
 154----------------------------------------------------------------------
 155* Need to get rid of the raw scanning stuff.  Instead, always use
 156  a get next directory entry approach.  The only thing left that uses
 157  raw scanning is the directory renaming code.
 158
 159
 160POSSIBLE PROBLEMS
 161----------------------------------------------------------------------
 162* vfat_valid_longname does not properly checked reserved names.
 163* When a volume name is the same as a directory name in the root
 164  directory of the filesystem, the directory name sometimes shows
 165  up as an empty file.
 166* autoconv option does not work correctly.
 167
 168BUG REPORTS
 169----------------------------------------------------------------------
 170If you have trouble with the VFAT filesystem, mail bug reports to
 171chaffee@bmrc.cs.berkeley.edu.  Please specify the filename
 172and the operation that gave you trouble.
 173
 174TEST SUITE
 175----------------------------------------------------------------------
 176If you plan to make any modifications to the vfat filesystem, please
 177get the test suite that comes with the vfat distribution at
 178
 179  http://web.archive.org/web/*/http://bmrc.berkeley.edu/
 180  people/chaffee/vfat.html
 181
 182This tests quite a few parts of the vfat filesystem and additional
 183tests for new features or untested features would be appreciated.
 184
 185NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
 186----------------------------------------------------------------------
 187(This documentation was provided by Galen C. Hunt <gchunt@cs.rochester.edu>
 188 and lightly annotated by Gordon Chaffee).
 189
 190This document presents a very rough, technical overview of my
 191knowledge of the extended FAT file system used in Windows NT 3.5 and
 192Windows 95.  I don't guarantee that any of the following is correct,
 193but it appears to be so.
 194
 195The extended FAT file system is almost identical to the FAT
 196file system used in DOS versions up to and including 6.223410239847
 197:-).  The significant change has been the addition of long file names.
 198These names support up to 255 characters including spaces and lower
 199case characters as opposed to the traditional 8.3 short names.
 200
 201Here is the description of the traditional FAT entry in the current
 202Windows 95 filesystem:
 203
 204        struct directory { // Short 8.3 names 
 205                unsigned char name[8];          // file name 
 206                unsigned char ext[3];           // file extension 
 207                unsigned char attr;             // attribute byte 
 208                unsigned char lcase;            // Case for base and extension
 209                unsigned char ctime_ms;         // Creation time, milliseconds
 210                unsigned char ctime[2];         // Creation time
 211                unsigned char cdate[2];         // Creation date
 212                unsigned char adate[2];         // Last access date
 213                unsigned char reserved[2];      // reserved values (ignored) 
 214                unsigned char time[2];          // time stamp 
 215                unsigned char date[2];          // date stamp 
 216                unsigned char start[2];         // starting cluster number 
 217                unsigned char size[4];          // size of the file 
 218        };
 219
 220The lcase field specifies if the base and/or the extension of an 8.3
 221name should be capitalized.  This field does not seem to be used by
 222Windows 95 but it is used by Windows NT.  The case of filenames is not
 223completely compatible from Windows NT to Windows 95.  It is not completely
 224compatible in the reverse direction, however.  Filenames that fit in
 225the 8.3 namespace and are written on Windows NT to be lowercase will
 226show up as uppercase on Windows 95.
 227
 228Note that the "start" and "size" values are actually little
 229endian integer values.  The descriptions of the fields in this
 230structure are public knowledge and can be found elsewhere.
 231
 232With the extended FAT system, Microsoft has inserted extra
 233directory entries for any files with extended names.  (Any name which
 234legally fits within the old 8.3 encoding scheme does not have extra
 235entries.)  I call these extra entries slots.  Basically, a slot is a
 236specially formatted directory entry which holds up to 13 characters of
 237a file's extended name.  Think of slots as additional labeling for the
 238directory entry of the file to which they correspond.  Microsoft
 239prefers to refer to the 8.3 entry for a file as its alias and the
 240extended slot directory entries as the file name. 
 241
 242The C structure for a slot directory entry follows:
 243
 244        struct slot { // Up to 13 characters of a long name 
 245                unsigned char id;               // sequence number for slot 
 246                unsigned char name0_4[10];      // first 5 characters in name 
 247                unsigned char attr;             // attribute byte
 248                unsigned char reserved;         // always 0 
 249                unsigned char alias_checksum;   // checksum for 8.3 alias 
 250                unsigned char name5_10[12];     // 6 more characters in name
 251                unsigned char start[2];         // starting cluster number
 252                unsigned char name11_12[4];     // last 2 characters in name
 253        };
 254
 255If the layout of the slots looks a little odd, it's only
 256because of Microsoft's efforts to maintain compatibility with old
 257software.  The slots must be disguised to prevent old software from
 258panicking.  To this end, a number of measures are taken:
 259
 260        1) The attribute byte for a slot directory entry is always set
 261           to 0x0f.  This corresponds to an old directory entry with
 262           attributes of "hidden", "system", "read-only", and "volume
 263           label".  Most old software will ignore any directory
 264           entries with the "volume label" bit set.  Real volume label
 265           entries don't have the other three bits set.
 266
 267        2) The starting cluster is always set to 0, an impossible
 268           value for a DOS file.
 269
 270Because the extended FAT system is backward compatible, it is
 271possible for old software to modify directory entries.  Measures must
 272be taken to ensure the validity of slots.  An extended FAT system can
 273verify that a slot does in fact belong to an 8.3 directory entry by
 274the following:
 275
 276        1) Positioning.  Slots for a file always immediately proceed
 277           their corresponding 8.3 directory entry.  In addition, each
 278           slot has an id which marks its order in the extended file
 279           name.  Here is a very abbreviated view of an 8.3 directory
 280           entry and its corresponding long name slots for the file
 281           "My Big File.Extension which is long":
 282
 283                <proceeding files...>
 284                <slot #3, id = 0x43, characters = "h is long">
 285                <slot #2, id = 0x02, characters = "xtension whic">
 286                <slot #1, id = 0x01, characters = "My Big File.E">
 287                <directory entry, name = "MYBIGFIL.EXT">
 288
 289           Note that the slots are stored from last to first.  Slots
 290           are numbered from 1 to N.  The Nth slot is or'ed with 0x40
 291           to mark it as the last one.
 292
 293        2) Checksum.  Each slot has an "alias_checksum" value.  The
 294           checksum is calculated from the 8.3 name using the
 295           following algorithm:
 296
 297                for (sum = i = 0; i < 11; i++) {
 298                        sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
 299                }
 300
 301        3) If there is free space in the final slot, a Unicode NULL (0x0000) 
 302           is stored after the final character.  After that, all unused 
 303           characters in the final slot are set to Unicode 0xFFFF.
 304
 305Finally, note that the extended name is stored in Unicode.  Each Unicode
 306character takes two bytes.
 307
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