path: root/Documentation/filesystems/vfat.txt
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authorAnton Arapov <>2012-04-16 10:05:28 +0200
committerAnton Arapov <>2012-04-16 10:05:28 +0200
commitb4b6116a13633898cf868f2f103c96a90c4c20f8 (patch)
tree93d1b7e2cfcdf473d8d4ff3ad141fa864f8491f6 /Documentation/filesystems/vfat.txt
parentedd4be777c953e5faafc80d091d3084b4343f5d3 (diff)
fedora kernel: d9aad82f3319f3cfd1aebc01234254ef0c37ad84v3.3.2-1
Signed-off-by: Anton Arapov <>
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+To use the vfat filesystem, use the filesystem type 'vfat'. i.e.
+ mount -t vfat /dev/fd0 /mnt
+No special partition formatter is required. mkdosfs will work fine
+if you want to format from within Linux.
+uid=### -- Set the owner of all files on this filesystem.
+ The default is the uid of current process.
+gid=### -- Set the group of all files on this filesystem.
+ The default is the gid of current process.
+umask=### -- The permission mask (for files and directories, see umask(1)).
+ The default is the umask of current process.
+dmask=### -- The permission mask for the directory.
+ The default is the umask of current process.
+fmask=### -- The permission mask for files.
+ The default is the umask of current process.
+allow_utime=### -- This option controls the permission check of mtime/atime.
+ 20 - If current process is in group of file's group ID,
+ you can change timestamp.
+ 2 - Other users can change timestamp.
+ The default is set from `dmask' option. (If the directory is
+ writable, utime(2) is also allowed. I.e. ~dmask & 022)
+ Normally utime(2) checks current process is owner of
+ the file, or it has CAP_FOWNER capability. But FAT
+ filesystem doesn't have uid/gid on disk, so normal
+ check is too unflexible. With this option you can
+ relax it.
+codepage=### -- Sets the codepage number for converting to shortname
+ characters on FAT filesystem.
+ By default, FAT_DEFAULT_CODEPAGE setting is used.
+iocharset=<name> -- Character set to use for converting between the
+ encoding is used for user visible filename and 16 bit
+ Unicode characters. Long filenames are stored on disk
+ in Unicode format, but Unix for the most part doesn't
+ know how to deal with Unicode.
+ By default, FAT_DEFAULT_IOCHARSET setting is used.
+ There is also an option of doing UTF-8 translations
+ with the utf8 option.
+ NOTE: "iocharset=utf8" is not recommended. If unsure,
+ you should consider the following option instead.
+utf8=<bool> -- UTF-8 is the filesystem safe version of Unicode that
+ is used by the console. It can be enabled for the
+ filesystem with this option. If 'uni_xlate' gets set,
+ UTF-8 gets disabled.
+uni_xlate=<bool> -- Translate unhandled Unicode characters to special
+ escaped sequences. This would let you backup and
+ restore filenames that are created with any Unicode
+ characters. Until Linux supports Unicode for real,
+ this gives you an alternative. Without this option,
+ a '?' is used when no translation is possible. The
+ escape character is ':' because it is otherwise
+ illegal on the vfat filesystem. The escape sequence
+ that gets used is ':' and the four digits of hexadecimal
+ unicode.
+nonumtail=<bool> -- When creating 8.3 aliases, normally the alias will
+ end in '~1' or tilde followed by some number. If this
+ option is set, then if the filename is
+ "longfilename.txt" and "longfile.txt" does not
+ currently exist in the directory, 'longfile.txt' will
+ be the short alias instead of 'longfi~1.txt'.
+usefree -- Use the "free clusters" value stored on FSINFO. It'll
+ be used to determine number of free clusters without
+ scanning disk. But it's not used by default, because
+ recent Windows don't update it correctly in some
+ case. If you are sure the "free clusters" on FSINFO is
+ correct, by this option you can avoid scanning disk.
+quiet -- Stops printing certain warning messages.
+check=s|r|n -- Case sensitivity checking setting.
+ s: strict, case sensitive
+ r: relaxed, case insensitive
+ n: normal, default setting, currently case insensitive
+nocase -- This was deprecated for vfat. Use shortname=win95 instead.
+ -- Shortname display/create setting.
+ lower: convert to lowercase for display,
+ emulate the Windows 95 rule for create.
+ win95: emulate the Windows 95 rule for display/create.
+ winnt: emulate the Windows NT rule for display/create.
+ mixed: emulate the Windows NT rule for display,
+ emulate the Windows 95 rule for create.
+ Default setting is `mixed'.
+tz=UTC -- Interpret timestamps as UTC rather than local time.
+ This option disables the conversion of timestamps
+ between local time (as used by Windows on FAT) and UTC
+ (which Linux uses internally). This is particularly
+ useful when mounting devices (like digital cameras)
+ that are set to UTC in order to avoid the pitfalls of
+ local time.
+showexec -- If set, the execute permission bits of the file will be
+ allowed only if the extension part of the name is .EXE,
+ .COM, or .BAT. Not set by default.
+debug -- Can be set, but unused by the current implementation.
+sys_immutable -- If set, ATTR_SYS attribute on FAT is handled as
+ IMMUTABLE flag on Linux. Not set by default.
+flush -- If set, the filesystem will try to flush to disk more
+ early than normal. Not set by default.
+rodir -- FAT has the ATTR_RO (read-only) attribute. On Windows,
+ the ATTR_RO of the directory will just be ignored,
+ and is used only by applications as a flag (e.g. it's set
+ for the customized folder).
+ If you want to use ATTR_RO as read-only flag even for
+ the directory, set this option.
+ -- specify FAT behavior on critical errors: panic, continue
+ without doing anything or remount the partition in
+ read-only mode (default behavior).
+<bool>: 0,1,yes,no,true,false
+* Need to get rid of the raw scanning stuff. Instead, always use
+ a get next directory entry approach. The only thing left that uses
+ raw scanning is the directory renaming code.
+* vfat_valid_longname does not properly checked reserved names.
+* When a volume name is the same as a directory name in the root
+ directory of the filesystem, the directory name sometimes shows
+ up as an empty file.
+* autoconv option does not work correctly.
+If you have trouble with the VFAT filesystem, mail bug reports to Please specify the filename
+and the operation that gave you trouble.
+If you plan to make any modifications to the vfat filesystem, please
+get the test suite that comes with the vfat distribution at
+ people/chaffee/vfat.html
+This tests quite a few parts of the vfat filesystem and additional
+tests for new features or untested features would be appreciated.
+(This documentation was provided by Galen C. Hunt <>
+ and lightly annotated by Gordon Chaffee).
+This document presents a very rough, technical overview of my
+knowledge of the extended FAT file system used in Windows NT 3.5 and
+Windows 95. I don't guarantee that any of the following is correct,
+but it appears to be so.
+The extended FAT file system is almost identical to the FAT
+file system used in DOS versions up to and including 6.223410239847
+:-). The significant change has been the addition of long file names.
+These names support up to 255 characters including spaces and lower
+case characters as opposed to the traditional 8.3 short names.
+Here is the description of the traditional FAT entry in the current
+Windows 95 filesystem:
+ struct directory { // Short 8.3 names
+ unsigned char name[8]; // file name
+ unsigned char ext[3]; // file extension
+ unsigned char attr; // attribute byte
+ unsigned char lcase; // Case for base and extension
+ unsigned char ctime_ms; // Creation time, milliseconds
+ unsigned char ctime[2]; // Creation time
+ unsigned char cdate[2]; // Creation date
+ unsigned char adate[2]; // Last access date
+ unsigned char reserved[2]; // reserved values (ignored)
+ unsigned char time[2]; // time stamp
+ unsigned char date[2]; // date stamp
+ unsigned char start[2]; // starting cluster number
+ unsigned char size[4]; // size of the file
+ };
+The lcase field specifies if the base and/or the extension of an 8.3
+name should be capitalized. This field does not seem to be used by
+Windows 95 but it is used by Windows NT. The case of filenames is not
+completely compatible from Windows NT to Windows 95. It is not completely
+compatible in the reverse direction, however. Filenames that fit in
+the 8.3 namespace and are written on Windows NT to be lowercase will
+show up as uppercase on Windows 95.
+Note that the "start" and "size" values are actually little
+endian integer values. The descriptions of the fields in this
+structure are public knowledge and can be found elsewhere.
+With the extended FAT system, Microsoft has inserted extra
+directory entries for any files with extended names. (Any name which
+legally fits within the old 8.3 encoding scheme does not have extra
+entries.) I call these extra entries slots. Basically, a slot is a
+specially formatted directory entry which holds up to 13 characters of
+a file's extended name. Think of slots as additional labeling for the
+directory entry of the file to which they correspond. Microsoft
+prefers to refer to the 8.3 entry for a file as its alias and the
+extended slot directory entries as the file name.
+The C structure for a slot directory entry follows:
+ struct slot { // Up to 13 characters of a long name
+ unsigned char id; // sequence number for slot
+ unsigned char name0_4[10]; // first 5 characters in name
+ unsigned char attr; // attribute byte
+ unsigned char reserved; // always 0
+ unsigned char alias_checksum; // checksum for 8.3 alias
+ unsigned char name5_10[12]; // 6 more characters in name
+ unsigned char start[2]; // starting cluster number
+ unsigned char name11_12[4]; // last 2 characters in name
+ };
+If the layout of the slots looks a little odd, it's only
+because of Microsoft's efforts to maintain compatibility with old
+software. The slots must be disguised to prevent old software from
+panicking. To this end, a number of measures are taken:
+ 1) The attribute byte for a slot directory entry is always set
+ to 0x0f. This corresponds to an old directory entry with
+ attributes of "hidden", "system", "read-only", and "volume
+ label". Most old software will ignore any directory
+ entries with the "volume label" bit set. Real volume label
+ entries don't have the other three bits set.
+ 2) The starting cluster is always set to 0, an impossible
+ value for a DOS file.
+Because the extended FAT system is backward compatible, it is
+possible for old software to modify directory entries. Measures must
+be taken to ensure the validity of slots. An extended FAT system can
+verify that a slot does in fact belong to an 8.3 directory entry by
+the following:
+ 1) Positioning. Slots for a file always immediately proceed
+ their corresponding 8.3 directory entry. In addition, each
+ slot has an id which marks its order in the extended file
+ name. Here is a very abbreviated view of an 8.3 directory
+ entry and its corresponding long name slots for the file
+ "My Big File.Extension which is long":
+ <proceeding files...>
+ <slot #3, id = 0x43, characters = "h is long">
+ <slot #2, id = 0x02, characters = "xtension whic">
+ <slot #1, id = 0x01, characters = "My Big File.E">
+ <directory entry, name = "MYBIGFIL.EXT">
+ Note that the slots are stored from last to first. Slots
+ are numbered from 1 to N. The Nth slot is or'ed with 0x40
+ to mark it as the last one.
+ 2) Checksum. Each slot has an "alias_checksum" value. The
+ checksum is calculated from the 8.3 name using the
+ following algorithm:
+ for (sum = i = 0; i < 11; i++) {
+ sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
+ }
+ 3) If there is free space in the final slot, a Unicode NULL (0x0000)
+ is stored after the final character. After that, all unused
+ characters in the final slot are set to Unicode 0xFFFF.
+Finally, note that the extended name is stored in Unicode. Each Unicode
+character takes two bytes.