fs/fat: comment fix, fat_bits can be also 32

[linux-2.6-block.git] / Documentation / filesystems / vfat.txt

USING VFAT
----------------------------------------------------------------------
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.

VFAT MOUNT OPTIONS
----------------------------------------------------------------------
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=lower|win95|winnt|mixed
	      -- 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.
time_offset=minutes
	      -- Set offset for conversion of timestamps from local time
		 used by FAT to UTC. I.e. <minutes> minutes will be subtracted
		 from each timestamp to convert it to UTC used internally by
		 Linux. This is useful when time zone set in sys_tz is
		 not the time zone used by the filesystem. Note that this
		 option still does not provide correct time stamps in all
		 cases in presence of DST - time stamps in a different DST
		 setting will be off by one hour.

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.

errors=panic|continue|remount-ro
	      -- specify FAT behavior on critical errors: panic, continue
		 without doing anything or remount the partition in
		 read-only mode (default behavior).

discard       -- If set, issues discard/TRIM commands to the block
		 device when blocks are freed. This is useful for SSD devices
		 and sparse/thinly-provisoned LUNs.

nfs=stale_rw|nostale_ro
		Enable this only if you want to export the FAT filesystem
		over NFS.

		stale_rw: This option maintains an index (cache) of directory
		inodes by i_logstart which is used by the nfs-related code to
		improve look-ups. Full file operations (read/write) over NFS is
		supported but with cache eviction at NFS server, this could
		result in ESTALE issues.

		nostale_ro: This option bases the inode number and filehandle
		on the on-disk location of a file in the MS-DOS directory entry.
		This ensures that ESTALE will not be returned after a file is
		evicted from the inode cache. However, it means that operations
		such as rename, create and unlink could cause filehandles that
		previously pointed at one file to point at a different file,
		potentially causing data corruption. For this reason, this
		option also mounts the filesystem readonly.

		To maintain backward compatibility, '-o nfs' is also accepted,
		defaulting to stale_rw

dos1xfloppy  -- If set, use a fallback default BIOS Parameter Block
		configuration, determined by backing device size. These static
		parameters match defaults assumed by DOS 1.x for 160 kiB,
		180 kiB, 320 kiB, and 360 kiB floppies and floppy images.


<bool>: 0,1,yes,no,true,false

TODO
----------------------------------------------------------------------
* 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.


POSSIBLE PROBLEMS
----------------------------------------------------------------------
* 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.

BUG REPORTS
----------------------------------------------------------------------
If you have trouble with the VFAT filesystem, mail bug reports to
chaffee@bmrc.cs.berkeley.edu.  Please specify the filename
and the operation that gave you trouble.

TEST SUITE
----------------------------------------------------------------------
If you plan to make any modifications to the vfat filesystem, please
get the test suite that comes with the vfat distribution at

  http://web.archive.org/web/*/http://bmrc.berkeley.edu/
  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.

NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
----------------------------------------------------------------------
(This documentation was provided by Galen C. Hunt <gchunt@cs.rochester.edu>
 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.