Documentation/filesystems/vfat.txt - third_party/linux - Git at Google

 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
 ----------------------------------------------------------------------
 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.

 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

 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 `lower'.

 <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://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.
	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
	----------------------------------------------------------------------
	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.

	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

	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 `lower'.

	<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://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.