VFAT

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, seeumask(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 iswritable, utime(2) is also allowed. i.e. ~dmask & 022.

Normally utime(2) checks current process is owner ofthe file, or it has CAP_FOWNER capability. But FATfilesystem doesn’t have uid/gid on disk, so normalcheck is too unflexible. With this option you canrelax it.

codepage=###
Sets the codepage number for converting to shortnamecharacters on FAT filesystem.By default, FAT_DEFAULT_CODEPAGE setting is used.
iocharset=<name>

Character set to use for converting between theencoding is used for user visible filename and 16 bitUnicode characters. Long filenames are stored on diskin Unicode format, but Unix for the most part doesn’tknow how to deal with Unicode.By default, FAT_DEFAULT_IOCHARSET setting is used.

There is also an option of doing UTF-8 translationswith the utf8 option.

Note

iocharset=utf8 is not recommended. If unsure, you should considerthe utf8 option instead.

utf8=<bool>
UTF-8 is the filesystem safe version of Unicode thatis used by the console. It can be enabled or disabledfor the filesystem with this option.If ‘uni_xlate’ gets set, UTF-8 gets disabled.By default, FAT_DEFAULT_UTF8 setting is used.
uni_xlate=<bool>
Translate unhandled Unicode characters to specialescaped sequences. This would let you backup andrestore filenames that are created with any Unicodecharacters. Until Linux supports Unicode for real,this gives you an alternative. Without this option,a ‘?’ is used when no translation is possible. Theescape character is ‘:’ because it is otherwiseillegal on the vfat filesystem. The escape sequencethat gets used is ‘:’ and the four digits of hexadecimalunicode.
nonumtail=<bool>
When creating 8.3 aliases, normally the alias willend in ‘~1’ or tilde followed by some number. If thisoption is set, then if the filename is“longfilename.txt” and “longfile.txt” does notcurrently exist in the directory, longfile.txt willbe the short alias instead of longfi~1.txt.
usefree
Use the “free clusters” value stored on FSINFO. It willbe used to determine number of free clusters withoutscanning disk. But it’s not used by default, becauserecent Windows don’t update it correctly in somecase. If you are sure the “free clusters” on FSINFO iscorrect, 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. Useshortname=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 ismixed.

tz=UTC
Interpret timestamps as UTC rather than local time.This option disables the conversion of timestampsbetween local time (as used by Windows on FAT) and UTC(which Linux uses internally). This is particularlyuseful when mounting devices (like digital cameras)that are set to UTC in order to avoid the pitfalls oflocal time.
time_offset=minutes
Set offset for conversion of timestamps from local timeused by FAT to UTC. I.e. <minutes> minutes will be subtractedfrom each timestamp to convert it to UTC used internally byLinux. This is useful when time zone set insys_tz isnot the time zone used by the filesystem. Note that thisoption still does not provide correct time stamps in allcases in presence of DST - time stamps in a different DSTsetting will be off by one hour.
showexec
If set, the execute permission bits of the file will beallowed 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 asIMMUTABLE flag on Linux. Not set by default.
flush
If set, the filesystem will try to flush to disk moreearly 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 setfor the customized folder).

If you want to use ATTR_RO as read-only flag even forthe directory, set this option.

errors=panic|continue|remount-ro
specify FAT behavior on critical errors: panic, continuewithout doing anything or remount the partition inread-only mode (default behavior).
discard
If set, issues discard/TRIM commands to the blockdevice when blocks are freed. This is useful for SSD devicesand sparse/thinly-provisoned LUNs.
nfs=stale_rw|nostale_ro

Enable this only if you want to export the FAT filesystemover NFS.

stale_rw: This option maintains an index (cache) of directoryinodes byi_logstart which is used by the nfs-related code toimprove look-ups. Full file operations (read/write) over NFS issupported but with cache eviction at NFS server, this couldresult in ESTALE issues.

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

To maintain backward compatibility,'-onfs' is also accepted,defaulting to “stale_rw”.

dos1xfloppy <bool>: 0,1,yes,no,true,false
If set, use a fallback default BIOS Parameter Blockconfiguration, determined by backing device size. These staticparameters match defaults assumed by DOS 1.x for 160 kiB,180 kiB, 320 kiB, and 360 kiB floppies and floppy images.

LIMITATION

The fallocated region of file is discarded at umount/evict timewhen using fallocate with FALLOC_FL_KEEP_SIZE.So, User should assume that fallocated region can be discarded atlast close if there is memory pressure resulting in eviction ofthe inode from the memory. As a result, for any dependency onthe fallocated region, user should make sure to recheck fallocateafter reopening the file.

TODO

Need to get rid of the raw scanning stuff. Instead, always usea get next directory entry approach. The only thing left that usesraw 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 rootdirectory of the filesystem, the directory name sometimes showsup as an empty file.
  • autoconv option does not work correctly.

TEST SUITE

If you plan to make any modifications to the vfat filesystem, pleaseget 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 additionaltests for new features or untested features would be appreciated.

NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM

This documentation was provided by Galen C. Huntgchunt@cs.rochester.edu andlightly annotated by Gordon Chaffee.

This document presents a very rough, technical overview of myknowledge of the extended FAT file system used in Windows NT 3.5 andWindows 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 FATfile system used in DOS versions up to and including6.223410239847:-). The significant change has been the addition of long file names.These names support up to 255 characters including spaces and lowercase characters as opposed to the traditional 8.3 short names.

Here is the description of the traditional FAT entry in the currentWindows 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 8.3name should be capitalized. This field does not seem to be used byWindows 95 but it is used by Windows NT. The case of filenames is notcompletely compatible from Windows NT to Windows 95. It is not completelycompatible in the reverse direction, however. Filenames that fit inthe 8.3 namespace and are written on Windows NT to be lowercase willshow up as uppercase on Windows 95.

Note

Note that thestart andsize values are actually littleendian integer values. The descriptions of the fields in thisstructure are public knowledge and can be found elsewhere.

With the extended FAT system, Microsoft has inserted extradirectory entries for any files with extended names. (Any name whichlegally fits within the old 8.3 encoding scheme does not have extraentries.) I call these extra entries slots. Basically, a slot is aspecially formatted directory entry which holds up to 13 characters ofa file’s extended name. Think of slots as additional labeling for thedirectory entry of the file to which they correspond. Microsoftprefers to refer to the 8.3 entry for a file as its alias and theextended 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 onlybecause of Microsoft’s efforts to maintain compatibility with oldsoftware. The slots must be disguised to prevent old software frompanicking. To this end, a number of measures are taken:

  1. The attribute byte for a slot directory entry is always setto 0x0f. This corresponds to an old directory entry withattributes of “hidden”, “system”, “read-only”, and “volumelabel”. Most old software will ignore any directoryentries with the “volume label” bit set. Real volume labelentries don’t have the other three bits set.
  2. The starting cluster is always set to 0, an impossiblevalue for a DOS file.

Because the extended FAT system is backward compatible, it ispossible for old software to modify directory entries. Measures mustbe taken to ensure the validity of slots. An extended FAT system canverify that a slot does in fact belong to an 8.3 directory entry bythe following:

  1. Positioning. Slots for a file always immediately proceedtheir corresponding 8.3 directory entry. In addition, eachslot has an id which marks its order in the extended filename. Here is a very abbreviated view of an 8.3 directoryentry 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

    Note that the slots are stored from last to first. Slotsare numbered from 1 to N. The Nth slot isor'ed with0x40 to mark it as the last one.

  2. Checksum. Each slot has an alias_checksum value. Thechecksum is calculated from the 8.3 name using thefollowing 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 UnicodeNULL(0x0000)is stored after the final character. After that, all unusedcharacters in the final slot are set to Unicode 0xFFFF.

Finally, note that the extended name is stored in Unicode. Each Unicodecharacter takes either two or four bytes, UTF-16LE encoded.