tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
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linux
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----------------------------------------------------------------------
11umask=### -- The permission mask (for files and directories, see umask(1)).
12 The default is the umask of current process.
13
14dmask=### -- The permission mask for the directory.
15 The default is the umask of current process.
16
17fmask=### -- The permission mask for files.
18 The default is the umask of current process.
19
20codepage=### -- Sets the codepage number for converting to shortname
21 characters on FAT filesystem.
22 By default, FAT_DEFAULT_CODEPAGE setting is used.
23
24iocharset=name -- Character set to use for converting between the
25 encoding is used for user visible filename and 16 bit
26 Unicode characters. Long filenames are stored on disk
27 in Unicode format, but Unix for the most part doesn't
28 know how to deal with Unicode.
29 By default, FAT_DEFAULT_IOCHARSET setting is used.
30
31 There is also an option of doing UTF-8 translations
32 with the utf8 option.
33
34 NOTE: "iocharset=utf8" is not recommended. If unsure,
35 you should consider the following option instead.
36
37utf8=<bool> -- UTF-8 is the filesystem safe version of Unicode that
38 is used by the console. It can be enabled for the
39 filesystem with this option. If 'uni_xlate' gets set,
40 UTF-8 gets disabled.
41
42uni_xlate=<bool> -- Translate unhandled Unicode characters to special
43 escaped sequences. This would let you backup and
44 restore filenames that are created with any Unicode
45 characters. Until Linux supports Unicode for real,
46 this gives you an alternative. Without this option,
47 a '?' is used when no translation is possible. The
48 escape character is ':' because it is otherwise
49 illegal on the vfat filesystem. The escape sequence
50 that gets used is ':' and the four digits of hexadecimal
51 unicode.
52
53nonumtail=<bool> -- When creating 8.3 aliases, normally the alias will
54 end in '~1' or tilde followed by some number. If this
55 option is set, then if the filename is
56 "longfilename.txt" and "longfile.txt" does not
57 currently exist in the directory, 'longfile.txt' will
58 be the short alias instead of 'longfi~1.txt'.
59
60usefree -- Use the "free clusters" value stored on FSINFO. It'll
61 be used to determine number of free clusters without
62 scanning disk. But it's not used by default, because
63 recent Windows don't update it correctly in some
64 case. If you are sure the "free clusters" on FSINFO is
65 correct, by this option you can avoid scanning disk.
66
67quiet -- Stops printing certain warning messages.
68
69check=s|r|n -- Case sensitivity checking setting.
70 s: strict, case sensitive
71 r: relaxed, case insensitive
72 n: normal, default setting, currently case insensitive
73
74shortname=lower|win95|winnt|mixed
75 -- Shortname display/create setting.
76 lower: convert to lowercase for display,
77 emulate the Windows 95 rule for create.
78 win95: emulate the Windows 95 rule for display/create.
79 winnt: emulate the Windows NT rule for display/create.
80 mixed: emulate the Windows NT rule for display,
81 emulate the Windows 95 rule for create.
82 Default setting is `lower'.
83
84<bool>: 0,1,yes,no,true,false
85
86TODO
87----------------------------------------------------------------------
88* Need to get rid of the raw scanning stuff. Instead, always use
89 a get next directory entry approach. The only thing left that uses
90 raw scanning is the directory renaming code.
91
92
93POSSIBLE PROBLEMS
94----------------------------------------------------------------------
95* vfat_valid_longname does not properly checked reserved names.
96* When a volume name is the same as a directory name in the root
97 directory of the filesystem, the directory name sometimes shows
98 up as an empty file.
99* autoconv option does not work correctly.
100
101BUG REPORTS
102----------------------------------------------------------------------
103If you have trouble with the VFAT filesystem, mail bug reports to
104chaffee@bmrc.cs.berkeley.edu. Please specify the filename
105and the operation that gave you trouble.
106
107TEST SUITE
108----------------------------------------------------------------------
109If you plan to make any modifications to the vfat filesystem, please
110get the test suite that comes with the vfat distribution at
111
112 http://bmrc.berkeley.edu/people/chaffee/vfat.html
113
114This tests quite a few parts of the vfat filesystem and additional
115tests for new features or untested features would be appreciated.
116
117NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
118----------------------------------------------------------------------
119(This documentation was provided by Galen C. Hunt <gchunt@cs.rochester.edu>
120 and lightly annotated by Gordon Chaffee).
121
122This document presents a very rough, technical overview of my
123knowledge of the extended FAT file system used in Windows NT 3.5 and
124Windows 95. I don't guarantee that any of the following is correct,
125but it appears to be so.
126
127The extended FAT file system is almost identical to the FAT
128file system used in DOS versions up to and including 6.223410239847
129:-). The significant change has been the addition of long file names.
130These names support up to 255 characters including spaces and lower
131case characters as opposed to the traditional 8.3 short names.
132
133Here is the description of the traditional FAT entry in the current
134Windows 95 filesystem:
135
136 struct directory { // Short 8.3 names
137 unsigned char name[8]; // file name
138 unsigned char ext[3]; // file extension
139 unsigned char attr; // attribute byte
140 unsigned char lcase; // Case for base and extension
141 unsigned char ctime_ms; // Creation time, milliseconds
142 unsigned char ctime[2]; // Creation time
143 unsigned char cdate[2]; // Creation date
144 unsigned char adate[2]; // Last access date
145 unsigned char reserved[2]; // reserved values (ignored)
146 unsigned char time[2]; // time stamp
147 unsigned char date[2]; // date stamp
148 unsigned char start[2]; // starting cluster number
149 unsigned char size[4]; // size of the file
150 };
151
152The lcase field specifies if the base and/or the extension of an 8.3
153name should be capitalized. This field does not seem to be used by
154Windows 95 but it is used by Windows NT. The case of filenames is not
155completely compatible from Windows NT to Windows 95. It is not completely
156compatible in the reverse direction, however. Filenames that fit in
157the 8.3 namespace and are written on Windows NT to be lowercase will
158show up as uppercase on Windows 95.
159
160Note that the "start" and "size" values are actually little
161endian integer values. The descriptions of the fields in this
162structure are public knowledge and can be found elsewhere.
163
164With the extended FAT system, Microsoft has inserted extra
165directory entries for any files with extended names. (Any name which
166legally fits within the old 8.3 encoding scheme does not have extra
167entries.) I call these extra entries slots. Basically, a slot is a
168specially formatted directory entry which holds up to 13 characters of
169a file's extended name. Think of slots as additional labeling for the
170directory entry of the file to which they correspond. Microsoft
171prefers to refer to the 8.3 entry for a file as its alias and the
172extended slot directory entries as the file name.
173
174The C structure for a slot directory entry follows:
175
176 struct slot { // Up to 13 characters of a long name
177 unsigned char id; // sequence number for slot
178 unsigned char name0_4[10]; // first 5 characters in name
179 unsigned char attr; // attribute byte
180 unsigned char reserved; // always 0
181 unsigned char alias_checksum; // checksum for 8.3 alias
182 unsigned char name5_10[12]; // 6 more characters in name
183 unsigned char start[2]; // starting cluster number
184 unsigned char name11_12[4]; // last 2 characters in name
185 };
186
187If the layout of the slots looks a little odd, it's only
188because of Microsoft's efforts to maintain compatibility with old
189software. The slots must be disguised to prevent old software from
190panicking. To this end, a number of measures are taken:
191
192 1) The attribute byte for a slot directory entry is always set
193 to 0x0f. This corresponds to an old directory entry with
194 attributes of "hidden", "system", "read-only", and "volume
195 label". Most old software will ignore any directory
196 entries with the "volume label" bit set. Real volume label
197 entries don't have the other three bits set.
198
199 2) The starting cluster is always set to 0, an impossible
200 value for a DOS file.
201
202Because the extended FAT system is backward compatible, it is
203possible for old software to modify directory entries. Measures must
204be taken to ensure the validity of slots. An extended FAT system can
205verify that a slot does in fact belong to an 8.3 directory entry by
206the following:
207
208 1) Positioning. Slots for a file always immediately proceed
209 their corresponding 8.3 directory entry. In addition, each
210 slot has an id which marks its order in the extended file
211 name. Here is a very abbreviated view of an 8.3 directory
212 entry and its corresponding long name slots for the file
213 "My Big File.Extension which is long":
214
215 <proceeding files...>
216 <slot #3, id = 0x43, characters = "h is long">
217 <slot #2, id = 0x02, characters = "xtension whic">
218 <slot #1, id = 0x01, characters = "My Big File.E">
219 <directory entry, name = "MYBIGFIL.EXT">
220
221 Note that the slots are stored from last to first. Slots
222 are numbered from 1 to N. The Nth slot is or'ed with 0x40
223 to mark it as the last one.
224
225 2) Checksum. Each slot has an "alias_checksum" value. The
226 checksum is calculated from the 8.3 name using the
227 following algorithm:
228
229 for (sum = i = 0; i < 11; i++) {
230 sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
231 }
232
233 3) If there is free space in the final slot, a Unicode NULL (0x0000)
234 is stored after the final character. After that, all unused
235 characters in the final slot are set to Unicode 0xFFFF.
236
237Finally, note that the extended name is stored in Unicode. Each Unicode
238character takes two bytes.