tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
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 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
60quiet -- Stops printing certain warning messages.
61
62check=s|r|n -- Case sensitivity checking setting.
63 s: strict, case sensitive
64 r: relaxed, case insensitive
65 n: normal, default setting, currently case insensitive
66
67shortname=lower|win95|winnt|mixed
68 -- Shortname display/create setting.
69 lower: convert to lowercase for display,
70 emulate the Windows 95 rule for create.
71 win95: emulate the Windows 95 rule for display/create.
72 winnt: emulate the Windows NT rule for display/create.
73 mixed: emulate the Windows NT rule for display,
74 emulate the Windows 95 rule for create.
75 Default setting is `lower'.
76
77<bool>: 0,1,yes,no,true,false
78
79TODO
80----------------------------------------------------------------------
81* Need to get rid of the raw scanning stuff. Instead, always use
82 a get next directory entry approach. The only thing left that uses
83 raw scanning is the directory renaming code.
84
85
86POSSIBLE PROBLEMS
87----------------------------------------------------------------------
88* vfat_valid_longname does not properly checked reserved names.
89* When a volume name is the same as a directory name in the root
90 directory of the filesystem, the directory name sometimes shows
91 up as an empty file.
92* autoconv option does not work correctly.
93
94BUG REPORTS
95----------------------------------------------------------------------
96If you have trouble with the VFAT filesystem, mail bug reports to
97chaffee@bmrc.cs.berkeley.edu. Please specify the filename
98and the operation that gave you trouble.
99
100TEST SUITE
101----------------------------------------------------------------------
102If you plan to make any modifications to the vfat filesystem, please
103get the test suite that comes with the vfat distribution at
104
105 http://bmrc.berkeley.edu/people/chaffee/vfat.html
106
107This tests quite a few parts of the vfat filesystem and additional
108tests for new features or untested features would be appreciated.
109
110NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
111----------------------------------------------------------------------
112(This documentation was provided by Galen C. Hunt <gchunt@cs.rochester.edu>
113 and lightly annotated by Gordon Chaffee).
114
115This document presents a very rough, technical overview of my
116knowledge of the extended FAT file system used in Windows NT 3.5 and
117Windows 95. I don't guarantee that any of the following is correct,
118but it appears to be so.
119
120The extended FAT file system is almost identical to the FAT
121file system used in DOS versions up to and including 6.223410239847
122:-). The significant change has been the addition of long file names.
123These names support up to 255 characters including spaces and lower
124case characters as opposed to the traditional 8.3 short names.
125
126Here is the description of the traditional FAT entry in the current
127Windows 95 filesystem:
128
129 struct directory { // Short 8.3 names
130 unsigned char name[8]; // file name
131 unsigned char ext[3]; // file extension
132 unsigned char attr; // attribute byte
133 unsigned char lcase; // Case for base and extension
134 unsigned char ctime_ms; // Creation time, milliseconds
135 unsigned char ctime[2]; // Creation time
136 unsigned char cdate[2]; // Creation date
137 unsigned char adate[2]; // Last access date
138 unsigned char reserved[2]; // reserved values (ignored)
139 unsigned char time[2]; // time stamp
140 unsigned char date[2]; // date stamp
141 unsigned char start[2]; // starting cluster number
142 unsigned char size[4]; // size of the file
143 };
144
145The lcase field specifies if the base and/or the extension of an 8.3
146name should be capitalized. This field does not seem to be used by
147Windows 95 but it is used by Windows NT. The case of filenames is not
148completely compatible from Windows NT to Windows 95. It is not completely
149compatible in the reverse direction, however. Filenames that fit in
150the 8.3 namespace and are written on Windows NT to be lowercase will
151show up as uppercase on Windows 95.
152
153Note that the "start" and "size" values are actually little
154endian integer values. The descriptions of the fields in this
155structure are public knowledge and can be found elsewhere.
156
157With the extended FAT system, Microsoft has inserted extra
158directory entries for any files with extended names. (Any name which
159legally fits within the old 8.3 encoding scheme does not have extra
160entries.) I call these extra entries slots. Basically, a slot is a
161specially formatted directory entry which holds up to 13 characters of
162a file's extended name. Think of slots as additional labeling for the
163directory entry of the file to which they correspond. Microsoft
164prefers to refer to the 8.3 entry for a file as its alias and the
165extended slot directory entries as the file name.
166
167The C structure for a slot directory entry follows:
168
169 struct slot { // Up to 13 characters of a long name
170 unsigned char id; // sequence number for slot
171 unsigned char name0_4[10]; // first 5 characters in name
172 unsigned char attr; // attribute byte
173 unsigned char reserved; // always 0
174 unsigned char alias_checksum; // checksum for 8.3 alias
175 unsigned char name5_10[12]; // 6 more characters in name
176 unsigned char start[2]; // starting cluster number
177 unsigned char name11_12[4]; // last 2 characters in name
178 };
179
180If the layout of the slots looks a little odd, it's only
181because of Microsoft's efforts to maintain compatibility with old
182software. The slots must be disguised to prevent old software from
183panicking. To this end, a number of measures are taken:
184
185 1) The attribute byte for a slot directory entry is always set
186 to 0x0f. This corresponds to an old directory entry with
187 attributes of "hidden", "system", "read-only", and "volume
188 label". Most old software will ignore any directory
189 entries with the "volume label" bit set. Real volume label
190 entries don't have the other three bits set.
191
192 2) The starting cluster is always set to 0, an impossible
193 value for a DOS file.
194
195Because the extended FAT system is backward compatible, it is
196possible for old software to modify directory entries. Measures must
197be taken to ensure the validity of slots. An extended FAT system can
198verify that a slot does in fact belong to an 8.3 directory entry by
199the following:
200
201 1) Positioning. Slots for a file always immediately proceed
202 their corresponding 8.3 directory entry. In addition, each
203 slot has an id which marks its order in the extended file
204 name. Here is a very abbreviated view of an 8.3 directory
205 entry and its corresponding long name slots for the file
206 "My Big File.Extension which is long":
207
208 <proceeding files...>
209 <slot #3, id = 0x43, characters = "h is long">
210 <slot #2, id = 0x02, characters = "xtension whic">
211 <slot #1, id = 0x01, characters = "My Big File.E">
212 <directory entry, name = "MYBIGFIL.EXT">
213
214 Note that the slots are stored from last to first. Slots
215 are numbered from 1 to N. The Nth slot is or'ed with 0x40
216 to mark it as the last one.
217
218 2) Checksum. Each slot has an "alias_checksum" value. The
219 checksum is calculated from the 8.3 name using the
220 following algorithm:
221
222 for (sum = i = 0; i < 11; i++) {
223 sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
224 }
225
226 3) If there is free space in the final slot, a Unicode NULL (0x0000)
227 is stored after the final character. After that, all unused
228 characters in the final slot are set to Unicode 0xFFFF.
229
230Finally, note that the extended name is stored in Unicode. Each Unicode
231character takes two bytes.