1 This is the main documentation for the CVF-FAT filesystem extension. 18Nov1998
9 4. Description of the CVF-FAT interface
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15 1. The idea of CVF-FAT
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18 CVF-FAT is a FAT filesystem extension that provides a generic interface for
19 Compressed Volume Files in FAT partitions. Popular CVF software, for
20 example, are Microsoft's Doublespace/Drivespace and Stac's Stacker.
21 Using the CVF-FAT interface, it is possible to load a module that handles
22 all the low-level disk access that has to do with on-the-fly compression
23 and decompression. Any other part of FAT filesystem access is still handled
24 by the FAT, MSDOS or VFAT or even UMSDOS driver.
26 CVF access works by redirecting certain low-level routines from the FAT
27 driver to a loadable, CVF-format specific module. This module must fake
28 a normal FAT filesystem to the FAT driver while doing all the extra stuff
29 like compression and decompression silently.
33 ------------------------------------------------------------------------------
37 CVF filesystems cannot do bmap. It's impossible in principle. Thus
38 all actions that require bmap do not work (swapping, writable mmapping).
39 Read-only mmapping works because the FAT driver has a hack for this
40 situation :) Well, writable mmapping should now work using the readpage
41 interface function which has been hacked into the FAT driver just for
44 - attention, DOSEmu users
46 You may have to unmount all CVF partitions before running DOSEmu depending
47 on your configuration. If DOSEmu is configured to use wholedisk or
48 partition access (this is often the case to let DOSEmu access
49 compressed partitions) there's a risk of destroying your compressed
50 partitions or crashing your system because of confused drivers.
52 Note that it is always safe to redirect the compressed partitions with
53 lredir or emufs.sys. Refer to the DOSEmu documentation for details.
57 ------------------------------------------------------------------------------
59 The CVF-FAT extension currently adds the following options to the FAT
60 driver's standard options:
63 Forces the driver to use the CVF module "xxx" instead of auto-detection.
64 Without this option, the CVF-FAT interface asks all currently loaded
65 CVF modules whether they recognize the CVF. Therefore, this option is
66 only necessary if the CVF format is not recognized correctly
67 because of bugs or incompatibilities in the CVF modules. (It skips
68 the detect_cvf call.) "xxx" may be the text "none" (without the quotes)
69 to inhibit using any of the loaded CVF modules, just in case a CVF
70 module insists on mounting plain FAT filesystems by misunderstanding.
71 "xxx" may also be the text "autoload", which has a special meaning for
72 a module loader, but does not skip auto-detection.
74 If the kernel supports kmod, the cvf_format=xxx option also controls
75 on-demand CVF module loading. Without this option, nothing is loaded
76 on demand. With cvf_format=xxx, a module "xxx" is requested automatically
77 before mounting the compressed filesystem (unless "xxx" is "none"). In
78 case there is a difference between the CVF format name and the module
79 name, setup aliases in your modules configuration. If the string "xxx"
80 is "autoload", a non-existent module "cvf_autoload" is requested which
81 can be used together with a special modules configuration (alias and
82 pre-install statements) in order to load more than one CVF module, let
83 them detect automatically which kind of CVF is to be mounted, and only
84 keep the "right" module in memory. For examples please refer to the
85 dmsdos documentation (ftp and http addresses see below).
88 Option string passed to the CVF module. I.e. only the "yyy" is passed
89 (without the quotes). The documentation for each CVF module should
90 explain it since it is interpreted only by the CVF module. Note that
91 the string must not contain a comma (",") - this would lead to
92 misinterpretation by the FAT driver, which would recognize the text
93 after a comma as a FAT driver option and might get confused or print
94 strange error messages. The documentation for the CVF module should
95 offer a different separation symbol, for example the dot "." or the
96 plus sign "+", which is only valid inside the string "yyy".
99 4. Description of the CVF-FAT interface
100 ------------------------------------------------------------------------------
102 Assuming you want to write your own CVF module, you need to write a lot of
103 interface functions. Most of them are covered in the kernel documentation
104 you can find on the net, and thus won't be described here. They have been
105 marked with "[...]" :-) Take a look at include/linux/fat_cvf.h.
109 char* cvf_version_text;
110 unsigned long int flags;
111 int (*detect_cvf) (struct super_block*sb);
112 int (*mount_cvf) (struct super_block*sb,char*options);
113 int (*unmount_cvf) (struct super_block*sb);
115 void (*zero_out_cluster) (struct inode*, int clusternr);
118 This structure defines the capabilities of a CVF module. It must be filled
119 out completely by a CVF module. Consider it as a kind of form that is used
120 to introduce the module to the FAT/CVF-FAT driver.
124 A version id which must be unique. Choose one.
126 A human readable version string that should be one short word
127 describing the CVF format the module implements. This text is used
128 for the cvf_format option. This name must also be unique.
130 Bit coded flags, currently only used for a readpage/mmap hack that
131 provides both mmap and readpage functionality. If CVF_USE_READPAGE
132 is set, mmap is set to generic_file_mmap and readpage is caught
133 and redirected to the cvf_readpage function. If it is not set,
134 readpage is set to generic_readpage and mmap is caught and redirected
135 to cvf_mmap. (If you want writable mmap use the readpage interface.)
137 A function that is called to decide whether the filesystem is a CVF of
138 the type the module supports. The detect_cvf function must return 0
139 for "NO, I DON'T KNOW THIS GARBAGE" or anything >0 for "YES, THIS IS
140 THE KIND OF CVF I SUPPORT". The function must maintain the module
141 usage counters for safety, i.e. do MOD_INC_USE_COUNT at the beginning
142 and MOD_DEC_USE_COUNT at the end. The function *must not* assume that
143 successful recognition would lead to a call of the mount_cvf function
146 A function that sets up some values or initializes something additional
147 to what has to be done when a CVF is mounted. This is called at the
148 end of fat_read_super and must return 0 on success. Definitely, this
149 function must increment the module usage counter by MOD_INC_USE_COUNT.
150 This mount_cvf function is also responsible for interpreting a CVF
151 module specific option string (the "yyy" from the FAT mount option
152 "cvf_options=yyy") which cannot contain a comma (use for example the
153 dot "." as option separator symbol).
155 A function that is called when the filesystem is unmounted. Most likely
156 it only frees up some memory and calls MOD_DEC_USE_COUNT. The return
157 value might be ignored (it currently is ignored).
159 All other interface functions are "caught" FAT driver functions, i.e.
160 are executed by the FAT driver *instead* of the original FAT driver
161 functions. NULL means use the original FAT driver functions instead.
162 If you really want "no action", write a function that does nothing and
165 The zero_out_cluster function is called when the fat driver wants to
166 zero out a (new) cluster. This is important for directories (mkdir).
167 If it is NULL, the FAT driver defaults to overwriting the whole
168 cluster with zeros. Note that clusternr is absolute, not relative
169 to the provided inode.
172 1. The cvf_bmap function should be ignored. It really should never
173 get called from somewhere. I recommend redirecting it to a panic
174 or fatal error message so bugs show up immediately.
175 2. The cvf_writepage function is ignored. This is because the fat
176 driver doesn't support it. This might change in future. I recommend
177 setting it to NULL (i.e use default).
179 int register_cvf_format(struct cvf_format*cvf_format);
180 If you have just set up a variable containing the above structure,
181 call this function to introduce your CVF format to the FAT/CVF-FAT
182 driver. This is usually done in init_module. Be sure to check the
183 return value. Zero means success, everything else causes a kernel
184 message printed in the syslog describing the error that occurred.
186 - a module with the same version id is already registered or
187 - too many CVF formats. Hack fs/fat/cvf.c if you need more.
189 int unregister_cvf_format(struct cvf_format*cvf_format);
190 This is usually called in cleanup_module. Return value =0 means
191 success. An error only occurs if you try to unregister a CVF format
192 that has not been previously registered. The code uses the version id
193 to distinguish the modules, so be sure to keep it unique.
196 ------------------------------------------------------------------------------
198 Refer to the dmsdos module (the successor of the dmsdos filesystem) for a
199 sample implementation. It can currently be found at
201 ftp://fb9nt.uni-duisburg.de/pub/linux/dmsdos/dmsdos-x.y.z.tgz
202 ftp://sunsite.unc.edu/pub/Linux/system/Filesystems/dosfs/dmsdos-x.y.z.tgz
203 ftp://ftp.uni-stuttgart.de/pub/systems/linux/local/system/dmsdos-x.y.z.tgz
205 (where x.y.z is to be replaced with the actual version number). Full
206 documentation about dmsdos is included in the dmsdos package, but can also
209 http://fb9nt.uni-duisburg.de/mitarbeiter/gockel/software/dmsdos/index.html
210 http://www.yk.rim.or.jp/~takafumi/dmsdos/index.html (in Japanese).