vfs(n) Tcl-only Virtual File Systems vfs(n)
NAME
::vfs - Commands and Procedures to create virtual filesystems
SYNOPSIS
package require Tcl 8.4
package require vfs ?1.2.1?
vfs::filesystem info
vfs::filesystem mount
vfs::filesystem unmount
vfs::accessMode mode
vfs::matchDirectories types
vfs::matchFiles types
vfs::matchCorrectTypes types filelist ?inDir?
DESCRIPTION
The ::vfs package provides commands to query, mount and unmount virtual filesystems, and provides as
Tcl libraries some facilities for helping the writing of new virtual filesystems in Tcl. Once a vir-tual virtual
tual filesystem is in place, the standard Tcl file, glob, cd, pwd, open commands, including all their
C APIs in the Tcl library (e.g. Tcl_FSOpenFileChannel, Tcl_FSMatchInDirectory,...), can be used
within the filesystem (and indeed, properly written extensions such as Tk which may open or read
files will also transparently access the virtual filesystem). Because all of Tcl's FS activity
passes through a single layer, it can all be intercepted. This package does just that. Notice that
this is quite different to overloading the file command in Tcl. We are actually providing vfs
replacements for C commands like access, stat. By implementing just a handful of commands at this
low level, we ensure that all commands at higher levels function irrespective of what is going on
inside the FS layer.
Tcl's filesystem hooks operate on a per-process basis. This means every Tcl interpreter in the same
process/application sees the same filesystem, including any virtual filesystems.
The package require vfs command should be used to access this library. It automatically registers
the vfs hooks into Tcl's filesystem, and these will not be removed until Tcl exits (if desired, con-trol control
trol over this could be exposed to Tcl in the future). However, the vfs package will at that stage
not have any new filesystems mounted, so it will have little effect. Note that package require vfs
has two effects. First of all, when it is issued in any Tcl interpreter it will ensure the vfs hooks
have been registered with Tcl's core just once (and if any of those interpreters are later deleted,
the vfs hooks will still remain registered - they remain until Tcl exits). The second effect is to
provide the command vfs::filesystem which allows the interpreter to intercept filesystem commands and
handle them with Tcl code in that interpreter.
There are three somewhat unsupported subcommands of vfs::filesystem, fullynormalize path, posixerror
int, internalerror ?script?, which are used to normalize a path (including any final symlink), to
register a posix error code with a Tcl error, and to trap/report internal errors in tclvfs implemen-tations implementations
tations respectively.
vfs::filesystem mount ?-volume? path command
To use a virtual filesystem, it must be 'mounted'. Mounting involves declaring to the vfs
package that any subdirectories of a given path in the filesystem should be handled by the
given command which should be a Tcl command or procedure in the interpreter in which the
vfs::filesystem is executed. If the ?-volume? flag is given, the given mount point is also
registered with Tcl as a new volume (like a new drive which will appear in file volumes).
This is useful (and required for reasonable operation) for mounts like ftp:// For paths
mounted inside the native filesystem, it should of course not be given. The new filesystem
mounts will be observed immediately in all interpreters in the current process. If the inter-
preter is later deleted, all mounts which are intercepted by it will be automatically removed
(and will therefore affect the view of the filesystem seen by all interpreters).
vfs::filesystem unmount path
This unmounts the virtual filesystem which was mounted at path (hence removing it from Tcl's
filesystem), or throws an error if no filesystem was mounted there.
vfs::filesystem info ?path?
If no arguments are given, this returns a list of all filesystems mounted (in all inter-
preters). If a path argument is given, then the command to be used for that path is returned,
or an error is thrown if no vfs is mounted for that path. There is currently no facility for
examining in which interpreter each command will be evaluated.
vfs::filesystem fullynormalize path
Performs a full expansion of path, (as per 'file normalize'), but including following any
links in the last element of path.
IMPLEMENTING A TCL ONLY VFS
The vfs package will intercept every filesystem operation which falls within a given mount point, and
pass the operation on to the mount point's command in the interpreter which registered it. In general
this occurs by the C equivalent of an evaluation like this: eval $command [list $subcmd $root $rela-
tive $actualpath] $args.
Here subcmd may be any of the following: access, createdirectory, deletefile, fileattributes,
matchindirectory, open, removedirectory, stat, utime. If command takes appropriate action for each of
these cases, a complete, perfect virtual filesystem will be achieved, indistinguishable to Tcl from
the native filesystem. (CAVEATS: right now I don't expose to Tcl all the permission-related flags of
'glob').
The remaining arguments specify a file path on which to operate (all commands operate on one of
these), and any additional arguments which may be required to carry out the action. The file path is
specified by three arguments: root is the part of the path which lies outside this filesystem's mount
point, relative is the part of the path which lies inside this filesytem, and actualpath is the orig-
inal (unnormalized) name of the path which was used in the current command wherever it originated (in
Tcl or C). For example, if C:/foo/bar/mount.zip/xxx/yyy is a path in your filesystem, where
mount.zip is a zip archive which has been mounted (on top of itself) and contains xxx/yyy, and the
current working directory is inside xxx, and we evaluate a command like file exists yyy, then
rootwill be C:/foo/bar/mount.zip, relative will be xxx/yyy, and actualpath will be yyy. The file sep-
arator between the root and relative is omitted.
Note that most filesystem operations will only require the relative argument to work correctly, but
the other arguments are actually required for correct operation of some subcommands.
Almost all of these commands should either return correctly (i.e. with a TCL_OK result at the C
level) or they should use vfs::filesystem posixerror to signal the appropriate posix error code. If
a Tcl error is thrown, that should be considered a bug, but it will be interpreted as an unknown
posix error in the filesystem call. The exceptions to these rules are those filesystem commands
which are able to specify a Tcl error message directly: open (when an interpreter is given),
matchindirectory and fileattributes (for a set or get operation only). These three commands are
allowed to throw any Tcl error message which will be passed along to the caller, or they may throw a
posix error which will be handled appropriately.
The actual commands are as follows (where r-r-a represents the standard argument triplet of root,
relative and actualpath):
command access r-r-a mode
Return TCL_OK or throw a posix error depending on whether the given access mode (which is an
integer) is compatible with the file.
command createdirectory r-r-a
Create a directory with the given name. The command can assume that all sub-directories in
the path exist and are valid, and that the actual desired path does not yet exist (Tcl takes
care of all of that for us).
command deletefile r-r-a
Delete the given file.
command fileattributes r-r-a ?index? ?value?
If neither index nor value is given, then return a list of all acceptable attribute names. If
index is given, but no value, then retrieve the value of the index'th attribute (counting in
order over the list returned when no argument is given) for the given file. If a value is
also given then set the index'th attribute of the given file to that value.
command matchindirectory r-r-a pattern types
Return the list of files or directories in the given path (which is always the name of an
existing directory), which match the pattern and are compatible with the types given. It is
very important that the command correctly handle types requests for directories only (and
files only), because to handle any kind of recursive globbing, Tcl will actually generate
requests for directory-only matches from the filesystem. See vfs::matchDirectories below for
help.
command open r-r-a mode permissions
For this command, mode is any of "r", "w", "a", "w+", "a+". If the open involves creating a
file, then permissions dictates what modes to create it with. If the open operation was not
successful, an error should be thrown. If the open operation is successful, the command
should return a list of either one or two items. The first item (which is obligatory) is the
name of the channel which has been created. The second item, if given, is a Tcl-callback to
be used when the channel is closed, so that the vfs can clean up as appropriate. This call-
back will be evaluated by Tcl just before the channel is closed. The channel will still
exist, and all available data will have been flushed into it. The callback can, for example,
seek to the beginning of the channel, read its contents and store that contents elsewhere
(e.g. compressed or on a remote ftp site, etc). The return code or any errors returned by the
callback are ignored (if the callback wishes to signal an error, it must do so asycnhronously,
with bgerror, for example), unless the 'internalerror' script has been specified, when they
are passed to that script for further action.
command removedirectory r-r-a recursive
Delete the given directory. recursive is either 0 or 1. If it is 1 then even if the directory
is non-empty, an attempt should be made to recursively delete it and its contents. If it is 0
and the directory is non-empty, a posix error (EEXIST) should be thrown.
command stat r-r-a
Return a list of even length containing field-name and value pairs for the contents of a stat
structure. The order is not important. The option names are dev (long), ino (long), mode
(int), nlink (long), uid (long), gid (long), size (long), atime (long), mtime (long), ctime
(long), type (string which is either "directory" or "file"), where the type of each argument
is given in brackets. The procedure should therefore return with something like return [list
dev 0 type file mtime 1234 ...].
command utime r-r-a actime mtime
Set the access and modification times of the given file (these are read with 'stat').
VFS HELPERS
The vfslib provides a number of Tcl procedures which can help with writing command procedures to han-
dle the above possibilities. These are:
vfs::accessMode mode
converts an integer access mode to a somewhat more preferable string, any of F X W XW R RX RW.
vfs::matchDirectories types
Does types want directories included?
vfs::matchFiles types
Does types want files included?
vfs::matchCorrectTypes types filelist ?inDir?
Returns that subset of the filelist (which are either absolute paths or names of files in
inDir) which are compatible with the types given.
VFS DEBUGGING
Use something like this to debug problems in your implementation: vfs::filesystem internalerror
report ; proc report {} { puts stderr $::errorInfo }
LIMITATIONS
There are very few limitations to the vfs code. One subtlety that you may encounter is if you mount
a case-sensitive virtual filesystem into a case-insensitive system (e.g. the standard Windows or
MacOS fs) and your code relies on case-insensitivity, then it will not run properly in the virtual
filesystem. Of course if your code relies on case-insensitivity, it wouldn't run under Tcl on Unix
either, so the best solution is to fix your code!
We may add link and lstat commands in the future to allow virtual filesystems to support reading and
writing links - this is supported by the C API, but has simply not been exposed to Tcl in this exten-
sion, yet.
The Tcl 'Tcl_FSMatchInDirectory' function takes a variety of type information in a Tcl_GlobTypeData
structure. We currently only expose the 'type' field from that structure (so the 'permissions' and
MacOS type/creator fields are ignored).
KEYWORDS
vfs, filesystem, file
Vfs 1.2.1 vfs(n)
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