--- --- vfs - Tcl-level Virtual Filesystems
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vfs(n) 1.0 "Tcl-level Virtual Filesystems"

Name

vfs - Filesystem management in Tcl

Description

The package vfs provides commands to query, mount and unmount virtual filesystems implemented in Tcl. This is further facilitated through the provison of helper commands in a tcl script library. See section "HANDLER ENVIRONMENT" of vfs-fsapi for more information.

Once a virtual filesystem is in place, the standard Tcl commands, like file, glob, cd, pwd, open, 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 filesystem activity passes through a single layer, every operation can be intercepted. This package does just that. This is also quite different from simply overloading the file command in Tcl. We are actually providing replacements for C commands like access, stat, etc. By implementing a small number of low-level commands we ensure that all commands at higher levels will function irrespective of what is going on inside the filesystem layer.

Tcl's filesystem hooks operate on a per-process basis. This means every Tcl interpreter in the same process/application will see the same filesystem, including any virtual filesystems.

To access this package use the command package require vfs. This automatically registers the vfs hooks into Tcl's filesystem and provides the command vfs::filesystem. The latter allows the registration of actual virtual filesystems. More in the upcoming section API. The hooks will not be removed until Tcl exits. If desired, control over this could be exposed to Tcl in the future. By and in itself the command above will have no further effect. Only after filesystem implementations have been registered and filesystems using them been mounted filesystem commands will actually be intercepted, and handled by the Tcl code of the mounted virtual filesystem.

API

vfs::filesystem mount ?-volume? path command

Mounts a virtual filesystem at path, making it useable. After completion of the call any access to a subdirectory of path will be handled by that filesystem. The filesystem is represented here by the command prefix which will be executed whenever an operation on a file or directory within path has to be performed.

Whether the command is implemented in C or Tcl is of no relevance as long as it adheres to the API specified in vfs-fsapi and is present in the interpreter where the mount operation is executed.

If the option -volume is specified the new mount point is also registered with Tcl as a new volume and will therefore from then on appear in the output of the command file volumes. This is useful (and required for reasonable operation) for mounts like ftp://. It should not be used for paths mounted inside the native filesystem.

The new filesystem mounts will be observed immediately in all interpreters in the current process. If the interpreter 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. An error is thrown if no filesystem was mounted there. After the completion of the operation the filesystem is not visible anymore, and any previous filesystem accessible through this path becomes accessible again.

vfs::filesystem info ?path?

A list of all filesystems mounted in all interpreters is returned, if no path argument was specified. Else the filesystem responsible for that path is examined and the command prefix used to handle all filesystem operations returned. An error is thrown if no filesystem 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). This includes the following of any links in the last element of path.

vfs::filesystem posixerror int

This command can be called by filesystem implementations during the execution of a filesystem operation to signal the posix error code of a failure. See also vfs-fsapi.

vfs::filesystem internalerror command

When used the specified command is registerd as the command to trap and report any internal errors thrown by filesystem implementations.

LIMITATIONS

The code of the package vfs has only a few limitations.

  1. One subtlety one has to be aware of is that mixing case-(in)sensitive filesystems and application code may yield unexpected results.

    For example mounting a case-sensitive virtual filesystem into a case-insensitive system (like the standard Windows or MacOS filesystems) and then using this with code relying on case-insensitivity problems will appear when accessing the virtual filesystem.

    Note that application code relying on case-insensitivity will not under Unix either, i.e. is inherently non-portable, and should be fixed.

  2. The C-API's for link and lstat are currently not exposed to the Tcl level. This may be done in the future to allow virtual filesystems implemented in Tcl to support the reading and writing of links.

  3. The public C-API filesystem function Tcl_FSMatchInDirectory is given a variety of type information in a Tcl_GlobTypeData structure. Currently only the type field of said strcuture is exposed to the tcl-level. Fields like permissions and MacOS type/creator are ignored.

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