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5.26.7 File Handlers

As noted earlier, Librep supports virtual files; that is it allows file names to be accessed that don’t reside on the local filing system, or aren’t normally valid as file names. This is achieved through the use of file handlers, Lisp functions that have signalled that they should be used to redirect all accesses to files whose names match a particular regular expression (see Regular Expressions).

For example, in Unix, a leading tilde ~ expands to the home directory. See below.

Variable: file-handler-alist

This variable contains a list of file handler declarations, each one of the form (regexp . function). Whenever a file operation is performed on a file whose name matches regexp, function is invoked to perform the action. The function is called as (function operation args…), where operation and args are from the original call.

For example if the file-handler-alist contains the entry ("^~" . tilde-file-handler), then all file operations on files starting with a tilde are redirected to the tilde-file-handler function.

Thus if a form (file-exists-p "~/foo") is executed, it would result in a call to tilde-file-handler as (tilde-file-handler 'file-exists-p "~/foo").

The list of operations that may be redirected to a file handler is: file-name-absolute-p, expand-file-name, local-file-name, canonical-file-name, file-name-nondirectory, file-name-directory, file-name-as-directory, directory-file-name, open-file, close-file, flush-file, seek-file, write-buffer-contents, read-file-contents, insert-file-contents, delete-file, rename-file, copy-file, copy-file-to-local-fs, copy-file-from-local-fs, make-directory, delete-directory, file-exists-p, file-regular-p, file-readable-p, file-writable-p, file-directory-p, file-symlink-p, file-owner-p, file-nlinks, file-size, file-modes, file-modes-as-string, set-file-modes, file-modtime, directory-files, make-symlink, read-symlink.

There are several undefined functions in this list. The write-buffer-contents, read-file-contents, and insert-file-contents pertain to the Jade text editor. The other two are defined as follows.

Operation: copy-file-to-local-fs file-name local-name

Called when copying files between file handlers, this operation should copy the file matching the handler file-name, to the file on the local file system local-name.

Operation: copy-file-from-local-fs local-name file-name

Called when copying files between file handlers, this operation should copy the local file file-name to the file matching the handler file-name.

To prevent infinite recursion, while a particular operation is being processed by a file handler, that operation may not be passed back to the same handler.

To allow file handlers to handle the open-file operation, it is possible to create file handles from arbitrary streams.

Function: make-file-from-stream file-name stream handler

Return a new file object that refers to the logical file called file-name, that is not in the local filing system. All access to the file object will be directed through the stream object stream, and the file handler function handler.

An alternative method of opening remote files is to use a temporary file in the local file system with either one (read or write modes), or two (append mode) synchronisations with the remote system. This is the method used by the FTP remote file backend (see the next section). It has the advantage of simplifying the seek-file operation.

Default File Handlers

Librep provides the following file handlers by default:

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