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Writing modules for the GNOME Virtual File System
Introduction
This chapter will introduce the basic concepts that are needed for writing GNOME Virtual File System modules.
GNOME VFS URIs (Uniform Resource Indicators)
The GNOME Virtual file system uses URIs similiar to the standard WWW URIs. The basic difference between a VFS URI and WWW URI is that, while with WWW URIs you can only use a single protocol for accessing a certain file, with GNOME VFS URIs you can combine different access methods in sequence.
For example, suppose you want to access file hello.c in a tar.gz file which is in turn accessible through FTP from a remote machine. In order to access this file, you would need to:
Connect to the FTP site.
Fetch the tar.gz file.
Decompress the tar.gz file using GZIP.
Extract hello.c from the resulting uncompressed tar file.
The GNOME Virtual File System lets you express this by combining the three access methods (i.e. tar, GZIP and FTP) into a single URI. Access methods are combined in the URI by using the `#' character, followed by the name for the access method and the subpath for that specific access method. The subpath can be omitted for those storage methods that do not need a path to retrieve the file. (For example, a GZIP file always contains a single uncompressed file, so no path is needed to locate the uncompressed file within the GZIP file. But on the other hand, the TAR method requires a path to locate a specific file or directory.)
For example, in the case we outlined above, the URI would look something like:
ftp://username:password@host.net/path/to/file.tar.gz#gzip#tar/path/to/hello.c |
Each method/subpath couple is called a URI element. When URI elements are combined like this, each URI element uses the previous one to access a base resource into which it will look up a file, using the subpath information. For this reason, we will say that each element is the parent element for the following one.
The first URI element, the one which has no parent, is called the toplevel element. It does not use the `#' character; instead, it uses the standard syntax of WWW URIs:
method://user:password@host/path/to/file |
This way, normal WWW URIs can be used with the GNOME Virtual File System.
Toplevel elements are also special because they let users specify user names, passwords and host names, while non-toplevel elements don't.
The GnomeVFSURI type
Within the GNOME Virtual File System library, URI elements are represented by a special type, GnomeVFSURI, which is meant to represent user-provided URIs in a machine-optimized way.
Every GnomeVFSURI contains the following information:
A reference counter
A pointer to the parent GnomeVFSURI URI element.
The subpath.
The name of the access method.
A pointer to a GnomeVFSMethod object, describing the access method (see below).
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