GIO is striving to provide a modern, easy-to-use VFS API that sits at the right level in the library stack. The goal is to overcome the shortcomings of GnomeVFS and provide an API that is so good that developers prefer it over raw POSIX calls. Among other things that means using GObject. It also means not cloning the POSIX API, but providing higher-level, document-centric interfaces.

The abstract file system model of GIO consists of a number of interfaces and base classes for I/O and files:

Then there is a number of stream classes, similar to the input and output stream hierarchies that can be found in frameworks like Java:

There are interfaces related to applications and the types of files they handle:

Beyond these, GIO provides facilities for file monitoring, asynchronous I/O and filename completion. In addition to the interfaces, GIO provides implementations for the local case. Implementations for various network file systems are provided by the GVFS package as loadable modules.

Other design choices which consciously break with the GnomeVFS design are to move backends out-of-process, which minimizes the dependency bloat and makes the whole system more robust. The backends are not included in GIO, but in the separate GVFS package. The GVFS package also contains the GVFS daemon, which spawn further mount daemons for each individual connection.

Figure 1. GIO in the GTK+ library stack

The GIO model of I/O is stateful: if an application establishes e.g. a SFTP connection to a server, it becomes available to all applications in the session; the user does not have to enter his password over and over again.

One of the big advantages of putting the VFS in the GLib layer is that GTK+ can directly use it, e.g. in the filechooser.