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<div class="chapter">
<div class="titlepage"><div><div><h2 class="title">
<a name="id-1.3.2"></a>Introduction</h2></div></div></div>
<p>
    GIO is striving to provide a modern, easy-to-use VFS API that sits
    at the right level in the library stack, as well as other generally
    useful APIs for desktop applications (such as networking and
    D-Bus support). 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.
  </p>
<p>
    The abstract file system model of GIO consists of a number of
    interfaces and base classes for I/O and files:
    </p>
<div class="variablelist"><table border="0" class="variablelist">
<colgroup>
<col align="left" valign="top">
<col>
</colgroup>
<tbody>
<tr>
<td><p><span class="term">GFile</span></p></td>
<td><p>reference to a file</p></td>
</tr>
<tr>
<td><p><span class="term">GFileInfo</span></p></td>
<td><p>information about a file or filesystem</p></td>
</tr>
<tr>
<td><p><span class="term">GFileEnumerator</span></p></td>
<td><p>list files in directories</p></td>
</tr>
<tr>
<td><p><span class="term">GDrive</span></p></td>
<td><p>represents a drive</p></td>
</tr>
<tr>
<td><p><span class="term">GVolume</span></p></td>
<td><p>represents a file system in an abstract way</p></td>
</tr>
<tr>
<td><p><span class="term">GMount</span></p></td>
<td><p>represents a mounted file system</p></td>
</tr>
</tbody>
</table></div>
<p>
    Then there is a number of stream classes, similar to the input and
    output stream hierarchies that can be found in frameworks like Java:
    </p>
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<td><p><span class="term">GInputStream</span></p></td>
<td><p>read data</p></td>
</tr>
<tr>
<td><p><span class="term">GOutputStream</span></p></td>
<td><p>write data</p></td>
</tr>
<tr>
<td><p><span class="term">GIOStream</span></p></td>
<td><p>read and write data</p></td>
</tr>
<tr>
<td><p><span class="term">GSeekable</span></p></td>
<td><p>interface optionally implemented by streams to support seeking</p></td>
</tr>
</tbody>
</table></div>
<p>
    There are interfaces related to applications and the types
    of files they handle:
    </p>
<div class="variablelist"><table border="0" class="variablelist">
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<td><p><span class="term">GAppInfo</span></p></td>
<td><p>information about an installed application</p></td>
</tr>
<tr>
<td><p><span class="term">GIcon</span></p></td>
<td><p>abstract type for file and application icons</p></td>
</tr>
</tbody>
</table></div>
<p>
    There is a framework for storing and retrieving application settings:
    </p>
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<td><p><span class="term">GSettings</span></p></td>
<td><p>stores and retrieves application settings</p></td>
</tr></tbody>
</table></div>
<p>
    There is support for network programming, including connectivity monitoring,
    name resolution, lowlevel socket APIs and highlevel client and server
    helper classes:
    </p>
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<td><p><span class="term">GSocket</span></p></td>
<td><p>lowlevel platform independent socket object</p></td>
</tr>
<tr>
<td><p><span class="term">GResolver</span></p></td>
<td><p>asynchronous and cancellable DNS resolver</p></td>
</tr>
<tr>
<td><p><span class="term">GSocketClient</span></p></td>
<td><p>high-level network client helper</p></td>
</tr>
<tr>
<td><p><span class="term">GSocketService</span></p></td>
<td><p>high-level network server helper</p></td>
</tr>
<tr>
<td><p><span class="term">GSocketConnection</span></p></td>
<td><p>network connection stream</p></td>
</tr>
<tr>
<td><p><span class="term">GNetworkMonitor</span></p></td>
<td><p>network connectivity monitoring</p></td>
</tr>
</tbody>
</table></div>
<p>
    There is support for connecting to D-Bus,
    sending and receiving messages, owning and watching bus names,
    and making objects available on the bus:
    </p>
<div class="variablelist"><table border="0" class="variablelist">
<colgroup>
<col align="left" valign="top">
<col>
</colgroup>
<tbody>
<tr>
<td><p><span class="term">GDBusConnection</span></p></td>
<td><p>a D-Bus connection</p></td>
</tr>
<tr>
<td><p><span class="term">GDBusMethodInvocation</span></p></td>
<td><p>for handling remote calls</p></td>
</tr>
<tr>
<td><p><span class="term">GDBusServer</span></p></td>
<td><p>helper for accepting connections</p></td>
</tr>
<tr>
<td><p><span class="term">GDBusProxy</span></p></td>
<td><p>proxy to access D-Bus interfaces on a remote object</p></td>
</tr>
</tbody>
</table></div>
<p>
    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.
  </p>
<p>
    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.
  </p>
<div class="figure">
<a name="gvfs-overview"></a><p class="title"><b>Figure 1. GIO in the GTK+ library stack</b></p>
<div class="figure-contents"><div><img src="gvfs-overview.png" alt="GIO in the GTK+ library stack"></div></div>
</div>
<br class="figure-break"><p>
    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.
  </p>
<p>
    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.
  </p>
</div>
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