glib/docs/reference/gio/overview.xml

<part>
  <title>GIO Overview</title>

  <chapter>
    <title>Introduction</title>

  <para>
    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.
  </para>

  <para>
    The abstract file system model of GIO consists of a number of
    interfaces and base classes for I/O and files:
    <variablelist>
       <varlistentry>
         <term>GFile</term>
         <listitem><para>reference to a file</para></listitem>
       </varlistentry>
       <varlistentry>
         <term>GFileInfo</term>
         <listitem><para>information about a file or filesystem</para></listitem>
       </varlistentry>
       <varlistentry>
         <term>GFileEnumerator</term>
         <listitem><para>list files in directories</para></listitem>
       </varlistentry>
       <varlistentry>
         <term>GDrive</term>
         <listitem><para>represents a drive</para></listitem>
       </varlistentry>
       <varlistentry>
         <term>GVolume</term>
         <listitem><para>represents a file system in an abstract way</para></listitem>
       </varlistentry>
       <varlistentry>
         <term>GMount</term>
         <listitem><para>represents a mounted file system</para></listitem>
       </varlistentry>
    </variablelist>
    Then there is a number of stream classes, similar to the input and
    output stream hierarchies that can be found in frameworks like Java:
    <variablelist>
       <varlistentry>
         <term>GInputStream</term>
         <listitem><para>read data</para></listitem>
       </varlistentry>
       <varlistentry>
         <term>GOutputStream</term>
         <listitem><para>write data</para></listitem>
       </varlistentry>
       <varlistentry>
         <term>GIOStream</term>
         <listitem><para>read and write data</para></listitem>
       </varlistentry>
       <varlistentry>
         <term>GSeekable</term>
         <listitem><para>interface optionally implemented by streams to support seeking</para></listitem>
       </varlistentry>
    </variablelist>
    There are interfaces related to applications and the types
    of files they handle:
    <variablelist>
       <varlistentry>
          <term>GAppInfo</term>
          <listitem><para>information about an installed application</para></listitem>
       </varlistentry>
       <varlistentry>
          <term>GIcon</term>
          <listitem><para>abstract type for file and application icons</para></listitem>
       </varlistentry>
    </variablelist>
    There is a framework for storing and retrieving application settings:
    <variablelist>
       <varlistentry>
          <term>GSettings</term>
          <listitem><para>stores and retrieves application settings</para></listitem>
       </varlistentry>
    </variablelist>
    There is support for network programming, including name resolution, lowlevel socket
    APIs and highlevel client and server helper classes:
    <variablelist>
       <varlistentry>
          <term>GSocket</term>
          <listitem><para>lowlevel platform independent socket object</para></listitem>
       </varlistentry>
       <varlistentry>
          <term>GResolver</term>
          <listitem><para>asynchronous and cancellable DNS resolver</para></listitem>
       </varlistentry>
       <varlistentry>
          <term>GSocketClient</term>
          <listitem><para>high-level network client helper</para></listitem>
       </varlistentry>
       <varlistentry>
          <term>GSocketService</term>
          <listitem><para>high-level network server helper</para></listitem>
       </varlistentry>
       <varlistentry>
          <term>GSocketConnection</term>
          <listitem><para>network connection stream</para></listitem>
       </varlistentry>
    </variablelist>
    There is support for connecting to <link linkend="http://www.freedesktop.org/wiki/Software/dbus">D-Bus</link>,
    sending and receiving messages, owning and watching bus names,
    and making objects available on the bus:
    <variablelist>
       <varlistentry>
          <term>GDBusConnection</term>
          <listitem><para>a D-Bus connection</para></listitem>
       </varlistentry>

       <varlistentry>
          <term>GDBusMethodInvocation</term>
          <listitem><para>for handling remove calls</para></listitem>
       </varlistentry>

       <varlistentry>
          <term>GDBusServer</term>
          <listitem><para>helper for accepting connections</para></listitem>
       </varlistentry>

       <varlistentry>
          <term>GDBusProxy</term>
          <listitem><para>proxy to access D-Bus interfaces on a remote object</para></listitem>
       </varlistentry>

    </variablelist>
    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.
  </para>

  <para>
    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.
  </para>

  <figure id="gvfs-overview">
    <title>GIO in the GTK+ library stack</title>
    <graphic fileref="gvfs-overview.png" format="PNG"></graphic>
  </figure>

  <para>
    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.
  </para>
  <para>
    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.
  </para>
  </chapter>

  <chapter>
    <title>Compiling GIO applications</title>

    <para>
      GIO comes with a <filename>gio-2.0.pc</filename> file that you
      should use together with <literal>pkg-config</literal> to obtain
      the necessary information about header files and libraries. See
      the <literal>pkg-config</literal> man page or the GLib documentation
      for more information on how to use <literal>pkg-config</literal>
      to compile your application.
    </para>

    <para>
      If you are using GIO on UNIX-like systems, you may want to use
      UNIX-specific GIO interfaces such as #GUnixInputStream,
      #GUnixOutputStream, #GUnixMount or #GDesktopAppInfo.
      To do so, use the <filename>gio-unix-2.0.pc</filename> file
      instead of <filename>gio-2.0.pc</filename>
    </para>

    <para>
      Since GIO is based on GObject, you need to call g_type_init()
      before you can use any GIO functions. If your application uses
      GTK+, this is already taken care of by gtk_init().
    </para>
  </chapter>

  <chapter>
    <title>Running GIO applications</title>

    <para>
      GIO inspects a few of environment variables in addition to the
      ones used by GLib.
    </para>

    <formalpara>
      <title><envar>XDG_DATA_HOME</envar>, <envar>XDG_DATA_DIRS</envar></title>

      <para>
        GIO uses these environment variables to locate MIME information.
        For more information, see the <ulink url="http://freedesktop.org/Standards/shared-mime-info-spec">Shared MIME-info Database</ulink>
        and the <ulink url="http://freedesktop.org/Standards/basedir-spec">Base Directory Specification</ulink>.
      </para>
    </formalpara>

    <formalpara>
      <title><envar>GVFS_DISABLE_FUSE</envar></title>

      <para>
        This variable can be set to keep #Gvfs from starting the fuse backend,
        which may be unwanted or unnecessary in certain situations.
      </para>
    </formalpara>

    <para>
      The following environment variables are only useful for debugging
      GIO itself or modules that it loads. They should not be set in a
      production environment.
    </para>
    <formalpara>
      <title><envar>GIO_USE_VFS</envar></title>

      <para>
        This environment variable can be set to the name of a #GVfs
        implementation to override the default for debugging purposes.
        The #GVfs implementation for local files that is included in GIO
        has the name "local", the implementation in the gvfs module has
        the name "gvfs".
      </para>
    </formalpara>

    <formalpara>
      <title><envar>GIO_USE_VOLUME_MONITOR</envar></title>

      <para>
        This variable can be set to the name of a #GVolumeMonitor
        implementation to override the default for debugging purposes.
        The #GVolumeMonitor implementation for local files that is included
        in GIO has the name "unix", the hal-based implementation in the
        gvfs module has the name "hal".
      </para>
    </formalpara>

    <formalpara>
      <title><envar>GIO_USE_URI_ASSOCIATION</envar></title>

      <para>
        This variable can be set to the name of a #GDesktopAppInfoLookup
        implementation to override the default for debugging purposes.
        GIO does not include a #GDesktopAppInfoLookup implementation,
        the GConf-based implementation in the gvfs module has the name
        "gconf".
      </para>
    </formalpara>

    <formalpara>
      <title><envar>GVFS_INOTIFY_DIAG</envar></title>

      <para>
        When this environment variable is set and GIO has been built
        with inotify support, a dump of diagnostic inotify information
        will be written every 20 seconds to a file named
        <filename>/tmp/gvfsdid.<replaceable>pid</replaceable></filename>.
      </para>
    </formalpara>

    <formalpara>
      <title><envar>GIO_EXTRA_MODULES</envar></title>

      <para>
        When this environment variable is set to a path, or a set of
        paths separated by a colon, GIO will attempt to load
        modules from within the path.
      </para>
    </formalpara>

    <formalpara>
      <title><envar>GSETTINGS_BACKEND</envar></title>

      <para>
        This variable can be set to the name of a #GSettingsBackend
        implementation to override the default for debugging purposes.
        The memory-based implementation that is included in GIO has
        the name "memory", the one in dconf has the name "dconf-settings".
      </para>
    </formalpara>

    <formalpara>
      <title><envar>GSETTINGS_SCHEMA_DIR</envar></title>

      <para>
        This variable can be set to the name of a directory that is
        considered in addition to the <filename>glib-2.0/schemas</filename>
        subdirectories of the XDG system data dirs when looking
        for compiled schemas for #GSettings.
      </para>
    </formalpara>

   <formalpara>
      <title><envar>DBUS_SYSTEM_BUS_ADDRESS</envar></title>

      <para>
        This variable is consulted to find the address of the D-Bus system
        bus. For the format of D-Bus addresses, see the D-Bus
        <ulink url="http://dbus.freedesktop.org/doc/dbus-specification.html#addresses">specification</ulink>.
      </para>
      <para>
        Setting this variable overrides platform-specific ways of determining
        the system bus address.
      </para>
   </formalpara>

   <formalpara>
      <title><envar>DBUS_SESSION_BUS_ADDRESS</envar></title>

      <para>
        This variable is consulted to find the address of the D-Bus session bus.
      </para>
      <para>
        Setting this variable overrides platform-specific ways of determining
        the session bus address.
      </para>
   </formalpara>

   <formalpara>
      <title><envar>DBUS_STARTER_BUS_TYPE</envar></title>

      <para>
        This variable is consulted to find out the 'starter' bus for an
        application that has been started via D-Bus activation. The possible
        values are 'system' or 'session'.
      </para>
   </formalpara>

   <formalpara>
      <title><envar>G_DBUS_DEBUG</envar></title>

      <para>
         This variable can be set to a list of debug options, which
         cause GLib to print out different types of debugging
         information when using the D-Bus routines.
         <variablelist>
           <varlistentry>
             <term>transport</term>
             <listitem><para>Show IO activity (e.g. reads and writes)</para></listitem>
           </varlistentry>
           <varlistentry>
             <term>message</term>
             <listitem><para>Show all sent and received D-Bus messages</para></listitem>
           </varlistentry>
           <varlistentry>
             <term>payload</term>
             <listitem><para>Show payload for all sent and received D-Bus messages (implies message)</para></listitem>
           </varlistentry>
           <varlistentry>
             <term>call</term>
             <listitem><para>Trace g_dbus_connection_call() and g_dbus_connection_call_sync() API usage</para></listitem>
           </varlistentry>
           <varlistentry>
             <term>signal</term>
             <listitem><para>Show when a D-Bus signal is received</para></listitem>
           </varlistentry>
           <varlistentry>
             <term>incoming</term>
             <listitem><para>Show when an incoming D-Bus method call is received</para></listitem>
           </varlistentry>
           <varlistentry>
             <term>return</term>
             <listitem><para>Show when a reply is returned via the #GDBusMethodInvocation API</para></listitem>
           </varlistentry>
           <varlistentry>
             <term>emission</term>
             <listitem><para>Trace g_dbus_connection_emit_signal() API usage</para></listitem>
           </varlistentry>
           <varlistentry>
             <term>authentication</term>
             <listitem><para>Show information about connection authentication</para></listitem>
           </varlistentry>
           <varlistentry>
             <term>address</term>
             <listitem><para>Show information about D-Bus address lookups and autolaunching</para></listitem>
           </varlistentry>
         </variablelist>
         The special value <literal>all</literal> can be used to turn
         on all debug options. The special value
         <literal>help</literal> can be used to print a list of
         supported options to standard output.
      </para>
   </formalpara>

   <formalpara>
      <title><envar>G_DBUS_COOKIE_SHA1_KEYRING_DIR</envar></title>

      <para>
        Can be used to override the directory used to store the
        keyring used in the <literal>DBUS_COOKIE_SHA1</literal>
        authentication mechanism. Normally the directory used is
        <filename>.dbus-keyrings</filename> in the user's home
        directory.
      </para>
   </formalpara>

   <formalpara>
      <title><envar>G_DBUS_COOKIE_SHA1_KEYRING_DIR_IGNORE_PERMISSION</envar></title>

      <para>
        If set, the permissions of the directory used to store the
        keyring used in the <literal>DBUS_COOKIE_SHA1</literal>
        authentication mechanism won't be checked. Normally the
        directory must be readable only by the user.
      </para>
   </formalpara>
  </chapter>

  <chapter id="extending-gio">
    <title>Extending GIO</title>

    <para>
      A lot of the functionality that is accessible through GIO
      is implemented in loadable modules, and modules provide a convenient
      way to extend GIO. In addition to the #GIOModule API which supports
      writing such modules, GIO has a mechanism to define extension points,
      and register implementations thereof, see #GIOExtensionPoint.
    </para>
    <para>
      The following extension points are currently defined by GIO:
    </para>

    <formalpara>
       <title>G_VFS_EXTENSION_POINT_NAME</title>

       <para>
          Allows to override the functionality of the #GVfs class.
          Implementations of this extension point must be derived from #GVfs.
          GIO uses the implementation with the highest priority that is active,
          see g_vfs_is_active().
       </para>
       <para>
          GIO implements this extension point for local files, gvfs contains
          an implementation that supports all the backends in gvfs.
       </para>
   </formalpara>

   <formalpara>
      <title>G_VOLUME_MONITOR_EXTENSION_POINT_NAME</title>

      <para>
         Allows to add more volume monitors.
         Implementations of this extension point must be derived from
         #GVolumeMonitor. GIO uses all registered extensions.
      </para>
      <para>
        gvfs contains an implementation that works together with the #GVfs
        implementation in gvfs.
      </para>
   </formalpara>

   <formalpara>
      <title>G_NATIVE_VOLUME_MONITOR_EXTENSION_POINT_NAME</title>

      <para>
         Allows to override the 'native' volume monitor.
         Implementations of this extension point must be derived from
         #GNativeVolumeMonitor. GIO uses the implementation with
         the highest priority that is supported, as determined by the
         is_supported() vfunc in #GVolumeMonitorClass.
      </para>
      <para>
         GIO implements this extension point for local mounts,
         gvfs contains a hal-based implementation.
      </para>
   </formalpara>

   <formalpara>
      <title>G_LOCAL_FILE_MONITOR_EXTENSION_POINT_NAME</title>

      <para>
        Allows to override the file monitor implementation for
        local files. Implementations of this extension point must
        be derived from #GLocalFileMonitor. GIO uses the implementation
        with the highest priority that is supported, as determined by the
        is_supported() vfunc in #GLocalFileMonitorClass.
      </para>
      <para>
        GIO uses this extension point internally, to switch between
        its fam-based and inotify-based file monitoring implementations.
      </para>
   </formalpara>

   <formalpara>
      <title>G_LOCAL_DIRECTORY_MONITOR_EXTENSION_POINT_NAME</title>

      <para>
        Allows to override the directory monitor implementation for
        local files. Implementations of this extension point must be
        derived from #GLocalDirectoryMonitor. GIO uses the implementation
        with the highest priority that is supported, as determined by the
        is_supported() vfunc in #GLocalDirectoryMonitorClass.
      </para>
      <para>
        GIO uses this extension point internally, to switch between
        its fam-based and inotify-based directory monitoring implementations.
      </para>
   </formalpara>

   <formalpara>
      <title>G_DESKTOP_APP_INFO_LOOKUP_EXTENSION_POINT_NAME</title>

      <para>
        Unix-only. Allows to provide a way to associate default handlers
        with URI schemes. Implementations of this extension point must
        implement the #GDesktopAppInfoLookup interface. GIO uses the
        implementation with the highest priority.
      </para>
      <para>
        This extension point has been discontinued in GLib 2.28. It is
        still available to keep API and ABI stability, but GIO is no
        longer using it for default handlers. Instead, the mime handler
        mechanism is used, together with x-scheme-handler pseudo-mimetypes.
      </para>
   </formalpara>

   <formalpara>
      <title>G_SETTINGS_BACKEND_EXTENSION_POINT_NAME</title>

      <para>
        Allows to provide an alternative storage for #GSettings.
        Implementations of this extension point must derive from the
        #GSettingsBackend type. GIO contains a keyfile-based
        implementation of this extension point, another one is provided
        by dconf.
      </para>
   </formalpara>

   <formalpara>
     <title>G_PROXY_EXTENSION_POINT_NAME</title>

     <para>
       Allows to provide implementations for network proxying.
       Implementations of this extension point must provide the
       #GProxy interface, and must be named after the network
       protocol they are proxying.
     </para>
     <para>
       glib-networking contains an implementation of this extension
       point based on libproxy.
     </para>
   </formalpara>

   <formalpara>
     <title>G_TLS_BACKEND_EXTENSION_POINT_NAME</title>

     <para>
       Allows to provide implementations for TLS support.
       Implementations of this extension point must implement
       the #GTlsBackend interface.
     </para>
     <para>
       glib-networking contains an implementation of this extension
       point.
     </para>
   </formalpara>

  </chapter>
</part>