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<div class="chapter">
<div class="titlepage"><div><div><h2 class="title">
<a name="chapter-signal"></a>The GObject messaging system</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="sect1"><a href="chapter-signal.html#closure">Closures</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="chapter-signal.html#id-1.3.5.2.5">C Closures</a></span></dt>
<dt><span class="sect2"><a href="chapter-signal.html#id-1.3.5.2.6">Non-C closures (for the fearless)</a></span></dt>
</dl></dd>
<dt><span class="sect1"><a href="signal.html">Signals</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="signal.html#signal-registration">Signal registration</a></span></dt>
<dt><span class="sect2"><a href="signal.html#signal-connection">Signal connection</a></span></dt>
<dt><span class="sect2"><a href="signal.html#signal-emission">Signal emission</a></span></dt>
<dt><span class="sect2"><a href="signal.html#signal-detail">The <span class="emphasis"><em>detail</em></span> argument</a></span></dt>
</dl></dd>
</dl></div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="closure"></a>Closures</h2></div></div></div>
<p>
      Closures are central to the concept of asynchronous signal delivery
      which is widely used throughout GTK+ and GNOME applications. A closure is an 
      abstraction, a generic representation of a callback. It is a small structure
      which contains three objects:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p>a function pointer (the callback itself) whose prototype looks like:
</p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
    <tbody>
      <tr>
        <td class="listing_lines" align="right"><pre>1</pre></td>
        <td class="listing_code"><pre class="programlisting"><span class="usertype">return_type</span><span class="normal"> </span><span class="function">function_callback</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">… </span><span class="symbol">,</span><span class="normal"> </span><span class="usertype">gpointer</span><span class="normal"> user_data</span><span class="symbol">);</span></pre></td>
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    </tbody>
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</div>

<p>
        </p>
</li>
<li class="listitem"><p>
           the <em class="parameter"><code>user_data</code></em> pointer which is passed to the callback upon invocation of the closure
          </p></li>
<li class="listitem"><p>
           a function pointer which represents the destructor of the closure: whenever the
           closure's refcount reaches zero, this function will be called before the closure
           structure is freed.
          </p></li>
</ul></div>
<p>
    </p>
<p>
      The <a class="link" href="gobject-Closures.html#GClosure" title="struct GClosure"><span class="type">GClosure</span></a> structure represents the common functionality of all
      closure implementations: there exists a different closure implementation for
      each separate runtime which wants to use the GObject type system.
      <a href="#ftn.id-1.3.5.2.3.2" class="footnote" name="id-1.3.5.2.3.2"><sup class="footnote">[4]</sup></a>
      The GObject library provides a simple <a class="link" href="gobject-Closures.html#GCClosure" title="struct GCClosure"><span class="type">GCClosure</span></a> type which
      is a specific implementation of closures to be used with C/C++ callbacks.
    </p>
<p>
      A <a class="link" href="gobject-Closures.html#GClosure" title="struct GClosure"><span class="type">GClosure</span></a> provides simple services:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
          Invocation (<code class="function"><a class="link" href="gobject-Closures.html#g-closure-invoke" title="g_closure_invoke ()">g_closure_invoke</a></code>): this is what closures 
          were created for: they hide the details of callback invocation from the
          callback invoker.</p></li>
<li class="listitem"><p>
          Notification: the closure notifies listeners of certain events such as
          closure invocation, closure invalidation and closure finalization. Listeners
          can be registered with <code class="function"><a class="link" href="gobject-Closures.html#g-closure-add-finalize-notifier" title="g_closure_add_finalize_notifier ()">g_closure_add_finalize_notifier</a></code>
          (finalization notification), <code class="function"><a class="link" href="gobject-Closures.html#g-closure-add-invalidate-notifier" title="g_closure_add_invalidate_notifier ()">g_closure_add_invalidate_notifier</a></code> 
          (invalidation notification) and 
          <code class="function"><a class="link" href="gobject-Closures.html#g-closure-add-marshal-guards" title="g_closure_add_marshal_guards ()">g_closure_add_marshal_guards</a></code> (invocation notification).
          There exist symmetric deregistration functions for finalization and invalidation
          events (<code class="function"><a class="link" href="gobject-Closures.html#g-closure-remove-finalize-notifier" title="g_closure_remove_finalize_notifier ()">g_closure_remove_finalize_notifier</a></code> and
          <code class="function"><a class="link" href="gobject-Closures.html#g-closure-remove-invalidate-notifier" title="g_closure_remove_invalidate_notifier ()">g_closure_remove_invalidate_notifier</a></code>) but not for the invocation 
          process.
          <a href="#ftn.id-1.3.5.2.4.2.2.1.6" class="footnote" name="id-1.3.5.2.4.2.2.1.6"><sup class="footnote">[5]</sup></a></p></li>
</ul></div>
<p>
    </p>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.3.5.2.5"></a>C Closures</h3></div></div></div>
<p>
        If you are using C or C++
        to connect a callback to a given event, you will either use simple <a class="link" href="gobject-Closures.html#GCClosure" title="struct GCClosure"><span class="type">GCClosure</span></a>s
        which have a pretty minimal API or the even simpler <code class="function"><a class="link" href="gobject-Signals.html#g-signal-connect" title="g_signal_connect()">g_signal_connect</a></code> 
        functions (which will be presented a bit later).
      </p>
<p>
        <code class="function"><a class="link" href="gobject-Closures.html#g-cclosure-new" title="g_cclosure_new ()">g_cclosure_new</a></code> will create a new closure which can invoke the
        user-provided callback_func with the user-provided
        <em class="parameter"><code>user_data</code></em> as its last parameter. When the closure
        is finalized (second stage of the destruction process), it will invoke
        the <em class="parameter"><code>destroy_data</code></em> function if the user has
        supplied one.
      </p>
<p>
        <code class="function"><a class="link" href="gobject-Closures.html#g-cclosure-new-swap" title="g_cclosure_new_swap ()">g_cclosure_new_swap</a></code> will create a new closure which can invoke the
        user-provided <em class="parameter"><code>callback_func</code></em> with the
        user-provided <em class="parameter"><code>user_data</code></em> as its first parameter
        (instead of being the 
        last parameter as with <code class="function"><a class="link" href="gobject-Closures.html#g-cclosure-new" title="g_cclosure_new ()">g_cclosure_new</a></code>). When the closure
        is finalized (second stage of the destruction process), it will invoke
        the <em class="parameter"><code>destroy_data</code></em> function if the user has
        supplied one.
      </p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.3.5.2.6"></a>Non-C closures (for the fearless)</h3></div></div></div>
<p>
        As was explained above, closures hide the details of callback invocation. In C,
        callback invocation is just like function invocation: it is a matter of creating
        the correct stack frame for the called function and executing a <span class="emphasis"><em>call</em></span>
        assembly instruction.
      </p>
<p>
        C closure marshallers transform the array of GValues which represent 
        the parameters to the target function into a C-style function parameter list, invoke
        the user-supplied C function with this new parameter list, get the return value of the
        function, transform it into a GValue and return this GValue to the marshaller caller.
      </p>
<p>
        A generic C closure marshaller is available as
        <a class="link" href="gobject-Closures.html#g-cclosure-marshal-generic" title="g_cclosure_marshal_generic ()"><code class="function">g_cclosure_marshal_generic</code></a>
        which implements marshalling for all function types using libffi. Custom
        marshallers for different types are not needed apart from performance
        critical code where the libffi-based marshaller may be too slow.
      </p>
<p>
        An example of a custom marshaller is given below, illustrating how
        <span class="type">GValue</span>s can be converted to a C function call. The
        marshaller is for a C function which takes an integer as its first
        parameter and returns void.
</p>
<div class="informalexample">
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    <tbody>
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        <td class="listing_lines" align="right"><pre>1
2
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25</pre></td>
        <td class="listing_code"><pre class="programlisting"><span class="function"><a href="gobject-Closures.html#g-cclosure-marshal-VOID--INT">g_cclosure_marshal_VOID__INT</a></span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">GClosure</span><span class="normal">     </span><span class="symbol">*</span><span class="normal"><a href="chapter-signal.html#closure">closure</a></span><span class="symbol">,</span>
<span class="normal">                              </span><span class="usertype">GValue</span><span class="normal">       </span><span class="symbol">*</span><span class="normal">return_value</span><span class="symbol">,</span>
<span class="normal">                              </span><span class="usertype">guint</span><span class="normal">         n_param_values</span><span class="symbol">,</span>
<span class="normal">                              </span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">GValue</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">param_values</span><span class="symbol">,</span>
<span class="normal">                              </span><span class="usertype">gpointer</span><span class="normal">      invocation_hint</span><span class="symbol">,</span>
<span class="normal">                              </span><span class="usertype">gpointer</span><span class="normal">      marshal_data</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="keyword">typedef</span><span class="normal"> </span><span class="type">void</span><span class="normal"> </span><span class="symbol">(*</span><span class="normal">GMarshalFunc_VOID__INT</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">gpointer</span><span class="normal">     data1</span><span class="symbol">,</span>
<span class="normal">                                          </span><span class="usertype">gint</span><span class="normal">         arg_1</span><span class="symbol">,</span>
<span class="normal">                                          </span><span class="usertype">gpointer</span><span class="normal">     data2</span><span class="symbol">);</span>
<span class="normal">  </span><span class="keyword">register</span><span class="normal"> </span><span class="usertype">GMarshalFunc_VOID__INT</span><span class="normal"> callback</span><span class="symbol">;</span>
<span class="normal">  </span><span class="keyword">register</span><span class="normal"> </span><span class="usertype">GCClosure</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">cc </span><span class="symbol">=</span><span class="normal"> </span><span class="symbol">(</span><span class="normal"><a href="gobject-Closures.html#GCClosure">GCClosure</a></span><span class="symbol">*)</span><span class="normal"> <a href="chapter-signal.html#closure">closure</a></span><span class="symbol">;</span>
<span class="normal">  </span><span class="keyword">register</span><span class="normal"> </span><span class="usertype">gpointer</span><span class="normal"> data1</span><span class="symbol">,</span><span class="normal"> data2</span><span class="symbol">;</span>

<span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">n_param_values </span><span class="symbol">==</span><span class="normal"> </span><span class="number">2</span><span class="symbol">);</span>

<span class="normal">  data1 </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="gobject-Generic-values.html#g-value-peek-pointer">g_value_peek_pointer</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">param_values </span><span class="symbol">+</span><span class="normal"> </span><span class="number">0</span><span class="symbol">);</span>
<span class="normal">  data2 </span><span class="symbol">=</span><span class="normal"> <a href="chapter-signal.html#closure">closure</a></span><span class="symbol">-&gt;</span><span class="normal">data</span><span class="symbol">;</span>

<span class="normal">  callback </span><span class="symbol">=</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">GMarshalFunc_VOID__INT</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">marshal_data </span><span class="symbol">?</span><span class="normal"> marshal_data </span><span class="symbol">:</span><span class="normal"> cc</span><span class="symbol">-&gt;</span><span class="normal">callback</span><span class="symbol">);</span>

<span class="normal">  </span><span class="function">callback</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">data1</span><span class="symbol">,</span>
<span class="normal">            </span><span class="function">g_marshal_value_peek_int</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">param_values </span><span class="symbol">+</span><span class="normal"> </span><span class="number">1</span><span class="symbol">),</span>
<span class="normal">            data2</span><span class="symbol">);</span>
<span class="cbracket">}</span></pre></td>
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    </tbody>
  </table>
</div>

<p>
      </p>
<p>
        There exist other kinds of marshallers, for example there is a generic
        Python marshaller which is used by all Python closures (a Python closure
        is used to invoke a callback written in Python). This Python marshaller
        transforms the input GValue list representing the function parameters
        into a Python tuple which is the equivalent structure in Python.
      </p>
</div>
</div>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.id-1.3.5.2.3.2" class="footnote"><p><a href="#id-1.3.5.2.3.2" class="para"><sup class="para">[4] </sup></a>
        In practice, closures sit at the boundary of language runtimes: if you are
        writing Python code and one of your Python callbacks receives a signal from
        a GTK+ widget, the C code in GTK+ needs to execute your Python
        code. The closure invoked by the GTK+ object invokes the Python callback:
        it behaves as a normal C object for GTK+ and as a normal Python object for
        Python code.
      </p></div>
<div id="ftn.id-1.3.5.2.4.2.2.1.6" class="footnote"><p><a href="#id-1.3.5.2.4.2.2.1.6" class="para"><sup class="para">[5] </sup></a>
            Closures are reference counted and notify listeners of their destruction in a two-stage
            process: the invalidation notifiers are invoked before the finalization notifiers.
          </p></div>
</div>
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