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<div class="chapter">
<div class="titlepage"><div><div><h1 class="title">
<a name="chapter-multi-threaded-programs"></a>Chapter 29. Multi-threaded programs</h1></div></div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<ul class="toc">
<li><span class="section"><a href="chapter-multi-threaded-programs.html#sec-the-constraints">The constraints</a></span></li>
<li><span class="section"><a href="sec-using-glib-dispatcher.html">Using Glib::Dispatcher</a></span></li>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="sec-the-constraints"></a>The constraints</h2></div></div></div>
<p>
Care is required when writing programs based on <span class="application">gtkmm</span> using
multiple threads of execution, arising from the fact that
<span class="application">libsigc++</span>, and in particular
<code class="classname">sigc::trackable</code>, are not thread-safe. That's
because none of the complex interactions that occur behind the scenes
when using <span class="application">libsigc++</span> are protected by a
mutex or other means of synchronization.
<a href="#ftn.id7073" class="footnote" name="id7073"><sup class="footnote">[1]</sup></a>
</p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="the-rules"></a>The rules</h3></div></div></div>
<p>
This requires a number of rules to be observed when writing
multi-threaded programs using <span class="application">gtkmm</span>. These are set out below, but
one point to note is that extra care is required when deriving classes
from <code class="classname">sigc::trackable</code>, because the effects are
unintuitive (see particularly points 4 and 5 below).
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
<p>
Use <code class="classname">Glib::Dispatcher</code> to invoke <span class="application">gtkmm</span> functions
from worker threads (this is dealt with in more detail in the next
section).
</p>
</li>
<li class="listitem">
<p>
A <code class="classname">sigc::signal</code> object should be regarded as
owned by the thread which created it. Only that thread should connect
a <code class="classname">sigc::slot</code> object to the signal object, and
only that thread should <code class="methodname">emit()</code> or call
<code class="methodname">operator()()</code> on the signal, or null any
connected <code class="classname">sigc::slot</code> object. It follows
(amongst other things) that any signal object provided by a <span class="application">gtkmm</span>
widget should only be operated on in the main GUI thread and any
object deriving from <code class="classname">sigc::trackable</code> having its
non-static methods referenced by slots connected to the signal object
should only be destroyed in that thread.
</p>
</li>
<li class="listitem">
<p>
Any <code class="classname">sigc::connection</code> object should be regarded
as owned by the thread in which the method returning the
<code class="classname">sigc::connection</code> object was called. Only that
thread should call <code class="classname">sigc::connection</code> methods on
the object.
</p>
</li>
<li class="listitem">
<p>
A <code class="classname">sigc::slot</code> object created by a call to
<code class="function">sigc::mem_fun()</code> which references a method of a
class deriving from <code class="classname">sigc::trackable</code> should
never be copied to another thread, nor destroyed by a different thread
than the one which created it.
</p>
</li>
<li class="listitem">
<p>
If a particular class object derives from
<code class="classname">sigc::trackable</code>, only one thread should create
<code class="classname">sigc::slot</code> objects representing any of the
class's non-static methods by calling
<code class="function">sigc::mem_fun()</code>. The first thread to create such
a slot should be regarded as owning the relevant object for the
purpose of creating further slots referencing <span class="emphasis"><em>any</em></span>
of its non-static methods using that function, or nulling those slots
by disconnecting them or destroying the trackable object.
</p>
</li>
<li class="listitem">
<p>
Although <span class="application">glib</span> is itself thread-safe, any
<span class="application">glibmm</span> wrappers which use
<span class="application">libsigc++</span> will not be. So for example, only
the thread in which a main loop runs should call
<code class="methodname">Glib::SignalIdle::connect()</code>,
<code class="methodname">Glib::SignalIO::connect()</code>,
<code class="methodname">Glib::SignalTimeout::connect()</code>,
<code class="methodname">Glib::SignalTimeout::connect_seconds</code>
for that main loop, or manipulate any
<code class="classname">sigc::connection</code> object returned by them.
</p>
<p>
The connect*_once() variants,
<code class="methodname">Glib::SignalIdle::connect_once()</code>,
<code class="methodname">Glib::SignalTimeout::connect_once()</code>,
<code class="methodname">Glib::SignalTimeout::connect_seconds_once()</code>,
are thread-safe for any case where the slot is not created by a call to
<code class="function">sigc::mem_fun()</code> which represents a method of a class
deriving from <code class="classname">sigc::trackable</code>.
</p>
</li>
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<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.id7073" class="footnote">
<p><a href="#id7073" class="para"><sup class="para">[1] </sup></a>
These interactions arise from the fact that, amongst other things, a
class inheriting from <code class="classname">sigc::trackable</code> will, via
that inheritance, have a <code class="classname">std::list</code> object
keeping track of slots created by calls to
<code class="function">sigc::mem_fun()</code> representing any of its
non-static methods (more particularly it keeps a list of callbacks
which will null the connected slots on its destruction). Each
<code class="classname">sigc::slot</code> object also keeps, via
<code class="classname">sigc::slot_rep</code>, its own
<code class="classname">sigc::trackable</code> object to track any
<code class="classname">sigc::connection</code> objects which it needs to
inform about its demise, and also has a function to deregister itself
from any <code class="classname">sigc::trackable</code> on disconnection or
destruction. <code class="classname">sigc::signal</code> objects also keep
lists of slots, which will be updated by a call to their
<code class="methodname">connect()</code> method or calls to any
<code class="classname">sigc::connection</code> object relating to such a
connection.
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