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<section id="threading">
<h1>Threading<a class="headerlink" href="#threading" title="Permalink to this headline">#</a></h1>
<section id="summary">
<h2>Summary<a class="headerlink" href="#summary" title="Permalink to this headline">#</a></h2>
<ul class="simple">
<li><p>Do not use threads if at all possible.</p></li>
<li><p>If threads have to be used, use <code class="docutils literal notranslate"><span class="pre">GTask</span></code> or <code class="docutils literal notranslate"><span class="pre">GThreadPool</span></code> and isolate the
threaded code as much as possible.</p></li>
<li><p>Use <code class="docutils literal notranslate"><span class="pre">g_thread_join()</span></code> to avoid leaking thread resources if using
<code class="docutils literal notranslate"><span class="pre">GThread</span></code> manually.</p></li>
<li><p>Be careful about the <code class="docutils literal notranslate"><span class="pre">GMainContext</span></code> which code is executed in if using
threads. Executing code in the wrong context can cause race conditions, or
block the main loop.</p></li>
</ul>
</section>
<section id="when-to-use-threading">
<h2>When to use threading<a class="headerlink" href="#when-to-use-threading" title="Permalink to this headline">#</a></h2>
<p>When writing projects using GLib, the default approach should be to never use
threads. Instead, make proper use of the GLib main context which, through the
use of asynchronous operations, allows most blocking I/O operations to continue
in the background while the main context continues to process other events.
Analysis, review and debugging of threaded code becomes very hard, very quickly.</p>
<p>Threading should only be necessary when using an external library which has
blocking functions which need to be called from GLib code. If the library
provides a non-blocking alternative, or one which integrates with a <code class="docutils literal notranslate"><span class="pre">poll()</span></code>
loop, that should be used in preference. If the blocking function really must be
used, a thin wrapper should be written for it to convert it to the normal
<code class="docutils literal notranslate"><span class="pre">GAsyncResult</span></code> style of GLib asynchronous function, running the blocking
operation in a worker thread.</p>
<p>For example, the following blocking function:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">int</span><span class="w"></span>
<span class="nf">some_blocking_function</span><span class="w"> </span><span class="p">(</span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">param1</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">param2</span><span class="p">);</span><span class="w"></span>
</pre></div>
</div>
<p>Should be wrapped by this pair of functions:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span><span class="w"></span>
<span class="nf">some_blocking_function_async</span><span class="w"> </span><span class="p">(</span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">param1</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">param2</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">GCancellable</span><span class="w"> </span><span class="o">*</span><span class="n">cancellable</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">GAsyncReadyCallback</span><span class="w"> </span><span class="n">callback</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">gpointer</span><span class="w"> </span><span class="n">user_data</span><span class="p">);</span><span class="w"></span>
<span class="kt">int</span><span class="w"></span>
<span class="nf">some_blocking_function_finish</span><span class="w"> </span><span class="p">(</span><span class="n">GAsyncResult</span><span class="w"> </span><span class="o">*</span><span class="n">result</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">GError</span><span class="w"> </span><span class="o">**</span><span class="n">error</span><span class="p">);</span><span class="w"></span>
</pre></div>
</div>
<p>With an implementation like:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cm">/* Closure for the call’s parameters. */</span><span class="w"></span>
<span class="k">typedef</span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">param1</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">param2</span><span class="p">;</span><span class="w"></span>
<span class="p">}</span><span class="w"> </span><span class="n">SomeBlockingFunctionData</span><span class="p">;</span><span class="w"></span>
<span class="k">static</span><span class="w"> </span><span class="kt">void</span><span class="w"></span>
<span class="nf">some_blocking_function_data_free</span><span class="w"> </span><span class="p">(</span><span class="n">SomeBlockingFunctionData</span><span class="w"> </span><span class="o">*</span><span class="n">data</span><span class="p">)</span><span class="w"></span>
<span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">free_param</span><span class="w"> </span><span class="p">(</span><span class="n">data</span><span class="o">-&gt;</span><span class="n">param1</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="n">free_param</span><span class="w"> </span><span class="p">(</span><span class="n">data</span><span class="o">-&gt;</span><span class="n">param2</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="n">g_free</span><span class="w"> </span><span class="p">(</span><span class="n">data</span><span class="p">);</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
<span class="k">static</span><span class="w"> </span><span class="kt">void</span><span class="w"></span>
<span class="nf">some_blocking_function_thread_cb</span><span class="w"> </span><span class="p">(</span><span class="n">GTask</span><span class="w"> </span><span class="o">*</span><span class="n">task</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">gpointer</span><span class="w"> </span><span class="n">source_object</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">gpointer</span><span class="w"> </span><span class="n">task_data</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">GCancellable</span><span class="w"> </span><span class="o">*</span><span class="n">cancellable</span><span class="p">)</span><span class="w"></span>
<span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">SomeBlockingFunctionData</span><span class="w"> </span><span class="o">*</span><span class="n">data</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">task_data</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">retval</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="cm">/* Handle cancellation. */</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">g_task_return_error_if_cancelled</span><span class="w"> </span><span class="p">(</span><span class="n">task</span><span class="p">))</span><span class="w"></span>
<span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">return</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="cm">/* Run the blocking function. */</span><span class="w"></span>
<span class="w"> </span><span class="n">retval</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">some_blocking_function</span><span class="w"> </span><span class="p">(</span><span class="n">data</span><span class="o">-&gt;</span><span class="n">param1</span><span class="p">,</span><span class="w"> </span><span class="n">data</span><span class="o">-&gt;</span><span class="n">param2</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="n">g_task_return_int</span><span class="w"> </span><span class="p">(</span><span class="n">task</span><span class="p">,</span><span class="w"> </span><span class="n">retval</span><span class="p">);</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
<span class="kt">void</span><span class="w"></span>
<span class="nf">some_blocking_function_async</span><span class="w"> </span><span class="p">(</span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">param1</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">param2</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">GCancellable</span><span class="w"> </span><span class="o">*</span><span class="n">cancellable</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">GAsyncReadyCallback</span><span class="w"> </span><span class="n">callback</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">gpointer</span><span class="w"> </span><span class="n">user_data</span><span class="p">)</span><span class="w"></span>
<span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">GTask</span><span class="w"> </span><span class="o">*</span><span class="n">task</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nb">NULL</span><span class="p">;</span><span class="w"> </span><span class="cm">/* owned */</span><span class="w"></span>
<span class="w"> </span><span class="n">SomeBlockingFunctionData</span><span class="w"> </span><span class="o">*</span><span class="n">data</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nb">NULL</span><span class="p">;</span><span class="w"> </span><span class="cm">/* owned */</span><span class="w"></span>
<span class="w"> </span><span class="n">g_return_if_fail</span><span class="w"> </span><span class="p">(</span><span class="n">validate_param</span><span class="w"> </span><span class="p">(</span><span class="n">param1</span><span class="p">));</span><span class="w"></span>
<span class="w"> </span><span class="n">g_return_if_fail</span><span class="w"> </span><span class="p">(</span><span class="n">validate_param</span><span class="w"> </span><span class="p">(</span><span class="n">param2</span><span class="p">));</span><span class="w"></span>
<span class="w"> </span><span class="n">g_return_if_fail</span><span class="w"> </span><span class="p">(</span><span class="n">cancellable</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="nb">NULL</span><span class="w"> </span><span class="o">||</span><span class="w"> </span><span class="n">G_IS_CANCELLABLE</span><span class="w"> </span><span class="p">(</span><span class="n">cancellable</span><span class="p">));</span><span class="w"></span>
<span class="w"> </span><span class="n">task</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">g_task_new</span><span class="w"> </span><span class="p">(</span><span class="nb">NULL</span><span class="p">,</span><span class="w"> </span><span class="n">cancellable</span><span class="p">,</span><span class="w"> </span><span class="n">callback</span><span class="p">,</span><span class="w"> </span><span class="n">user_data</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="n">g_task_set_source_tag</span><span class="w"> </span><span class="p">(</span><span class="n">task</span><span class="p">,</span><span class="w"> </span><span class="n">some_blocking_function_async</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="cm">/* Cancellation should be handled manually using mechanisms specific to</span>
<span class="cm"> * some_blocking_function(). */</span><span class="w"></span>
<span class="w"> </span><span class="n">g_task_set_return_on_cancel</span><span class="w"> </span><span class="p">(</span><span class="n">task</span><span class="p">,</span><span class="w"> </span><span class="n">FALSE</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="cm">/* Set up a closure containing the call’s parameters. Copy them to avoid</span>
<span class="cm"> * locking issues between the calling thread and the worker thread. */</span><span class="w"></span>
<span class="w"> </span><span class="n">data</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">g_new0</span><span class="w"> </span><span class="p">(</span><span class="n">SomeBlockingFunctionData</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="n">data</span><span class="o">-&gt;</span><span class="n">param1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">copy_param</span><span class="w"> </span><span class="p">(</span><span class="n">param1</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="n">data</span><span class="o">-&gt;</span><span class="n">param2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">copy_param</span><span class="w"> </span><span class="p">(</span><span class="n">param2</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="n">g_task_set_task_data</span><span class="w"> </span><span class="p">(</span><span class="n">task</span><span class="p">,</span><span class="w"> </span><span class="n">data</span><span class="p">,</span><span class="w"> </span><span class="n">some_blocking_function_data_free</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="cm">/* Run the task in a worker thread and return immediately while that continues</span>
<span class="cm"> * in the background. When it’s done it will call @callback in the current</span>
<span class="cm"> * thread default main context. */</span><span class="w"></span>
<span class="w"> </span><span class="n">g_task_run_in_thread</span><span class="w"> </span><span class="p">(</span><span class="n">task</span><span class="p">,</span><span class="w"> </span><span class="n">some_blocking_function_thread_cb</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="n">g_object_unref</span><span class="w"> </span><span class="p">(</span><span class="n">task</span><span class="p">);</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
<span class="kt">int</span><span class="w"></span>
<span class="nf">some_blocking_function_finish</span><span class="w"> </span><span class="p">(</span><span class="n">GAsyncResult</span><span class="w"> </span><span class="o">*</span><span class="n">result</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">GError</span><span class="w"> </span><span class="o">**</span><span class="n">error</span><span class="p">)</span><span class="w"></span>
<span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">g_return_val_if_fail</span><span class="w"> </span><span class="p">(</span><span class="n">g_task_is_valid</span><span class="w"> </span><span class="p">(</span><span class="n">result</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">some_blocking_function_async</span><span class="p">),</span><span class="w"> </span><span class="mi">-1</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="n">g_return_val_if_fail</span><span class="w"> </span><span class="p">(</span><span class="n">error</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="nb">NULL</span><span class="w"> </span><span class="o">||</span><span class="w"> </span><span class="o">*</span><span class="n">error</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="nb">NULL</span><span class="p">,</span><span class="w"> </span><span class="mi">-1</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">g_task_propagate_int</span><span class="w"> </span><span class="p">(</span><span class="n">G_TASK</span><span class="w"> </span><span class="p">(</span><span class="n">result</span><span class="p">),</span><span class="w"> </span><span class="n">error</span><span class="p">);</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>See the <a class="reference external" href="https://docs.gtk.org/gio/class.Task.html">GTask documentation</a>
for more details.</p>
</section>
<section id="using-threading">
<h2>Using threading<a class="headerlink" href="#using-threading" title="Permalink to this headline">#</a></h2>
<p>If <code class="docutils literal notranslate"><span class="pre">GTask</span></code> is not suitable for writing the worker thread, a more low-level
approach must be used. This should be considered very carefully, as it is very
easy to get threading code wrong in ways which will unpredictably cause bugs at
runtime, cause deadlocks, or consume too many resources and terminate the
program.</p>
<p>A full manual on writing threaded code is beyond the scope of this document, but
here are a number of guidelines to follow which should reduce the potential for
bugs in threading code. The overriding principle is to reduce the amount of code
and data which can be affected by threading—for example, reducing the number of
threads, the complexity of worker thread implementation, and the amount of data
shared between threads.</p>
<ul class="simple">
<li><p>Use <code class="docutils literal notranslate"><span class="pre">GThreadPool</span></code> instead of manually creating threads if possible.
<code class="docutils literal notranslate"><span class="pre">GThreadPool</span></code> supports a work queue, limits on the number of spawned
threads, and automatically joins finished threads so they are not leaked.</p></li>
<li><p>If it is not possible to use a GThreadPool (which is rarely the case):</p>
<ul>
<li><p>Use <code class="docutils literal notranslate"><span class="pre">g_thread_try_new()</span></code> to spawn threads, instead of <code class="docutils literal notranslate"><span class="pre">g_thread_new()</span></code>,
so errors due to the system running out of threads can be handled
gracefully rather than unconditionally aborting the program.</p></li>
<li><p>Explicitly join threads using <code class="docutils literal notranslate"><span class="pre">g_thread_join()</span></code> to avoid leaking the
thread resources.</p></li>
</ul>
</li>
<li><p>Use message passing to transfer data between threads, rather than manual
locking with mutexes. <code class="docutils literal notranslate"><span class="pre">GThreadPool</span></code> explicitly supports this with
<code class="docutils literal notranslate"><span class="pre">g_thread_pool_push()</span></code>.</p></li>
<li><p>If mutexes must be used:</p>
<ul>
<li><p>Isolate threading code as much as possible, keeping mutexes private
within classes, and tightly bound to very specific class members.</p></li>
<li><p>All mutexes should be clearly commented beside their declaration,
indicating which other structures or variables they protect access to.
Similarly, those variables should be commented saying that they should
only be accessed with that mutex held.</p></li>
</ul>
</li>
<li><p>Be careful about interactions between main contexts and threads. For example,
<code class="docutils literal notranslate"><span class="pre">g_timeout_add_seconds()</span></code> adds a timeout <em>to be executed in the global
default main context</em>, which is being run in the main thread, not necessarily
the current thread. Getting this wrong can mean that work intended for a
worker thread accidentally ends up being executed in the main thread anyway.</p></li>
</ul>
</section>
<section id="debugging">
<h2>Debugging<a class="headerlink" href="#debugging" title="Permalink to this headline">#</a></h2>
<p>Debugging threading issues is tricky, both because they are hard to reproduce,
and because they are hard to reason about. This is one of the big reasons for
avoiding using threads in the first place.</p>
<p>However, if a threading issue does arise, Valgrind’s <a class="reference external" href="https://valgrind.org/docs/manual/drd-manual.html">drd</a> and <a class="reference external" href="https://valgrind.org/docs/manual/hg-manual.html">helgrind</a> tools are useful.</p>
</section>
</section>
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<li><a class="reference internal" href="#">Threading</a><ul>
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