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|
||
<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">-></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">-></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">-></span><span class="n">param1</span><span class="p">,</span><span class="w"> </span><span class="n">data</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">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">-></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">-></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>
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||
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Contents
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<ul>
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<li><a class="reference internal" href="#">Threading</a><ul>
|
||
<li><a class="reference internal" href="#summary">Summary</a></li>
|
||
<li><a class="reference internal" href="#when-to-use-threading">When to use threading</a></li>
|
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<li><a class="reference internal" href="#using-threading">Using threading</a></li>
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<li><a class="reference internal" href="#debugging">Debugging</a></li>
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