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<li class="toctree-l2"><a class="reference internal" href="../introduction/languages.html">Programming Languages</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../introduction/builder.html">GNOME Builder</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../introduction/flatpak.html">Flatpak</a></li>
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<li class="toctree-l1 has-children"><a class="reference internal" href="../guidelines.html">Guidelines</a><input class="toctree-checkbox" id="toctree-checkbox-3" name="toctree-checkbox-3" role="switch" type="checkbox"/><label for="toctree-checkbox-3"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul>
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<li class="toctree-l2 has-children"><a class="reference internal" href="../guidelines/programming.html">Programming Guidelines</a><input class="toctree-checkbox" id="toctree-checkbox-4" name="toctree-checkbox-4" role="switch" type="checkbox"/><label for="toctree-checkbox-4"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul>
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<li class="toctree-l3"><a class="reference internal" href="../guidelines/programming/coding-style.html">C Coding Style</a></li>
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<li class="toctree-l3"><a class="reference internal" href="../guidelines/programming/memory-management.html">Managing Memory</a></li>
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<li class="toctree-l3"><a class="reference internal" href="../guidelines/accessibility/coding-guidelines.html">Coding Guidelines for Supporting Accessibility</a></li>
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<li class="toctree-l3"><a class="reference internal" href="../guidelines/accessibility/custom-widgets.html">Making Custom Components Accessible</a></li>
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<li class="toctree-l3"><a class="reference internal" href="../guidelines/localization/practices.html">Best Practices for Localization</a></li>
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<li class="toctree-l2 has-children"><a class="reference internal" href="../guidelines/maintainer.html">Maintainer Guidelines</a><input class="toctree-checkbox" id="toctree-checkbox-7" name="toctree-checkbox-7" role="switch" type="checkbox"/><label for="toctree-checkbox-7"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul>
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<li class="toctree-l3"><a class="reference internal" href="../guidelines/maintainer/api-stability.html">API Stability</a></li>
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<li class="toctree-l3"><a class="reference internal" href="../guidelines/maintainer/parallel-installability.html">Parallel Installability</a></li>
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<li class="toctree-l3"><a class="reference internal" href="../guidelines/maintainer/integrating.html">Integrating with GNOME</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../guidelines/devel-docs.html">Developer Documentation Style Guidelines</a></li>
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<li class="toctree-l4"><a class="reference internal" href="beginners/getting_started/content_view.html">Adding A Content View</a></li>
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<li class="toctree-l4"><a class="reference internal" href="beginners/getting_started/dark_mode.html">Forcing The Dark Color Scheme</a></li>
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<li class="toctree-l3 has-children"><a class="reference internal" href="beginners/components.html">UI components</a><input class="toctree-checkbox" id="toctree-checkbox-11" name="toctree-checkbox-11" role="switch" type="checkbox"/><label for="toctree-checkbox-11"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul>
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<li class="toctree-l4"><a class="reference internal" href="beginners/components/window.html">Windows</a></li>
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<li class="toctree-l4"><a class="reference internal" href="beginners/components/image.html">Images</a></li>
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<li class="toctree-l4"><a class="reference internal" href="beginners/components/switch.html">Switches</a></li>
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<li class="toctree-l2"><a class="reference internal" href="application.html">Using GtkApplication</a></li>
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<li class="toctree-l2"><a class="reference internal" href="save-state.html">Saving and Loading Window State</a></li>
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<li class="toctree-l2"><a class="reference internal" href="notifications.html">Using Notifications</a></li>
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<li class="toctree-l2"><a class="reference internal" href="deprecations.html">Dealing With Deprecations</a></li>
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<li class="toctree-l2 current current-page"><a class="current reference internal" href="#">Pre- and Post-Conditions</a></li>
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<li class="toctree-l2"><a class="reference internal" href="main-contexts.html">Main Contexts</a></li>
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<li class="toctree-l2"><a class="reference internal" href="lists.html">Using GLib Lists</a></li>
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<i class="icon"><svg><use href="#svg-toc"></use></svg></i>
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</label>
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<section id="pre-and-post-conditions">
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<h1>Pre- and Post-Conditions<a class="headerlink" href="#pre-and-post-conditions" title="Permalink to this headline">#</a></h1>
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<p>An important part of secure coding is ensuring that incorrect data does not
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propagate far through code — the further some malicious input can propagate,
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the more code it sees, and the greater potential there is for an exploit to be
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possible.</p>
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<p>A standard way of preventing the propagation of invalid data is to check all
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inputs to, and outputs from, all publicly visible functions in a library or
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module. There are two levels of checking:</p>
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<dl class="simple">
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<dt>Validation</dt><dd><p>Check for invalid input and return an error gracefully on failure.</p>
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</dd>
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||
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<dt>Assertions</dt><dd><p>Check for programmer errors and abort the program on failure.</p>
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</dd>
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||
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</dl>
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<section id="validation">
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<h2>Validation<a class="headerlink" href="#validation" title="Permalink to this headline">#</a></h2>
|
||
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<p>Validation is a complex topic, and is typically handled using <code class="docutils literal notranslate"><span class="pre">GError</span></code>. For more information on how to use <code class="docutils literal notranslate"><span class="pre">GError</span></code> in your API, see the <a class="reference external" href="https://docs.gtk.org/glib/error-reporting.html">GLib reference on error reporting</a>.</p>
|
||
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<p>The remainder of this section discusses pre- and post-condition assertions, which are purely for catching programmer errors.
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||
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A programmer error is where a function is called in a way which is documented as disallowed.
|
||
|
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For example, if <code class="docutils literal notranslate"><span class="pre">NULL</span></code> is passed to a parameter which is documented as requiring a non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code> value to be passed; or if a negative value is passed to a function which requires a positive value.
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||
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Programmer errors can happen on output too — for example, returning NULL when it is not documented to, or not setting a <code class="docutils literal notranslate"><span class="pre">GError</span></code> output when it fails.</p>
|
||
|
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</section>
|
||
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<section id="assertions">
|
||
|
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<h2>Assertions<a class="headerlink" href="#assertions" title="Permalink to this headline">#</a></h2>
|
||
|
|
<p>Adding pre- and post-condition assertions to code is as much about ensuring the behavior of each function is correctly and completely documented as it is about adding the assertions themselves.
|
||
|
|
All assertions should be documented, preferably by using the relevant <a class="reference internal" href="../guidelines/programming/introspection.html"><span class="doc">introspection annotations</span></a>, such as (<code class="docutils literal notranslate"><span class="pre">nullable</span></code>).</p>
|
||
|
|
<p>Pre- and post-condition assertions are implemented using <code class="docutils literal notranslate"><span class="pre">g_return_if_fail()</span></code> and <code class="docutils literal notranslate"><span class="pre">g_return_val_if_fail()</span></code>.</p>
|
||
|
|
<p>The pre-conditions should check each parameter at the start of the function, before any other code is executed (even retrieving the private data structure from a GObject, for example, since the GObject pointer could be <code class="docutils literal notranslate"><span class="pre">NULL</span></code>).
|
||
|
|
The post-conditions should check the return value and any output parameters at the end of the function — this requires a single <code class="docutils literal notranslate"><span class="pre">return</span></code> statement and use of goto to merge other control paths into it.
|
||
|
|
See <a class="reference internal" href="../guidelines/programming/memory-management.html#single-path-cleanup"><span class="std std-ref">Single-path cleanup</span></a> for an example.</p>
|
||
|
|
</section>
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</section>
|
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Contents
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<div class="toc-tree">
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<ul>
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<li><a class="reference internal" href="#">Pre- and Post-Conditions</a><ul>
|
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|
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<li><a class="reference internal" href="#validation">Validation</a></li>
|
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|
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<li><a class="reference internal" href="#assertions">Assertions</a></li>
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</ul>
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