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<section id="using-the-guards-library-in-c-and-c">
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<span id="using-the-guards-library-in-cpp"></span><h1>Using the guards library in C and C++<a class="headerlink" href="#using-the-guards-library-in-c-and-c" title="Link to this heading">¶</a></h1>
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<p>You can use the CodeQL guards library to identify conditional expressions that control the execution of other parts of a program in C and C++ codebases.</p>
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<section id="about-the-guards-library">
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<h2>About the guards library<a class="headerlink" href="#about-the-guards-library" title="Link to this heading">¶</a></h2>
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<p>The guards library (defined in <code class="docutils literal notranslate"><span class="pre">semmle.code.cpp.controlflow.Guards</span></code>) provides a class <a class="reference external" href="https://codeql.github.com/codeql-standard-libraries/cpp/semmle/code/cpp/controlflow/Guards.qll/type.Guards$GuardCondition.html">GuardCondition</a> representing Boolean values that are used to make control flow decisions.
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A <code class="docutils literal notranslate"><span class="pre">GuardCondition</span></code> is considered to guard a basic block if the block can only be reached if the <code class="docutils literal notranslate"><span class="pre">GuardCondition</span></code> is evaluated a certain way. For instance, in the following code, <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre"><</span> <span class="pre">10</span></code> is a <code class="docutils literal notranslate"><span class="pre">GuardCondition</span></code>, and it guards all the code before the return statement.</p>
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<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">if</span><span class="p">(</span><span class="n">x</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">10</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
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<span class="w"> </span><span class="n">f</span><span class="p">(</span><span class="n">x</span><span class="p">);</span>
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<span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">20</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
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<span class="w"> </span><span class="n">g</span><span class="p">(</span><span class="n">x</span><span class="p">);</span>
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<span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
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<span class="w"> </span><span class="n">h</span><span class="p">(</span><span class="n">x</span><span class="p">);</span>
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<span class="p">}</span>
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<span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
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</pre></div>
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</div>
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</section>
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<section id="the-controls-predicate">
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<h2>The <code class="docutils literal notranslate"><span class="pre">controls</span></code> predicate<a class="headerlink" href="#the-controls-predicate" title="Link to this heading">¶</a></h2>
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<p>The <code class="docutils literal notranslate"><span class="pre">controls</span></code> predicate helps determine which blocks are only run when the <code class="docutils literal notranslate"><span class="pre">GuardCondition</span></code> evaluates a certain way. <code class="docutils literal notranslate"><span class="pre">guard.controls(block,</span> <span class="pre">testIsTrue)</span></code> holds if <code class="docutils literal notranslate"><span class="pre">block</span></code> is only entered if the value of this condition is <code class="docutils literal notranslate"><span class="pre">testIsTrue</span></code>.</p>
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<p>In the following code sample, the call to <code class="docutils literal notranslate"><span class="pre">isValid</span></code> controls the calls to <code class="docutils literal notranslate"><span class="pre">performAction</span></code> and <code class="docutils literal notranslate"><span class="pre">logFailure</span></code> but not the return statement.</p>
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<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">if</span><span class="p">(</span><span class="n">isValid</span><span class="p">(</span><span class="n">accessToken</span><span class="p">))</span><span class="w"> </span><span class="p">{</span>
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<span class="w"> </span><span class="n">performAction</span><span class="p">();</span>
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<span class="w"> </span><span class="n">succeeded</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span>
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<span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
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<span class="w"> </span><span class="n">logFailure</span><span class="p">();</span>
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<span class="w"> </span><span class="n">succeeded</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="k">return</span><span class="w"> </span><span class="n">succeeded</span><span class="p">;</span>
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</pre></div>
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</div>
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<p>In the following code sample, the call to <code class="docutils literal notranslate"><span class="pre">isValid</span></code> controls the body of the
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<code class="docutils literal notranslate"><span class="pre">if</span></code> statement, and also the code after the <code class="docutils literal notranslate"><span class="pre">if</span></code>.</p>
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<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">if</span><span class="p">(</span><span class="o">!</span><span class="n">isValid</span><span class="p">(</span><span class="n">accessToken</span><span class="p">))</span><span class="w"> </span><span class="p">{</span>
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<span class="w"> </span><span class="n">logFailure</span><span class="p">();</span>
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<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="n">performAction</span><span class="p">();</span>
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<span class="k">return</span><span class="w"> </span><span class="n">succeeded</span><span class="p">;</span>
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</pre></div>
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</div>
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</section>
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<section id="the-ensureseq-and-ensureslt-predicates">
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<h2>The <code class="docutils literal notranslate"><span class="pre">ensuresEq</span></code> and <code class="docutils literal notranslate"><span class="pre">ensuresLt</span></code> predicates<a class="headerlink" href="#the-ensureseq-and-ensureslt-predicates" title="Link to this heading">¶</a></h2>
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<p>The <code class="docutils literal notranslate"><span class="pre">ensuresEq</span></code> and <code class="docutils literal notranslate"><span class="pre">ensuresLt</span></code> predicates are the main way of determining what, if any, guarantees the <code class="docutils literal notranslate"><span class="pre">GuardCondition</span></code> provides for a given basic block.</p>
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<section id="the-ensureseq-predicate">
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<h3>The <code class="docutils literal notranslate"><span class="pre">ensuresEq</span></code> predicate<a class="headerlink" href="#the-ensureseq-predicate" title="Link to this heading">¶</a></h3>
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<p>When <code class="docutils literal notranslate"><span class="pre">ensuresEq(left,</span> <span class="pre">right,</span> <span class="pre">k,</span> <span class="pre">block,</span> <span class="pre">true)</span></code> holds, then <code class="docutils literal notranslate"><span class="pre">block</span></code> is only executed if <code class="docutils literal notranslate"><span class="pre">left</span></code> was equal to <code class="docutils literal notranslate"><span class="pre">right</span> <span class="pre">+</span> <span class="pre">k</span></code> at their last evaluation. When <code class="docutils literal notranslate"><span class="pre">ensuresEq(left,</span> <span class="pre">right,</span> <span class="pre">k,</span> <span class="pre">block,</span> <span class="pre">false)</span></code> holds, then <code class="docutils literal notranslate"><span class="pre">block</span></code> is only executed if <code class="docutils literal notranslate"><span class="pre">left</span></code> was not equal to <code class="docutils literal notranslate"><span class="pre">right</span> <span class="pre">+</span> <span class="pre">k</span></code> at their last evaluation.</p>
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</section>
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<section id="the-ensureslt-predicate">
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<h3>The <code class="docutils literal notranslate"><span class="pre">ensuresLt</span></code> predicate<a class="headerlink" href="#the-ensureslt-predicate" title="Link to this heading">¶</a></h3>
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<p>When <code class="docutils literal notranslate"><span class="pre">ensuresLt(left,</span> <span class="pre">right,</span> <span class="pre">k,</span> <span class="pre">block,</span> <span class="pre">true)</span></code> holds, then <code class="docutils literal notranslate"><span class="pre">block</span></code> is only executed if <code class="docutils literal notranslate"><span class="pre">left</span></code> was strictly less than <code class="docutils literal notranslate"><span class="pre">right</span> <span class="pre">+</span> <span class="pre">k</span></code> at their last evaluation. When <code class="docutils literal notranslate"><span class="pre">ensuresLt(left,</span> <span class="pre">right,</span> <span class="pre">k,</span> <span class="pre">block,</span> <span class="pre">false)</span></code> holds, then <code class="docutils literal notranslate"><span class="pre">block</span></code> is only executed if <code class="docutils literal notranslate"><span class="pre">left</span></code> was greater than or equal to <code class="docutils literal notranslate"><span class="pre">right</span> <span class="pre">+</span> <span class="pre">k</span></code> at their last evaluation.</p>
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<p>In the following code sample, the comparison on the first line ensures that <code class="docutils literal notranslate"><span class="pre">index</span></code> is less than <code class="docutils literal notranslate"><span class="pre">size</span></code> in the “then” block, and that <code class="docutils literal notranslate"><span class="pre">index</span></code> is greater than or equal to <code class="docutils literal notranslate"><span class="pre">size</span></code> in the “else” block.</p>
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<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">if</span><span class="p">(</span><span class="n">index</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="n">size</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
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<span class="w"> </span><span class="n">ret</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">array</span><span class="p">[</span><span class="n">index</span><span class="p">];</span>
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<span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
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<span class="w"> </span><span class="n">ret</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">nullptr</span>
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<span class="p">}</span>
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<span class="k">return</span><span class="w"> </span><span class="n">ret</span><span class="p">;</span>
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</pre></div>
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</div>
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</section>
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</section>
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<section id="the-compareseq-and-compareslt-predicates">
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<h2>The <code class="docutils literal notranslate"><span class="pre">comparesEq</span></code> and <code class="docutils literal notranslate"><span class="pre">comparesLt</span></code> predicates<a class="headerlink" href="#the-compareseq-and-compareslt-predicates" title="Link to this heading">¶</a></h2>
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<p>The <code class="docutils literal notranslate"><span class="pre">comparesEq</span></code> and <code class="docutils literal notranslate"><span class="pre">comparesLt</span></code> predicates help determine if the <code class="docutils literal notranslate"><span class="pre">GuardCondition</span></code> evaluates to true.</p>
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<section id="the-compareseq-predicate">
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<h3>The <code class="docutils literal notranslate"><span class="pre">comparesEq</span></code> predicate<a class="headerlink" href="#the-compareseq-predicate" title="Link to this heading">¶</a></h3>
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<p><code class="docutils literal notranslate"><span class="pre">comparesEq(left,</span> <span class="pre">right,</span> <span class="pre">k,</span> <span class="pre">true,</span> <span class="pre">testIsTrue)</span></code> holds if <code class="docutils literal notranslate"><span class="pre">left</span></code> equals <code class="docutils literal notranslate"><span class="pre">right</span> <span class="pre">+</span> <span class="pre">k</span></code> when the expression evaluates to <code class="docutils literal notranslate"><span class="pre">testIsTrue</span></code>.</p>
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</section>
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<section id="the-compareslt-predicate">
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<h3>The <code class="docutils literal notranslate"><span class="pre">comparesLt</span></code> predicate<a class="headerlink" href="#the-compareslt-predicate" title="Link to this heading">¶</a></h3>
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<p><code class="docutils literal notranslate"><span class="pre">comparesLt(left,</span> <span class="pre">right,</span> <span class="pre">k,</span> <span class="pre">isLessThan,</span> <span class="pre">testIsTrue)</span></code> holds if <code class="docutils literal notranslate"><span class="pre">left</span> <span class="pre"><</span> <span class="pre">right</span> <span class="pre">+</span> <span class="pre">k</span></code> evaluates to <code class="docutils literal notranslate"><span class="pre">isLessThan</span></code> when the expression evaluates to <code class="docutils literal notranslate"><span class="pre">testIsTrue</span></code>.</p>
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</section>
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</section>
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<section id="further-reading">
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<h2>Further reading<a class="headerlink" href="#further-reading" title="Link to this heading">¶</a></h2>
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<ul class="simple">
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<li><p><a class="reference external" href="https://github.com/github/codeql/tree/main/cpp/ql/src">CodeQL queries for C and C++</a></p></li>
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<li><p><a class="reference external" href="https://github.com/github/codeql/tree/main/cpp/ql/examples">Example queries for C and C++</a></p></li>
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<li><p><a class="reference external" href="https://codeql.github.com/codeql-standard-libraries/cpp/">CodeQL library reference for C and C++</a></p></li>
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</ul>
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<ul class="simple">
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<li><p>“<a class="reference internal" href="../ql-language-reference/index.html#ql-language-reference"><span class="std std-ref">QL language reference</span></a>”</p></li>
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<li><p>“<a class="reference internal" href="../codeql-overview/codeql-tools.html#codeql-tools"><span class="std std-ref">CodeQL tools</span></a>”</p></li>
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</ul>
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