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     20 <h1>FAQ and How to Deal with Common False Positives</h1>
     21 
     22 <ol>
     23   <li><a href="#custom_assert">How do I tell the analyzer that I do not want the bug being
     24 reported here since my custom error handler will safely end the execution before
     25 the bug is reached?</a></li>
     26   <li><a href="#null_pointer">The analyzer reports a null dereference, but I know that the
     27 pointer is never null. How can I tell the analyzer that a pointer can never be
     28 null?</a></li>
     29   <li><a href="#use_assert">The analyzer assumes that a loop body is never entered.  How can I tell it that the loop body will be entered at least once?</a></li>
     30   <li><a href="#suppress_issue">How can I suppress a specific analyzer warning?</a></li>
     31   <li><a href="#exclude_code">How can I selectively exclude code the analyzer examines?</a></li>
     32 </ol>
     33 
     34 
     35 <h4 id="custom_assert" class="faq">Q: How do I tell the analyzer that I do not want the bug being
     36 reported here since my custom error handler will safely end the execution before
     37 the bug is reached?</h4>
     38 
     39 <img src="images/example_custom_assert.png" alt="example custom assert">
     40 
     41 <p>You can tell the analyzer that this path is unreachable by teaching it about your <a href = "annotations.html#custom_assertions" >custom assertion handlers</a>. For example, you can modify the code segment as following.</p>
     42 
     43 <pre class="code_example">
     44 void customAssert() <span class="code_highlight">__attribute__((analyzer_noreturn))</span>;
     45 int foo(int *b) {
     46   if (!b)
     47     customAssert();
     48   return *b;
     49 }</pre>
     50 
     51 
     52 <h4 id="null_pointer" class="faq">Q: The analyzer reports a null dereference, but I know that the
     53 pointer is never null. How can I tell the analyzer that a pointer can never be
     54 null?</h4>
     55 
     56 <img src="images/example_null_pointer.png" alt="example null pointer">
     57 
     58 <p>The reason the analyzer often thinks that a pointer can be null is because the preceding code checked compared it against null. So if you are absolutely sure that it cannot be null, remove the preceding check and, preferably, add an assertion as well. For example, in the code segment above, it will be sufficient to remove the <tt>if (!b)</tt> check. </p>
     59 
     60 <pre class="code_example">
     61 void usePointer(int *b);
     62 int foo(int *b) {
     63   usePointer(b);
     64   return *b;
     65 }</pre>
     66 
     67 <h4 id="use_assert" class="faq">Q: The analyzer assumes that a loop body is never entered.  How can I tell it that the loop body will be entered at least once?</h4>
     68 
     69 <img src="images/example_use_assert.png" alt="example use assert">
     70 
     71 <p>You can teach the analyzer facts about your code as well as document it by
     72 using assertions. In the contrived example above, the analyzer reports an error
     73 on the path which assumes that the loop is never entered. However, the owner of
     74 the code might know that the loop is always entered because the input parameter
     75 <tt>length</tt> is always greater than <tt>0</tt>. The false positive can be
     76 suppressed by asserting this knowledge, adding <tt>assert(length > 0)</tt> in
     77 the beginning of the function.</p>
     78 
     79 <pre class="code_example">
     80 int foo(int length) {
     81   int x = 0;
     82   <span class="code_highlight">assert(length > 0);</span>
     83   for (int i = 0; i < length; i++)
     84     x += 1;
     85   return length/x;
     86 }
     87 </pre>
     88 
     89 <h4 id="suppress_issue" class="faq">Q: How can I suppress a specific analyzer warning?</h4>
     90 
     91 <p>There is currently no solid mechanism for suppressing an analyzer warning,
     92 although this is currently being investigated. When you encounter an analyzer
     93 bug/false positive, check if it's one of the issues discussed above or if the
     94 analyzer <a href = "annotations.html#custom_assertions" >annotations</a> can 
     95 resolve the issue. Second, please <a href = "filing_bugs.html">report it</a> to 
     96 help us improve user experience. As the last resort, consider using <tt>__clang_analyzer__</tt> macro
     97 <a href = "faq.html#exclude_code" >described below</a>.</p>
     98 
     99 <h4 id="exclude_code" class="faq">Q: How can I selectively exclude code the analyzer examines?</h4>
    100 
    101 <p>When the static analyzer is using clang to parse source files, it implicitly 
    102 defines the preprocessor macro <tt>__clang_analyzer__</tt>. One can use this 
    103 macro to selectively exclude code the analyzer examines. Here is an example:
    104 
    105 <pre class="code_example">
    106 #ifndef __clang_analyzer__
    107 // Code not to be analyzed
    108 #endif
    109 </pre>
    110 
    111 This usage is discouraged because it makes the code dead to the analyzer from 
    112 now on. Instead, we prefer that users file bugs against the analyzer when it flags 
    113 false positives.
    114 </p>
    115 
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