external/bsd/llvm/dist/clang/www/analyzer/faq.html

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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> In the contrived example above, the analyzer has detected that the body of
  72 the loop is never entered for the case where <tt>length <= 0</tt>. In this
  73 particular example, you may know that the loop will always be entered because
  74 the input parameter <tt>length</tt> will be greater than zero in all calls to this
  75 function. You can teach the analyzer facts about your code as well as document
  76 it by using assertions. By adding <tt>assert(length > 0)</tt> in the beginning
  77 of the function, you tell the analyzer that your code is never expecting a zero
  78 or a negative value, so it won't need to test the correctness of those paths.
  79 </p>
  80
  81 <pre class="code_example">
  82 int foo(int length) {
  83   int x = 0;
  84   <span class="code_highlight">assert(length > 0);</span>
  85   for (int i = 0; i < length; i++)
  86     x += 1;
  87   return length/x;
  88 }
  89 </pre>
  90
  91 <h4 id="suppress_issue" class="faq">Q: How can I suppress a specific analyzer warning?</h4>
  92
  93 <p>There is currently no solid mechanism for suppressing an analyzer warning,
  94 although this is currently being investigated. When you encounter an analyzer
  95 bug/false positive, check if it's one of the issues discussed above or if the
  96 analyzer <a href = "annotations.html#custom_assertions" >annotations</a> can
  97 resolve the issue. Second, please <a href = "filing_bugs.html">report it</a> to
  98 help us improve user experience. As the last resort, consider using <tt>__clang_analyzer__</tt> macro
  99 <a href = "faq.html#exclude_code" >described below</a>.</p>
 100
 101 <h4 id="exclude_code" class="faq">Q: How can I selectively exclude code the analyzer examines?</h4>
 102
 103 <p>When the static analyzer is using clang to parse source files, it implicitly
 104 defines the preprocessor macro <tt>__clang_analyzer__</tt>. One can use this
 105 macro to selectively exclude code the analyzer examines. Here is an example:
 106
 107 <pre class="code_example">
 108 #ifndef __clang_analyzer__
 109 // Code not to be analyzed
 110 #endif
 111 </pre>
 112
 113 This usage is discouraged because it makes the code dead to the analyzer from
 114 now on. Instead, we prefer that users file bugs against the analyzer when it flags
 115 false positives.
 116 </p>
 117
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