1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3 <html> 4 <head> 5 <title>FAQ and How to Deal with Common False Positives</title> 6 <link type="text/css" rel="stylesheet" href="menu.css"> 7 <link type="text/css" rel="stylesheet" href="content.css"> 8 <script type="text/javascript" src="scripts/menu.js"></script> 9 <style type="text/css"> 10 tr:first-child { width:20%; } 11 </style> 12 </head> 13 <body> 14 15 <div id="page"> 16 <!--#include virtual="menu.html.incl"--> 17 18 <div id="content"> 19 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> 115 116 </div> 117 </div> 118 </body> 119 </html> 120 121