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="#dead_store">How do I tell the static analyzer that I don't care about a specific dead store?</a></li> 30 <li><a href="#unused_ivar">How do I tell the static analyzer that I don't care about a specific unused instance variable in Objective C?</a></li> 31 <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> 32 <li><a href="#suppress_issue">How can I suppress a specific analyzer warning?</a></li> 33 <li><a href="#exclude_code">How can I selectively exclude code the analyzer examines?</a></li> 34 </ol> 35 36 37 <h4 id="custom_assert" class="faq">Q: How do I tell the analyzer that I do not want the bug being 38 reported here since my custom error handler will safely end the execution before 39 the bug is reached?</h4> 40 41 <img src="images/example_custom_assert.png" alt="example custom assert"> 42 43 <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> 44 45 <pre class="code_example"> 46 void customAssert() <span class="code_highlight">__attribute__((analyzer_noreturn))</span>; 47 int foo(int *b) { 48 if (!b) 49 customAssert(); 50 return *b; 51 }</pre> 52 53 54 <h4 id="null_pointer" class="faq">Q: The analyzer reports a null dereference, but I know that the 55 pointer is never null. How can I tell the analyzer that a pointer can never be 56 null?</h4> 57 58 <img src="images/example_null_pointer.png" alt="example null pointer"> 59 60 <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> 61 62 <pre class="code_example"> 63 void usePointer(int *b); 64 int foo(int *b) { 65 usePointer(b); 66 return *b; 67 }</pre> 68 69 <h4 id="dead_store" class="faq">Q: How do I tell the static analyzer that I don't care about a specific dead store?</h4> 70 71 <p>When the analyzer sees that a value stored into a variable is never used, it's going to produce a message similar to this one: 72 <pre class="code_example">Value stored to 'x' is never read</pre> 73 You can use the <tt>(void)x;</tt> idiom to acknowledge that there is a dead store in your code but you do not want it to be reported in the future.</p> 74 75 <h4 id="unused_ivar" class="faq">Q: How do I tell the static analyzer that I don't care about a specific unused instance variable in Objective C?</h4> 76 77 <p>When the analyzer sees that a value stored into a variable is never used, it is going to produce a message similar to this one: 78 <pre class="code_example">Instance variable 'commonName' in class 'HappyBird' is never used by the methods in its @implementation</pre> 79 You can add <tt>__attribute__((unused))</tt> to the instance variable declaration to suppress the warning.</p> 80 81 <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> 82 83 <img src="images/example_use_assert.png" alt="example use assert"> 84 85 <p> In the contrived example above, the analyzer has detected that the body of 86 the loop is never entered for the case where <tt>length <= 0</tt>. In this 87 particular example, you may know that the loop will always be entered because 88 the input parameter <tt>length</tt> will be greater than zero in all calls to this 89 function. You can teach the analyzer facts about your code as well as document 90 it by using assertions. By adding <tt>assert(length > 0)</tt> in the beginning 91 of the function, you tell the analyzer that your code is never expecting a zero 92 or a negative value, so it won't need to test the correctness of those paths. 93 </p> 94 95 <pre class="code_example"> 96 int foo(int length) { 97 int x = 0; 98 <span class="code_highlight">assert(length > 0);</span> 99 for (int i = 0; i < length; i++) 100 x += 1; 101 return length/x; 102 } 103 </pre> 104 105 <h4 id="suppress_issue" class="faq">Q: How can I suppress a specific analyzer warning?</h4> 106 107 <p>There is currently no solid mechanism for suppressing an analyzer warning, 108 although this is currently being investigated. When you encounter an analyzer 109 bug/false positive, check if it's one of the issues discussed above or if the 110 analyzer <a href = "annotations.html#custom_assertions" >annotations</a> can 111 resolve the issue. Second, please <a href = "filing_bugs.html">report it</a> to 112 help us improve user experience. As the last resort, consider using <tt>__clang_analyzer__</tt> macro 113 <a href = "faq.html#exclude_code" >described below</a>.</p> 114 115 <h4 id="exclude_code" class="faq">Q: How can I selectively exclude code the analyzer examines?</h4> 116 117 <p>When the static analyzer is using clang to parse source files, it implicitly 118 defines the preprocessor macro <tt>__clang_analyzer__</tt>. One can use this 119 macro to selectively exclude code the analyzer examines. Here is an example: 120 121 <pre class="code_example"> 122 #ifndef __clang_analyzer__ 123 // Code not to be analyzed 124 #endif 125 </pre> 126 127 This usage is discouraged because it makes the code dead to the analyzer from 128 now on. Instead, we prefer that users file bugs against the analyzer when it flags 129 false positives. 130 </p> 131 132 </div> 133 </div> 134 </body> 135 </html> 136 137