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 24reported here since my custom error handler will safely end the execution before 25the bug is reached?</a></li> 26 <li><a href="#null_pointer">The analyzer reports a null dereference, but I know that the 27pointer is never null. How can I tell the analyzer that a pointer can never be 28null?</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="#unlocalized_string">How do I tell the static analyzer that I don't care about a specific unlocalized string?</a></li> 32 <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> 33 <li><a href="#suppress_issue">How can I suppress a specific analyzer warning?</a></li> 34 <li><a href="#exclude_code">How can I selectively exclude code the analyzer examines?</a></li> 35</ol> 36 37 38<h4 id="custom_assert" class="faq">Q: How do I tell the analyzer that I do not want the bug being 39reported here since my custom error handler will safely end the execution before 40the bug is reached?</h4> 41 42<img src="images/example_custom_assert.png" alt="example custom assert"> 43 44<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> 45 46<pre class="code_example"> 47void customAssert() <span class="code_highlight">__attribute__((analyzer_noreturn))</span>; 48int foo(int *b) { 49 if (!b) 50 customAssert(); 51 return *b; 52}</pre> 53 54 55<h4 id="null_pointer" class="faq">Q: The analyzer reports a null dereference, but I know that the 56pointer is never null. How can I tell the analyzer that a pointer can never be 57null?</h4> 58 59<img src="images/example_null_pointer.png" alt="example null pointer"> 60 61<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> 62 63<pre class="code_example"> 64void usePointer(int *b); 65int foo(int *b) { 66 usePointer(b); 67 return *b; 68}</pre> 69 70<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> 71 72<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: 73<pre class="code_example">Value stored to 'x' is never read</pre> 74You 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> 75 76<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> 77 78<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: 79<pre class="code_example">Instance variable 'commonName' in class 'HappyBird' is never used by the methods in its @implementation</pre> 80You can add <tt>__attribute__((unused))</tt> to the instance variable declaration to suppress the warning.</p> 81 82<h4 id="unlocalized_string" class="faq">Q: How do I tell the static analyzer that I don't care about a specific unlocalized string?</h4> 83 84<p>When the analyzer sees that an unlocalized string is passed to a method that will present that string to the user, it is going to produce a message similar to this one: 85<pre class="code_example">User-facing text should use localized string macro</pre> 86 87If your project deliberately uses unlocalized user-facing strings (for example, in a debugging UI that is never shown to users), you can suppress the analyzer warnings (and document your intent) with a function that just returns its input but is annotated to return a localized string: 88<pre class="code_example"> 89__attribute__((annotate("returns_localized_nsstring"))) 90static inline NSString *LocalizationNotNeeded(NSString *s) { 91 return s; 92} 93</pre> 94 95You can then call this function when creating your debugging UI: 96<pre class="code_example"> 97[field setStringValue:LocalizationNotNeeded(@"Debug")]; 98</pre> 99 100Some projects may also find it useful to use NSLocalizedString but add "DNL" or "Do Not Localize" to the string contents as a convention: 101<pre class="code_example"> 102UILabel *testLabel = [[UILabel alloc] init]; 103NSString *s = NSLocalizedString(@"Hello <Do Not Localize>", @"For debug purposes"); 104[testLabel setText:s]; 105</pre> 106</p> 107 108<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> 109 110<img src="images/example_use_assert.png" alt="example use assert"> 111 112<p> In the contrived example above, the analyzer has detected that the body of 113the loop is never entered for the case where <tt>length <= 0</tt>. In this 114particular example, you may know that the loop will always be entered because 115the input parameter <tt>length</tt> will be greater than zero in all calls to this 116function. You can teach the analyzer facts about your code as well as document 117it by using assertions. By adding <tt>assert(length > 0)</tt> in the beginning 118of the function, you tell the analyzer that your code is never expecting a zero 119or a negative value, so it won't need to test the correctness of those paths. 120</p> 121 122<pre class="code_example"> 123int foo(int length) { 124 int x = 0; 125 <span class="code_highlight">assert(length > 0);</span> 126 for (int i = 0; i < length; i++) 127 x += 1; 128 return length/x; 129} 130</pre> 131 132<h4 id="suppress_issue" class="faq">Q: How can I suppress a specific analyzer warning?</h4> 133 134<p>There is currently no solid mechanism for suppressing an analyzer warning, 135although this is currently being investigated. When you encounter an analyzer 136bug/false positive, check if it's one of the issues discussed above or if the 137analyzer <a href = "annotations.html#custom_assertions" >annotations</a> can 138resolve the issue. Second, please <a href = "filing_bugs.html">report it</a> to 139help us improve user experience. As the last resort, consider using <tt>__clang_analyzer__</tt> macro 140<a href = "faq.html#exclude_code" >described below</a>.</p> 141 142<h4 id="exclude_code" class="faq">Q: How can I selectively exclude code the analyzer examines?</h4> 143 144<p>When the static analyzer is using clang to parse source files, it implicitly 145defines the preprocessor macro <tt>__clang_analyzer__</tt>. One can use this 146macro to selectively exclude code the analyzer examines. Here is an example: 147 148<pre class="code_example"> 149#ifndef __clang_analyzer__ 150// Code not to be analyzed 151#endif 152</pre> 153 154This usage is discouraged because it makes the code dead to the analyzer from 155now on. Instead, we prefer that users file bugs against the analyzer when it flags 156false positives. 157</p> 158 159</div> 160</div> 161</body> 162</html> 163 164
