Blame - www/analyzer/faq.html - platform/external/clang

blob: 129bfb63bb1bda453d4d0cdc54a5695fddf7a639 [file] [log] [blame]

Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	1	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
				2	"http://www.w3.org/TR/html4/strict.dtd">
				3	<html>
				4	<head>
Anna Zaks	70186fc	2012-06-09 01:05:01 +0000	[diff] [blame]	5	<title>FAQ and How to Deal with Common False Positives</title>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	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">
Ted Kremenek	dea37f2	2012-06-09 20:10:45 +0000	[diff] [blame]	10	tr:first-child { width:20%; }
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	11	</style>
				12	</head>
				13	<body>
				14
				15	<div id="page">
				16	<!--#include virtual="menu.html.incl"-->
				17
				18	<div id="content">
				19
Ted Kremenek	c1cb12b	2012-06-09 20:10:42 +0000	[diff] [blame]	20	<h1>FAQ and How to Deal with Common False Positives</h1>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	21
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	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>
Anna Zaks	843f0e8	2012-06-16 00:30:21 +0000	[diff] [blame]	31	<li><a href="#exclude_code">How can I selectively exclude code the analyzer examines?</a></li>
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	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
Ted Kremenek	dea37f2	2012-06-09 20:10:45 +0000	[diff] [blame]	36	reported here since my custom error handler will safely end the execution before
				37	the bug is reached?</h4>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	38
				39	<img src="images/example_custom_assert.png" alt="example custom assert">
				40
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	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>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	42
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	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
Ted Kremenek	dea37f2	2012-06-09 20:10:45 +0000	[diff] [blame]	53	pointer is never null. How can I tell the analyzer that a pointer can never be
				54	null?</h4>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	55
				56	<img src="images/example_null_pointer.png" alt="example null pointer">
				57
Ted Kremenek	dea37f2	2012-06-09 20:10:45 +0000	[diff] [blame]	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>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	59
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	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>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	68
				69	<img src="images/example_use_assert.png" alt="example use assert">
				70
Anna Zaks	df53919	2013-03-01 06:38:16 +0000	[diff] [blame^]	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>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	80
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	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>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	90
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	91	<h4 id="suppress_issue" class="faq">Q: How can I suppress a specific analyzer warning?</h4>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	92
Anna Zaks	843f0e8	2012-06-16 00:30:21 +0000	[diff] [blame]	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>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	117
				118	</div>
				119	</div>
				120	</body>
				121	</html>
				122