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Anna Zaks	70186fc	2012-06-09 01:05:01 +0000	[diff] [blame]	5	<title>FAQ and How to Deal with Common False Positives</title>
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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
Ted Kremenek	dea37f2	2012-06-09 20:10:45 +0000	[diff] [blame]	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>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	78
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	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>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	88
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	89	<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]	90
Anna Zaks	843f0e8	2012-06-16 00:30:21 +0000	[diff] [blame^]	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>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	115
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