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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>
Pirama Arumuga Nainar	87d948e	2016-03-03 15:49:35 -0800	[diff] [blame]	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>
Pirama Arumuga Nainar	4967a71	2016-09-19 22:19:55 -0700	[diff] [blame]	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>
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	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>
Anna Zaks	843f0e8	2012-06-16 00:30:21 +0000	[diff] [blame]	34	<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]	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
Ted Kremenek	dea37f2	2012-06-09 20:10:45 +0000	[diff] [blame]	39	reported here since my custom error handler will safely end the execution before
				40	the bug is reached?</h4>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	41
				42	<img src="images/example_custom_assert.png" alt="example custom assert">
				43
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	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>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	45
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	46	<pre class="code_example">
				47	void customAssert() <span class="code_highlight">__attribute__((analyzer_noreturn))</span>;
				48	int 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
Ted Kremenek	dea37f2	2012-06-09 20:10:45 +0000	[diff] [blame]	56	pointer is never null. How can I tell the analyzer that a pointer can never be
				57	null?</h4>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	58
				59	<img src="images/example_null_pointer.png" alt="example null pointer">
				60
Ted Kremenek	dea37f2	2012-06-09 20:10:45 +0000	[diff] [blame]	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>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	62
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	63	<pre class="code_example">
				64	void usePointer(int *b);
				65	int foo(int *b) {
				66	usePointer(b);
				67	return *b;
				68	}</pre>
				69
Pirama Arumuga Nainar	87d948e	2016-03-03 15:49:35 -0800	[diff] [blame]	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>
				74	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>
				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>
				80	You can add <tt>__attribute__((unused))</tt> to the instance variable declaration to suppress the warning.</p>
				81
Pirama Arumuga Nainar	4967a71	2016-09-19 22:19:55 -0700	[diff] [blame]	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
				87	If 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")))
				90	static inline NSString LocalizationNotNeeded(NSString s) {
				91	return s;
				92	}
				93	</pre>
				94
				95	You can then call this function when creating your debugging UI:
				96	<pre class="code_example">
				97	[field setStringValue:LocalizationNotNeeded(@"Debug")];
				98	</pre>
				99
				100	Some 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">
				102	UILabel *testLabel = [[UILabel alloc] init];
				103	NSString *s = NSLocalizedString(@"Hello <Do Not Localize>", @"For debug purposes");
				104	[testLabel setText:s];
				105	</pre>
				106	</p>
				107
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	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>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	109
				110	<img src="images/example_use_assert.png" alt="example use assert">
				111
Anna Zaks	df53919	2013-03-01 06:38:16 +0000	[diff] [blame]	112	<p> In the contrived example above, the analyzer has detected that the body of
				113	the loop is never entered for the case where <tt>length <= 0</tt>. In this
				114	particular example, you may know that the loop will always be entered because
				115	the input parameter <tt>length</tt> will be greater than zero in all calls to this
				116	function. You can teach the analyzer facts about your code as well as document
				117	it by using assertions. By adding <tt>assert(length > 0)</tt> in the beginning
				118	of the function, you tell the analyzer that your code is never expecting a zero
				119	or a negative value, so it won't need to test the correctness of those paths.
				120	</p>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	121
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	122	<pre class="code_example">
				123	int 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>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	131
Anna Zaks	e06f5a0	2012-06-11 22:09:44 +0000	[diff] [blame]	132	<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]	133
Anna Zaks	843f0e8	2012-06-16 00:30:21 +0000	[diff] [blame]	134	<p>There is currently no solid mechanism for suppressing an analyzer warning,
				135	although this is currently being investigated. When you encounter an analyzer
				136	bug/false positive, check if it's one of the issues discussed above or if the
				137	analyzer <a href = "annotations.html#custom_assertions" >annotations</a> can
				138	resolve the issue. Second, please <a href = "filing_bugs.html">report it</a> to
				139	help 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
				145	defines the preprocessor macro <tt>__clang_analyzer__</tt>. One can use this
				146	macro 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
				154	This usage is discouraged because it makes the code dead to the analyzer from
				155	now on. Instead, we prefer that users file bugs against the analyzer when it flags
				156	false positives.
				157	</p>
Anna Zaks	568bdee	2012-06-09 01:04:54 +0000	[diff] [blame]	158
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