www/analyzer/faq.html - third_party/swift-clang - Git at Google

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 <h1>FAQ and How to Deal with Common False Positives</h1>

 <ol>
   <li><a href="#custom_assert">How do I tell the analyzer that I do not want the bug being
 reported here since my custom error handler will safely end the execution before
 the bug is reached?</a></li>
   <li><a href="#null_pointer">The analyzer reports a null dereference, but I know that the
 pointer is never null. How can I tell the analyzer that a pointer can never be
 null?</a></li>
   <li><a href="#dead_store">How do I tell the static analyzer that I don't care about a specific dead store?</a></li>
   <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>
   <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>
   <li><a href="#suppress_issue">How can I suppress a specific analyzer warning?</a></li>
   <li><a href="#exclude_code">How can I selectively exclude code the analyzer examines?</a></li>
 </ol>


 <h4 id="custom_assert" class="faq">Q: How do I tell the analyzer that I do not want the bug being
 reported here since my custom error handler will safely end the execution before
 the bug is reached?</h4>

 <img src="images/example_custom_assert.png" alt="example custom assert">

 <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>

 <pre class="code_example">
 void customAssert() <span class="code_highlight">__attribute__((analyzer_noreturn))</span>;
 int foo(int *b) {
   if (!b)
     customAssert();
   return *b;
 }</pre>


 <h4 id="null_pointer" class="faq">Q: The analyzer reports a null dereference, but I know that the
 pointer is never null. How can I tell the analyzer that a pointer can never be
 null?</h4>

 <img src="images/example_null_pointer.png" alt="example null pointer">

 <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>

 <pre class="code_example">
 void usePointer(int *b);
 int foo(int *b) {
   usePointer(b);
   return *b;
 }</pre>

 <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>

 <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:
 <pre class="code_example">Value stored to 'x' is never read</pre>
 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>

 <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>

 <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:
 <pre class="code_example">Instance variable 'commonName' in class 'HappyBird' is never used by the methods in its @implementation</pre>
 You can add <tt>__attribute__((unused))</tt> to the instance variable declaration to suppress the warning.</p>

 <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>

 <img src="images/example_use_assert.png" alt="example use assert">

 <p> In the contrived example above, the analyzer has detected that the body of
 the loop is never entered for the case where <tt>length <= 0</tt>. In this
 particular example, you may know that the loop will always be entered because
 the input parameter <tt>length</tt> will be greater than zero in all calls to this
 function. You can teach the analyzer facts about your code as well as document
 it by using assertions. By adding <tt>assert(length > 0)</tt> in the beginning
 of the function, you tell the analyzer that your code is never expecting a zero
 or a negative value, so it won't need to test the correctness of those paths.
 </p>

 <pre class="code_example">
 int foo(int length) {
   int x = 0;
   <span class="code_highlight">assert(length > 0);</span>
   for (int i = 0; i < length; i++)
     x += 1;
   return length/x;
 }
 </pre>

 <h4 id="suppress_issue" class="faq">Q: How can I suppress a specific analyzer warning?</h4>

 <p>There is currently no solid mechanism for suppressing an analyzer warning,
 although this is currently being investigated. When you encounter an analyzer
 bug/false positive, check if it's one of the issues discussed above or if the
 analyzer <a href = "annotations.html#custom_assertions" >annotations</a> can
 resolve the issue. Second, please <a href = "filing_bugs.html">report it</a> to
 help us improve user experience. As the last resort, consider using <tt>__clang_analyzer__</tt> macro
 <a href = "faq.html#exclude_code" >described below</a>.</p>

 <h4 id="exclude_code" class="faq">Q: How can I selectively exclude code the analyzer examines?</h4>

 <p>When the static analyzer is using clang to parse source files, it implicitly
 defines the preprocessor macro <tt>__clang_analyzer__</tt>. One can use this
 macro to selectively exclude code the analyzer examines. Here is an example:

 <pre class="code_example">
 #ifndef __clang_analyzer__
 // Code not to be analyzed
 #endif
 </pre>

 This usage is discouraged because it makes the code dead to the analyzer from
 now on. Instead, we prefer that users file bugs against the analyzer when it flags
 false positives.
 </p>

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	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
	"http://www.w3.org/TR/html4/strict.dtd">
	<html>
	<head>
	<title>FAQ and How to Deal with Common False Positives</title>
	<link type="text/css" rel="stylesheet" href="menu.css">
	<link type="text/css" rel="stylesheet" href="content.css">
	<script type="text/javascript" src="scripts/menu.js"></script>
	<style type="text/css">
	tr:first-child { width:20%; }
	</style>
	</head>
	<body>

	<div id="page">
	<!--#include virtual="menu.html.incl"-->

	<div id="content">

	<h1>FAQ and How to Deal with Common False Positives</h1>

	<ol>
	<li><a href="#custom_assert">How do I tell the analyzer that I do not want the bug being
	reported here since my custom error handler will safely end the execution before
	the bug is reached?</a></li>
	<li><a href="#null_pointer">The analyzer reports a null dereference, but I know that the
	pointer is never null. How can I tell the analyzer that a pointer can never be
	null?</a></li>
	<li><a href="#dead_store">How do I tell the static analyzer that I don't care about a specific dead store?</a></li>
	<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>
	<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>
	<li><a href="#suppress_issue">How can I suppress a specific analyzer warning?</a></li>
	<li><a href="#exclude_code">How can I selectively exclude code the analyzer examines?</a></li>
	</ol>


	<h4 id="custom_assert" class="faq">Q: How do I tell the analyzer that I do not want the bug being
	reported here since my custom error handler will safely end the execution before
	the bug is reached?</h4>

	<img src="images/example_custom_assert.png" alt="example custom assert">

	<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>

	<pre class="code_example">
	void customAssert() <span class="code_highlight">__attribute__((analyzer_noreturn))</span>;
	int foo(int *b) {
	if (!b)
	customAssert();
	return *b;
	}</pre>


	<h4 id="null_pointer" class="faq">Q: The analyzer reports a null dereference, but I know that the
	pointer is never null. How can I tell the analyzer that a pointer can never be
	null?</h4>

	<img src="images/example_null_pointer.png" alt="example null pointer">

	<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>

	<pre class="code_example">
	void usePointer(int *b);
	int foo(int *b) {
	usePointer(b);
	return *b;
	}</pre>

	<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>

	<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:
	<pre class="code_example">Value stored to 'x' is never read</pre>
	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>

	<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>

	<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:
	<pre class="code_example">Instance variable 'commonName' in class 'HappyBird' is never used by the methods in its @implementation</pre>
	You can add <tt>__attribute__((unused))</tt> to the instance variable declaration to suppress the warning.</p>

	<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>

	<img src="images/example_use_assert.png" alt="example use assert">

	<p> In the contrived example above, the analyzer has detected that the body of
	the loop is never entered for the case where <tt>length <= 0</tt>. In this
	particular example, you may know that the loop will always be entered because
	the input parameter <tt>length</tt> will be greater than zero in all calls to this
	function. You can teach the analyzer facts about your code as well as document
	it by using assertions. By adding <tt>assert(length > 0)</tt> in the beginning
	of the function, you tell the analyzer that your code is never expecting a zero
	or a negative value, so it won't need to test the correctness of those paths.
	</p>

	<pre class="code_example">
	int foo(int length) {
	int x = 0;
	<span class="code_highlight">assert(length > 0);</span>
	for (int i = 0; i < length; i++)
	x += 1;
	return length/x;
	}
	</pre>

	<h4 id="suppress_issue" class="faq">Q: How can I suppress a specific analyzer warning?</h4>

	<p>There is currently no solid mechanism for suppressing an analyzer warning,
	although this is currently being investigated. When you encounter an analyzer
	bug/false positive, check if it's one of the issues discussed above or if the
	analyzer <a href = "annotations.html#custom_assertions" >annotations</a> can
	resolve the issue. Second, please <a href = "filing_bugs.html">report it</a> to
	help us improve user experience. As the last resort, consider using <tt>__clang_analyzer__</tt> macro
	<a href = "faq.html#exclude_code" >described below</a>.</p>

	<h4 id="exclude_code" class="faq">Q: How can I selectively exclude code the analyzer examines?</h4>

	<p>When the static analyzer is using clang to parse source files, it implicitly
	defines the preprocessor macro <tt>__clang_analyzer__</tt>. One can use this
	macro to selectively exclude code the analyzer examines. Here is an example:

	<pre class="code_example">
	#ifndef __clang_analyzer__
	// Code not to be analyzed
	#endif
	</pre>

	This usage is discouraged because it makes the code dead to the analyzer from
	now on. Instead, we prefer that users file bugs against the analyzer when it flags
	false positives.
	</p>

	</div>
	</div>
	</body>
	</html>