148 lines
4.7 KiB
XML
148 lines
4.7 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
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<article>
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<articleinfo>
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<title>Writing Cppcheck rules</title>
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<author>
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<firstname>Daniel</firstname>
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<surname>Marjamaki</surname>
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<affiliation>
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<orgname>Cppcheck</orgname>
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</affiliation>
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</author>
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<pubdate>2010</pubdate>
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</articleinfo>
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<section>
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<title>Introduction</title>
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<para>This is a short and simple guide that describes how rules are
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written for Cppcheck.</para>
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<para>The patterns are defined with regular expressions. It is required
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that you know how regular expressions work.</para>
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</section>
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<section>
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<title>Data representation of the source code</title>
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<para>The data used by the rules are not the raw source code.
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<literal>Cppcheck</literal> will read the source code and process it
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before the rules are used.</para>
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<para>Cppcheck is designed to find bugs and dangerous code. Stylistic
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information (such as indentation, comments, etc) are filtered out at an
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early state. You don't need to worry about such stylistic information when
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you write rules.</para>
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<para>Between each token in the code there is always a space. For instance
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the raw code "1+f()" is processed into "1 + f ( )".</para>
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<para>The code is simplified in many ways.</para>
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</section>
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<section>
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<title>Creating a simple rule</title>
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<para>When creating a rule there are two steps:</para>
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<orderedlist>
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<listitem>
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<para>Create the regular expression</para>
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</listitem>
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<listitem>
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<para>Create a XML based rule file</para>
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</listitem>
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</orderedlist>
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<section>
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<title>Step 1 - Creating the regular expression</title>
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<para>Cppcheck uses the <literal>PCRE</literal> library to handle
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regular expressions. <literal>PCRE</literal> stands for "Perl Compatible
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Regular Expressions". The homepage for <literal>PCRE</literal> is
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<literal>http://www.pcre.org</literal>.</para>
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<para>Let's create a regular expression that checks for code such
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as:</para>
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<programlisting>if (p)
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free(p);</programlisting>
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<para>For such code the condition is often redundant (on most
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implementations it is valid to free a NULL pointer).</para>
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<para>The regular expression must be written for the simplified code. To
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see what the simplified code looks like you can create a source file
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with the code:</para>
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<programlisting>void f() {
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if (p)
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free(p);
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}</programlisting>
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<para>Save that code as <literal>dealloc.cpp</literal> and then use
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<literal>cppcheck --rule=".+" dealloc.cpp</literal>:</para>
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<programlisting>$ ./cppcheck --rule=".+" dealloc.cpp
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Checking dealloc.cpp...
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[dealloc.cpp:1]: (style) found ' void f ( ) { if ( p ) { free ( p ) ; } }'</programlisting>
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<para>The regular expression <literal>.+</literal> matches everything
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and the matching text is shown on the screen.</para>
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<para>From that output we can see that the simplified code is:</para>
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<programlisting> void f ( ) { if ( p ) { free ( p ) ; } }</programlisting>
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<para>Now that we know how the simplified code looks. We can create a
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regular expression that matches it properly:</para>
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<programlisting>$ cppcheck --rule="if \( p \) { free \( p \) ; }" dealloc.cpp
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Checking dealloc.cpp...
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[dealloc.cpp:2]: (style) found 'if ( p ) { free ( p ) ; }'</programlisting>
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</section>
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<section>
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<title>Step 2 - Create rule file</title>
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<para>A rule file is a simple XML file that contains:</para>
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<itemizedlist>
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<listitem>
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<para>a pattern to search for</para>
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</listitem>
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<listitem>
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<para>an error message that is reported when pattern is found</para>
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</listitem>
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</itemizedlist>
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<para>Here is a simple example:</para>
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<programlisting><?xml version="1.0"?>
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<rule version="1">
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<pattern>if \( p \) { free \( p \) ; }</pattern>
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<message>
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<id>redundantCondition</id>
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<severity>style</severity>
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<summary>Redundant condition. It is valid to free a NULL pointer.</summary>
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</message>
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</rule></programlisting>
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<para>If you save that xml data in <literal>dealloc.rule</literal> you
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can test this rule:</para>
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<programlisting>$ cppcheck --rule-file=dealloc.rule dealloc.cpp
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Checking dealloc.cpp...
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[dealloc.cpp:2]: (style) Redundant condition. It is valid to free a NULL pointer.</programlisting>
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</section>
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</section>
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</article>
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