cppcheck/man/writing-rules.docbook

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  <articleinfo>
    <title>Writing Cppcheck rules</title>

    <author>
      <firstname>Daniel</firstname>

      <surname>Marjamäki</surname>

      <affiliation>
        <orgname>Cppcheck</orgname>
      </affiliation>
    </author>

    <pubdate>2010</pubdate>
  </articleinfo>

  <section>
    <title>Introduction</title>

    <para>This is a short guide for developers who want to write Cppcheck
    rules.</para>

    <para>There are two ways to write rules.</para>

    <variablelist>
      <varlistentry>
        <term>Regular expressions</term>

        <listitem>
          <para>Simple rules can be created by using regular expressions. No
          compilation is required.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>C++</term>

        <listitem>
          <para>Advanced rules must be created with C++. These rules must be
          compiled and linked statically with Cppcheck.</para>
        </listitem>
      </varlistentry>
    </variablelist>

    <para>It is a good first step to use regular expressions. It is easier.
    You'll get results quicker. Therefore this guide will focus on regular
    expressions.</para>
  </section>

  <section>
    <title>Data representation of the source code</title>

    <para>The data used by the rules are not the raw source code.
    <literal>Cppcheck</literal> will read the source code and process it
    before the rules are used.</para>

    <para>Cppcheck is designed to find bugs and dangerous code. Stylistic
    information such as indentation, comments, etc are filtered out at an
    early state. You don't need to worry about such stylistic information when
    you write rules.</para>

    <para>Between each token in the code there is always a space. For instance
    the raw code "1+f()" is processed into "1 + f ( )".</para>

    <para>The code is simplified in many ways. For example:</para>

    <itemizedlist>
      <listitem>
        <para>The templates are instantiated</para>
      </listitem>

      <listitem>
        <para>The typedefs are handled</para>
      </listitem>

      <listitem>
        <para>There is no "else if". These are converted into "else {
        if.."</para>
      </listitem>

      <listitem>
        <para>The bodies of "if", "else", "while", "do" and "for" are always
        enclosed in "{" and "}"</para>
      </listitem>

      <listitem>
        <para>A declaration of multiple variables is split up into multiple
        variable declarations. "int a,b;" =&gt; "int a; int b;"</para>
      </listitem>

      <listitem>
        <para>There is no sizeof</para>
      </listitem>

      <listitem>
        <para>NULL is replaced with 0</para>
      </listitem>

      <listitem>
        <para>Static value flow analysis is made. Known values are inserted
        into the code.</para>
      </listitem>

      <listitem>
        <para>.. and many more</para>
      </listitem>
    </itemizedlist>

    <para>The simplifications are made in the <literal>Cppcheck</literal>
    <literal>Tokenizer</literal>. For more information see:
    <uri>http://cppcheck.sourceforge.net/doxyoutput/classTokenizer.html</uri></para>
  </section>

  <section>
    <title>Regular expressions</title>

    <para>Simple rules can be defined through regular expressions.</para>

    <para>Cppcheck uses the <literal>PCRE</literal> library to handle regular
    expressions. <literal>PCRE</literal> stands for "Perl Compatible Regular
    Expressions". The homepage for <literal>PCRE</literal> is
    <literal>http://www.pcre.org</literal>.</para>

    <section>
      <title>Creating the regular expression</title>

      <para>Let's create a regular expression that checks for:</para>

      <programlisting>if (p)
    free(p);</programlisting>

      <para>For such code the condition is often redundant, on most
      implementations it is valid to free a NULL pointer.</para>

      <para>The regular expression must be written for the simplified code. To
      see what the simplified code looks like you can create a source file
      with some code:</para>

      <programlisting>void f() {
    if (p)
        free(p);
}</programlisting>

      <para>Save that code as <literal>dealloc.cpp</literal> and use
      <literal>cppcheck --rule=".+" dealloc.cpp</literal>:</para>

      <programlisting>$ ./cppcheck --rule=".+" dealloc.cpp 
Checking dealloc.cpp...
[dealloc.cpp:1]: (style) found ' void f ( ) { if ( p ) { free ( p ) ; } }'</programlisting>

      <para>From that output we can see that the simplified code is:</para>

      <programlisting> void f ( ) { if ( p ) { free ( p ) ; } }</programlisting>

      <para>Now that we know how the simplified code looks for a simple test
      case, we can create a regular expression:</para>

      <programlisting>$ cppcheck --rule="if \( p \) { free \( p \) ; }" dealloc.cpp
Checking dealloc.cpp...
[dealloc.cpp:2]: (style) found 'if ( p ) { free ( p ) ; }'</programlisting>
    </section>

    <section>
      <title>Create rule file</title>

      <para>A rule consist of:</para>

      <itemizedlist>
        <listitem>
          <para>a pattern to search for</para>
        </listitem>

        <listitem>
          <para>an error message that is reported when pattern is found</para>
        </listitem>
      </itemizedlist>

      <para>Here is a simple example:</para>

      <programlisting>&lt;?xml version="1.0"?&gt;
&lt;rule version="1"&gt;
  &lt;pattern&gt;if \( p \) { free \( p \) ; }&lt;/pattern&gt;
  &lt;message&gt;
    &lt;id&gt;redundantCondition&lt;/id&gt;
    &lt;severity&gt;style&lt;/severity&gt;
    &lt;summary&gt;Redundant condition. It is valid to free a NULL pointer.&lt;/summary&gt;
  &lt;/message&gt;
&lt;/rule&gt;</programlisting>

      <para>If you save that xml data in <literal>dealloc.rule</literal> you
      can test this rule:</para>

      <programlisting>$ cppcheck --rule-file=dealloc.rule dealloc.cpp
Checking dealloc.cpp...
[dealloc.cpp:2]: (style) Redundant condition. It is valid to free a NULL pointer.</programlisting>
    </section>
  </section>
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