cppcheck/man/writing-rules.docbook

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<article>
  <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 and simple guide that describes how rules are
    written for Cppcheck.</para>

    <para>The patterns are defined with regular expressions. It is required
    that you know how regular expressions work.</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.</para>
  </section>

  <section>
    <title>Creating a simple rule</title>

    <para>When creating a rule there are two steps:</para>

    <orderedlist>
      <listitem>
        <para>Create the regular expression</para>
      </listitem>

      <listitem>
        <para>Create a XML based rule file</para>
      </listitem>
    </orderedlist>

    <section>
      <title>Step 1 - Creating the regular expression</title>

      <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>

      <para>Let's create a regular expression that checks for code such
      as:</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 the code:</para>

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

      <para>Save that code as <literal>dealloc.cpp</literal> and then 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>The regular expression <literal>.+</literal> matches everything
      and the matching text is shown on the screen.</para>

      <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. We can create a
      regular expression that matches it properly:</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>Step 2 - Create rule file</title>

      <para>A rule file is a simple XML file that contains:</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>
</article>