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. Cppcheck
    uses PCRE to handle regular expressions.</para>

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

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

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

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

      <para>The regular expression must match the simplified code. Create a
      source file that has the bad code:</para>

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

      <para>To see the simplified code you can 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>In short, the simplified code is:</para>

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

      <para>Now that the simplified code is known 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>

      <para>Feel free to improve the pattern. Above, the pointer name must be
      "p" to get a match.</para>
    </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 - this
          is optional, if none is given a default message is written.</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>The <literal>message</literal>, <literal>id</literal>,
      <literal>severity</literal> and <literal>summary</literal> elements are
      optional. But highly recommended.</para>

      <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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