Writing rules: Added part 3. Introduction to writing rules with C++.
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Daniel Marjamäki
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<?xml version="1.0" encoding="UTF-8"?>
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<article version="5.0" xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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xmlns:svg="http://www.w3.org/2000/svg"
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xmlns:m="http://www.w3.org/1998/Math/MathML"
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xmlns:html="http://www.w3.org/1999/xhtml"
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xmlns:db="http://docbook.org/ns/docbook">
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<info>
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<title>Writing Cppcheck rules</title>
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<subtitle>Part 3 - Introduction to writing rules with C++</subtitle>
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<author>
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<personname><firstname>Daniel</firstname><surname>Marjamäki</surname></personname>
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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>2011</pubdate>
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</info>
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<section>
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<title>Introduction</title>
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<para>The goal for this article is to introduce how
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<literal>Cppcheck</literal> rules are written with C++. With C++ it is
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possible to write more complex rules than is possible with regular
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expressions.</para>
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</section>
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<section>
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<title>Basics</title>
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<para>A C++ rule is written in a C++ function.</para>
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<para>Rules are organized into Check classes. For instance there is a
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class with the name <literal>CheckStl</literal> that contains various stl
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rules. The <literal>CheckOther</literal> can always be used if no other
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class suits you.</para>
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<para>When you have added your rule you must recompile Cppcheck before you
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can test it.</para>
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</section>
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<section>
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<title>Division by zero</title>
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<para>This simple regular expression will check for division by
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zero:</para>
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<programlisting>cppcheck --rule="/ 0"</programlisting>
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<para>Here is the corresponding C++ check:</para>
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<programlisting>// Detect division by zero
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void CheckOther::divisionByZero()
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{
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// Loop through all tokens
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for (const Token *tok = _tokenizer->tokens(); tok; tok = tok->next())
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{
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// check if there is a division by zero
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if (Token::Match(tok, "/ 0"))
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{
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// report error
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divisionByZeroError(tok);
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}
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}
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}
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// Report error
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void CheckOther::divisionByZeroError()
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{
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reportError(tok, Severity::error, "divisionByZero", "Division by zero");
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}</programlisting>
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<para>The <literal>Token::Match</literal> matches tokens against
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expressions. A few rules about Token::Match expressions are:</para>
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<itemizedlist>
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<listitem>
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<para>tokens are either completely matched or not matched at all. The
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token "abc" is not matched by "ab".</para>
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</listitem>
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<listitem>
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<para>Spaces are used as separators.</para>
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</listitem>
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<listitem>
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<para>With normal regular expressions there are special meanings for +
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* ? ( ). These are just normal characters in
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<literal>Token::Match</literal> patterns.</para>
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</listitem>
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</itemizedlist>
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</section>
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<section>
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<title>Condition before deallocation</title>
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<para>In the first <literal>Writing rules</literal> article I described a
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rule that looks for redundant conditions. Here is the regular expression
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that was shown:</para>
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<programlisting>if \( p \) { free \( p \) ; }</programlisting>
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<para>The corresponding <literal>Token::Match</literal> expression
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is:</para>
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<programlisting>if ( %var% ) { free ( %var% ) ; }</programlisting>
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<para>Any variable name is matched by <literal>%var%</literal>.</para>
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<para>Here is a C++ function:</para>
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<programlisting>// Find redundant condition before deallocation
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void CheckOther::dealloc()
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{
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// Loop through all tokens
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for (const Token *tok = _tokenizer->tokens(); tok; tok = tok->next())
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{
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// Is there a condition and a deallocation?
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if (Token::Match(tok, "if ( %var% ) { free ( %var% ) ; }"))
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{
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// Get variable name used in condition:
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const std::string varname1 = tok->strAt(2);
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// Get variable name used in deallocation:
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const std::string varname2 = tok->strAt(7);
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// Is the same variable used?
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if (varname1 == varname2)
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{
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// report warning
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deallocWarning(tok);
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}
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}
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}
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}
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// Report warning
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void CheckOther::deallocWarning()
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{
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reportError(tok, Severity::warning, "dealloc", "Redundant condition before deallocation");
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}</programlisting>
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<para>The strAt function is used to fetch strings from the token list. The
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parameter specifies the token offset.</para>
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</section>
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</article>
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