writing-rules-2.docbook

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<section id="writing-rules-2">
  <title>Part 2 - The Cppcheck data representation</title>

  <section>
    <title>Introduction</title>

    <para>In this article I will discuss the data representation that Cppcheck
    uses.</para>

    <para>The data representation that Cppcheck uses is specifically designed
    for static analysis. It is not intended to be generic and useful for other
    tasks.</para>
  </section>

  <section>
    <title>See the data</title>

    <para>There are two ways to look at the data representation at
    runtime.</para>

    <para>Using <parameter class="command">--rule=.+</parameter> is one way.
    All tokens are written on a line:</para>

    <programlisting> int a ; int b ;</programlisting>

    <para>Using <parameter class="command">--debug</parameter> is another way.
    The tokens are line separated in the same way as the original code:</para>

    <programlisting>1: int a@1 ;
2: int b@2 ;</programlisting>

    <para>In the <parameter class="command">--debug</parameter> output there are
    "@1" and "@2" shown. These are the
    variable ids (Cppcheck gives each variable a unique id). You can ignore
    these if you only plan to write rules with regular expressions, you can't
    use variable ids with regular expressions.</para>

    <para>In general, I will use the <parameter class="command">--rule=.+</parameter>
    output in this article because it is more compact.</para>
  </section>

  <section>
    <title>Some of the simplifications</title>

    <para>The data is simplified in many ways.</para>

    <section>
      <title>Preprocessing</title>

      <para>The Cppcheck data is preprocessed. There are no comments, #define,
      #include, etc.</para>

      <para>Original source code:</para>

      <programlisting>#define SIZE 123
char a[SIZE];</programlisting>

      <para>The Cppcheck data for that is:</para>

      <programlisting> char a [ 123 ] ;</programlisting>
    </section>

    <section>
      <title>typedef</title>

      <para>The typedefs are simplified.</para>

      <programlisting>typedef char s8;
s8 x;</programlisting>

      <para>The Cppcheck data for that is:</para>

      <programlisting> ; char x ;</programlisting>
    </section>

    <section>
      <title>Calculations</title>

      <para>Calculations are simplified.</para>

      <programlisting>int a[10 + 4];</programlisting>

      <para>The Cppcheck data for that is:</para>

      <programlisting> int a [ 14 ] ;</programlisting>
    </section>

    <section>
      <title>Variables</title>

      <section>
        <title>Variable declarations</title>

        <para>Variable declarations are simplified. Only one variable can be
        declared at a time. The initialization is also broken out into a
        separate statement.</para>

        <programlisting>int *a=0, b=2;</programlisting>

        <para>The Cppcheck data for that is:</para>

        <programlisting>int * a ; a = 0 ; int b ; b = 2 ;</programlisting>

        <para>This is even done in the global scope. Even though that is
        invalid in C/C++.</para>
      </section>

      <section>
        <title>Known variable values</title>

        <para>Known variable values are simplified.</para>

        <programlisting>void f()
{
    int x = 0;
    x++;
    array[x + 2] = 0;
}</programlisting>

        <para>The <parameter class="command">--debug</parameter> output for that
        is:</para>

        <programlisting>1: void f ( )
2: {
3: ; ;
4: ;
5: array [ 3 ] = 0 ;
6: }</programlisting>

        <para>The variable x is removed because it is not used after the
        simplification. It is therefore redundant.</para>

        <para>The "known values" doesn't have to be numeric. Variable aliases,
        pointer aliases, strings, etc should be handled too.</para>

        <para>Example code:</para>

        <programlisting>void f()
{
    char *a = strdup("hello");
    char *b = a;
    free(b);
}</programlisting>

        <para>The <parameter class="command">--debug</parameter> output for that
        is:</para>

        <programlisting>1: void f ( )
2: {
3: char * a@1 ; a@1 = strdup ( "hello" ) ;
4: ; ;
5: free ( a@1 ) ;
6: }</programlisting>
      </section>
    </section>

    <section>
      <title>if/for/while</title>

      <section>
        <title>Braces in if/for/while-body</title>

        <para>Cppcheck makes sure that there are always braces in if/for/while
        bodies.</para>

        <programlisting>    if (x)
        f1();</programlisting>

        <para>The Cppcheck data for that is:</para>

        <programlisting> if ( x ) { f1 ( ) ; }</programlisting>
      </section>

      <section>
        <title>No else if</title>

        <para>The simplified data representation doesn't have "else
        if".</para>

        <programlisting>void f(int x)
{
    if (x == 1)
        f1();
    else if (x == 2)
        f2();
}</programlisting>

        <para>The <parameter class="command">--debug</parameter> output:</para>

        <programlisting>1: void f ( int x@1 )
2: {
3: if ( x@1 == 1 ) {
4: f1 ( ) ; }
5: else { if ( x@1 == 2 ) {
6: f2 ( ) ; } }
7: }
</programlisting>
      </section>

      <section>
        <title>Condition is always true / false</title>

        <para>Conditions that are always true / false are simplified.</para>

        <programlisting>void f()
{
    if (true) {
        f1();
    }
}</programlisting>

        <para>The Cppcheck data is:</para>

        <programlisting> void f ( ) { { f1 ( ) ; } }</programlisting>

        <para>Another example:</para>

        <programlisting>void f()
{
    if (false) {
        f1();
    }
}</programlisting>

        <para>The debug output:</para>

        <programlisting> void f ( ) { }</programlisting>
      </section>

      <section>
        <title>Assignments</title>

        <para>Assignments within conditions are broken out from the
        condition.</para>

        <programlisting>void f()
{
    int x;
    if ((x = f1()) == 12) {
        f2();
    }
}</programlisting>

        <para>The <code>x=f1()</code> is broken out. The
        <parameter class="command">--debug</parameter> output:</para>

        <programlisting>1: void f ( )
2: {
3: int x@1 ;
4: x@1 = f1 ( ) ; if ( x@1 == 12 ) {
5: f2 ( ) ;
6: }
7: }</programlisting>

        <para>Replacing the "if" with "while" in the above example:</para>

        <programlisting>void f()
{
    int x;
    while ((x = f1()) == 12) {
        f2();
    }
}</programlisting>

        <para>The <literal>x=f1()</literal> is broken out twice. The
        <parameter class="command">--debug</parameter> output:</para>

        <programlisting>1: void f ( )
2: {
3: int x@1 ;
4: x@1 = f1 ( ) ; while ( x@1 == 12 ) {
5: f2 ( ) ; x@1 = f1 ( ) ;
5:
6: }
7: }</programlisting>
      </section>

      <section>
        <title>Comparison with &gt;</title>

        <para>Comparisons are simplified. The two conditions in this example
        are logically the same:</para>

        <programlisting>void f()
{
    if (x &lt; 2);
    if (2 &gt; x);
}</programlisting>

        <para>Cppcheck data doesn't use <literal>&gt;</literal> for
        comparisons. It is converted into <literal>&lt;</literal> instead. In
        the Cppcheck data there is no difference for <literal>2&gt;x</literal>
        and <literal>x&lt;2</literal>.</para>

        <programlisting>1:
2: void f ( )
3: {
4: if ( x &lt; 2 ) { ; }
5: if ( x &lt; 2 ) { ; }
6: }</programlisting>

        <para>A similar conversion happens when <literal>&gt;=</literal> is
        used.</para>
      </section>

      <section>
        <title>if (x) and if (!x)</title>

        <para>If possible a condition will be reduced to x or !x. Here is an
        example code:</para>

        <programlisting>void f()
{
    if (!x);
    if (NULL == x);
    if (x == 0);

    if (x);
    if (NULL != x);
    if (x != 0);
}</programlisting>

        <para>The <parameter class="command">--debug</parameter> output is:</para>

        <programlisting>1: void f ( )
2: {
3: if ( ! x ) { ; }
4: if ( ! x ) { ; }
5: if ( ! x ) { ; }
6:
7: if ( x ) { ; }
8: if ( x ) { ; }
9: if ( x ) { ; }
10: }</programlisting>
      </section>
    </section>
  </section>
</section>