cppcheck/man/cppcheck-design.docbook

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<info>
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<title>Cppcheck Design</title>
<author>
<personname><firstname>Daniel</firstname><surname>Marjamäki</surname></personname>
<affiliation>
<orgname>Cppcheck</orgname>
</affiliation>
</author>
<pubdate>2010</pubdate>
</info>
<section>
<title>Introduction</title>
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<para>The goal with this article is to give users an idea of how Cppcheck
works.</para>
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<para>Cppcheck is a static analysis tool that tries to completely avoid
false warnings. A false warning is when the tool reports that there is an
error even though there is no error.</para>
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<para>Cppcheck is a relatively simple tool. I hope that this article will
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highlight that it is possible to avoid false warnings even with simple
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analysis.</para>
</section>
<section>
<title>Limitations of static analysis</title>
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<para>There are many bugs in programs that are really hard to detect for
tools. Here is an example:</para>
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<programlisting> // calculate the number of days
int days = hours / 23;</programlisting>
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<para>A human programmer knows that there are 24 hours in a day and
therefore he could see that "23" is wrong. A tool will probably not know
that there are 24 hours in a day.</para>
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<para>A tool that tries to detect all bugs could write a warning message
for every calculation in the program. Then it will correctly report that
"hours / 23" is wrong but incorrectly warn about "hours / 24".</para>
<para>Cppcheck will only write a warning message if it can determine that
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the calculation is wrong. In this case, no error will be written.</para>
</section>
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<section>
<title>Control flow analysis</title>
<para>When you review code you will probably use "control flow analysis"
in your head to determine if there are bugs or not.</para>
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<para>Control flow analysis is when you try to determine what the possible
execution paths are.</para>
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<para>The control flow analysis in Cppcheck is quite simple.</para>
</section>
<section>
<title>Buffer overflows</title>
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<para>This is a simple description of how buffer overflows are detected by
Cppcheck.</para>
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<para>If an array is accessed out of bounds somewhere in its scope then an
error message will be written. An example code:</para>
<programlisting>void f()
{
char a[10];
if (x + y == 2) {
a[20] = 0;
}
}</programlisting>
<para>Cppcheck will report this message:</para>
<programlisting>Array 'a[10]' index 20 out of bounds</programlisting>
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<para>No control flow analysis is used. Cppcheck will not try to determine
how execution can reach the "a[20] = 0;" statement. It is assumed that all
statements are reachable. Cppcheck will detect the error even if it is
really impossible that "x + y == 2" is true. I still claim that this is a
correct warning because the statement is there and it has the
error.</para>
<para>Cppcheck will also investigate function calls. But then control flow
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analysis can be needed to avoid false warnings. Here is an example that
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logically is the same as the previous example:</para>
<para><programlisting>void f1(char *s)
{
s[20] = 0;
}
void f2()
{
char a[10];
if (x + y == 2) {
f1(a);
}
}</programlisting>Cppcheck will report this message:</para>
<programlisting>Array 'a[10]' index 20 out of bounds</programlisting>
<para>If the execution reaches the function call then there will be an
error.</para>
<para>But if the condition is moved into "f1" then it will be necessary to
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prove that "x+y==2" can be true when the function is called from "f2". No
error message is reported for this code:</para>
<para><programlisting>void f1(char *s)
{
if (x + y == 2) {
s[20] = 0;
}
}
void f2()
{
char a[10];
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f1(a);
}</programlisting></para>
</section>
<section>
<title>Memory leaks</title>
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<para>The check uses simple control-flow analysis. The control flow
analysis assumes that all conditions can always be either true or false.
It is assumed that all statements are reachable. Here is an
example:</para>
<programlisting>void f()
{
char *a = malloc(10);
if (x + y == 2) {
return;
}
free(a);
}</programlisting>
<para>Cppcheck will determine that there is a leak at the "return;"
statement:</para>
<programlisting>Memory leak: a</programlisting>
<para>Cppcheck doesn't try to determine how the execution reaches the
"return;" statement. It will only see that if the execution reaches the
"return;" then there will be a memory leak.</para>
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<para>Lack of advanced control-flow analysis means that many bugs are not
detected:</para>
<programlisting>void f(int x)
{
char *a = 0;
if (x == 10)
a = malloc(10);
if (x == 20)
free(a);
}</programlisting>
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<para>Cppcheck doesn't detect any error. The "all conditions can be either
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true/false" means that cppcheck doesn't know that "if (x==20)" is always
false when "if (x==10)" is true. So Cppcheck can't establish that there is
a leak. Many other static analysis tools will probably detect that there
will be a leak if x is 10.</para>
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</section>
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<section>
<title>Final thoughts</title>
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<para>You can not trust that Cppcheck will detect all bugs.</para>
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<para>Cppcheck will just find some bugs. It is likely that you won't find
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these bugs unless you use Cppcheck.</para>
</section>
</article>