Part 1 - Getting started
Daniel
Marjamäki
Cppcheck
2010
Introduction
This is a short and simple guide that describes how rules are
written for Cppcheck.
The patterns are defined with regular expressions. It is required
that you know how regular expressions work.
Data representation of the source code
The data used by the rules are not the raw source code.
Cppcheck will read the source code and process it
before the rules are used.
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.
Between each token in the code there is always a space. For instance
the raw code "1+f()" is processed into "1 + f ( )"
.
The code is simplified in many ways.
Creating a simple rule
When creating a rule there are two steps:
Create the regular expression
Create a XML based rule file
Step 1 - Creating the regular expression
Cppcheck uses the PCRE library to handle regular expressions.
PCRE stands for "Perl Compatible Regular Expressions".
The homepage for PCRE is
http://www.pcre.org/.
Let's create a regular expression that checks for code such
as:
if (p)
free(p);
For such code the condition is often redundant (on most
implementations it is valid to free a NULL pointer).
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:
void f() {
if (p)
free(p);
}
Save that code as dealloc.cpp and then use
cppcheck --rule=".+" dealloc.cpp:
$ ./cppcheck --rule=".+" dealloc.cpp
Checking dealloc.cpp...
[dealloc.cpp:1]: (style) found ' void f ( ) { if ( p ) { free ( p ) ; } }'
The regular expression .+ matches everything
and the matching text is shown on the screen.
From that output we can see that the simplified code is:
void f ( ) { if ( p ) { free ( p ) ; } }
Now that we know how the simplified code looks. We can create a
regular expression that matches it properly:
$ cppcheck --rule="if \( p \) { free \( p \) ; }" dealloc.cpp
Checking dealloc.cpp...
[dealloc.cpp:2]: (style) found 'if ( p ) { free ( p ) ; }'
Step 2 - Create rule file
A rule file is a simple XML file that contains:
a pattern to search for
an error message that is reported when pattern is found
Here is a simple example:
<?xml version="1.0"?>
<rule version="1">
<pattern>if \( p \) { free \( p \) ; }</pattern>
<message>
<id>redundantCondition</id>
<severity>style</severity>
<summary>Redundant condition. It is valid to free a NULL pointer.</summary>
</message>
</rule>
If you save that xml data in dealloc.rule you
can test this rule:
$ cppcheck --rule-file=dealloc.rule dealloc.cpp
Checking dealloc.cpp...
[dealloc.cpp:2]: (style) Redundant condition. It is valid to free a NULL pointer.