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referencing deallocated memory.
The class was defined basically as follows:
class ScriptResult
{
private:
std::string output;
public:
std::string get_output() const
{
return this->output;
}
};
The problem was when it was used like this, as in our script output
routines:
const char *s = sr.get_output().c_str();
printf("%s\n", s);
The reason is that the temporary std::string returned by get_output goes
out of scope after the line containing it, which invalidates the memory
pointed to by c_str(). By the time of the printf, s may be pointing to
deallocated memory.
This could have been fixed by returning a const reference that would
remain valid as long as the ScriptResult's output member is valid:
const std::string& get_output() const
{
return this->output;
}
However I noticed that get_output() was always immediately followed by a
c_str(), so I just had get_output return that instead, which has the
same period of validity.
This problem became visiable when compiling with Visual C++ 2010. The
first four bytes of script output in normal output would be garbage
(probably some kind of free list pointer). It didn't happen in XML
output, because the get_output-returned string happened to remain in
scope during that.
Here is some documentation for Nmap, but these files are much less comprehensive than what you'll find at the actual Nmap documentation site ( http://nmap.org ).