Re: [PATCH] Remove useless if-before-free tests.

From: Jim Meyering <jim@meyering.net>
Date: Feb 24, 2008, 18:15 UTC
Message-ID: <877igup6fl.fsf@rho.meyering.net>
In-Reply-To: <7vd4qo7fsc.fsf@gitster.siamese.dyndns.org>

Junio C Hamano <gitster@pobox.com> wrote: ...

Show 6 quoted lines

>> If you're interested in automating detection of the useless
>> tests, you might like the useless-if-before-free script in gnulib:
>> [it *does* detect brace-enclosed free statements, and has a --name=S
>>  option to make it detect free-like functions with different names]
>
> While I have your attention ;-)

Hi Jun,

No excuse required ;-)

> I am not interested in automating useless "if (x) free(x)" tests,

Yeah, that particular one is not a big deal, but whenever I take the time to make a sweeping change, I find it's worth a little more to automate a check to preserve the goal state.

> but one thing I recently wanted but did not know a handy
> tool for was to find all the calls to free() that free a pointer
> to an object of a particular type.

...

> I guess sparse could be hacked to do that, but do GNU folks have
> some checker like that?

A general purpose tool to do something like that would be very useful. I thought of cscope and eclipse, but as far as I know, neither of them can perform such a query.

This made me think of the dwarves package/paper:

    7 dwarves
    https://ols2006.108.redhat.com/2007/Reprints/melo-Reprint.pdf

This looked promising at first, but it annotates function _definitions_ for run-time data collection, while you want to look at uses, which can be done statically:

    3.4   ctracer

    A class tracer, ctracer is an experiment in creating valid source
    code from the DWARF information.  For ctracer a method is any
    function that receives as one of its parameters a pointer to a
    specified struct. It looks for all such methods and generates
    kprobes entry and exit functions. At these probe points it
    collects information about the data structure internal state,
    saving the values in its members in that point in time, and
    records it in a relay buffer. The data is later collected in
    userspace and post-processed, generating html + CSS callgraphs.

Too bad coverity is closed-source.  I'll bet it could do this easily.

Maybe hacking sparse is the way to go, after all.