On Fri, Feb 19, 2010 at 11:33:30AM +0100, Bernd Petrovitsch wrote:
> On Fre, 2010-02-19 at 13:10 +0300, Dan Carpenter wrote:
> > On Fri, Feb 19, 2010 at 09:29:05AM +0100, Clemens Ladisch wrote:
> > > > This doesn't change anything, but I think it makes the code clearer.
> > > > It silences a smatch warning:
> > > > sound/pci/oxygen/oxygen_mixer.c +91 dac_mute_put(7) warn: add some parenthesis here?
> > >
> > > That message doesn't say why some parentheses should be added.
> > > And it's a question; how do I give it the answer "no"? :-)
> > >
> > > > - changed = !value->value.integer.value[0] != chip->dac_mute;
> > > > + changed = (!value->value.integer.value[0]) != chip->dac_mute;
> > >
> > > This doesn't look any clearer to me; I don't think that the unary
> > > negation operator could be thought to have lower precedence than "!=".
> >
> > Well, it's hard to argue that it's more ambiguous. :P
> But it doesn't make the code clearer - unless you are a C novice. Unary
> operators generally bind stronger than others - be it "+", "-", "!",
> "~", "*".
> I would expect kernel programmers to know that (and I don't assume
> in-depth knowledge of operator precedence rules).
>
> > > Why does smatch warn about this combination? Do such errors actually
> > > happen:
> >
> > Yep. I have made some myself when writing smatch.
> >
> > For example here are some related bugs in the current kernel.
> >
> > drivers/staging/rtl8192u/ieee80211/ieee80211_wx.c
> > 721 if (!ext->ext_flags & IW_ENCODE_EXT_GROUP_KEY &&
> Well, I see potential bugs here and the if() should have been
> a) if (!(ext->ext_flags & IW_ENCODE_EXT_GROUP_KEY) &&
Yep. This is clearly what the code should say.
The problem in the original code is that IW_ENCODE_EXT_GROUP_KEY is not
equal to either 1 or to 0. (So that means the condition in the original
code is always false).
> b) if (!ext->ext_flags && IW_ENCODE_EXT_GROUP_KEY &&
> So you one has to look at the driver for the correct fix (and perhaps
> both of above are wrong).
>
> And I don't see what parenthesis around a logical negations can help
> with the above error example.
>
Basically often when people write:
if (!foo == bar) { ...
What they mean is:
if (!(foo == bar)) { ...
But if they really do mean the original code they could just write
this so it's clear to everyone:
if ((!foo) == bar) { ...
To me it's like "==" vs "=". Of course, every programmer knows the
what the difference is but it helps to have gcc warn about adding the
extra parenthesis. Maybe I suck, but it definitely has helped me in
then past.
I don't have strong feelings about this btw. The original code in
oxygyn_mixer works fine. I just was making some changes to smatch and
that was a new warning today. There is no method to my madness.
regards,
dan carpenter
> Bernd
> --
> Bernd Petrovitsch Email : bernd@xxxxxxxxxxxxxxxxxxx
> LUGA : http://www.luga.at
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