linux-kernel - Re: [PATCH] rtl2832: remove compiler warning

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Date:	Tue, 10 Feb 2015 21:35:52 -0200
From:	Mauro Carvalho Chehab <mchehab@....samsung.com>
To:	Antti Palosaari <crope@....fi>
Cc:	Luis de Bethencourt <luis@...ethencourt.com>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rtl2832: remove compiler warning

Em Tue, 10 Feb 2015 12:57:24 +0200
Antti Palosaari <crope@....fi> escreveu:

> On 02/09/2015 12:44 AM, Luis de Bethencourt wrote:
> > Cleaning the following compiler warning:
> > rtl2832.c:703:12: warning: 'tmp' may be used uninitialized in this function
> >
> > Even though it could never happen since if rtl2832_rd_demod_reg () doesn't set
> > tmp, this line would never run because we go to err. It is still nice to avoid
> > compiler warnings.
> >
> > Signed-off-by: Luis de Bethencourt <luis.bg@...sung.com>
> > ---
> >   drivers/media/dvb-frontends/rtl2832.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/media/dvb-frontends/rtl2832.c b/drivers/media/dvb-frontends/rtl2832.c
> > index 5d2d8f4..ad36d1c 100644
> > --- a/drivers/media/dvb-frontends/rtl2832.c
> > +++ b/drivers/media/dvb-frontends/rtl2832.c
> > @@ -685,7 +685,7 @@ static int rtl2832_read_status(struct dvb_frontend *fe, fe_status_t *status)
> >   	struct rtl2832_dev *dev = fe->demodulator_priv;
> >   	struct i2c_client *client = dev->client;
> >   	int ret;
> > -	u32 tmp;
> > +	u32 tmp = 0;
> >
> >   	dev_dbg(&client->dev, "\n");
> 
> I looked the code and I cannot see how it could used as uninitialized. 
> Dunno how it could be fixed properly.
> 
> Also, I think idiom to say compiler that variable could be uninitialized 
> is to store its own value. But I am fine with zero initialization too.
> 
> u32 tmp = tmp;

Actually, the right way is to declare it as:

	u32 uninitialized_var(tmp)

The syntax to suppress compiler warnings depends on the compiler:

include/linux/compiler-clang.h:#define uninitialized_var(x) x = *(&(x))
include/linux/compiler-gcc.h:#define uninitialized_var(x) x = x

Also, using uninitialized_var() better documents it.

Regards,
Mauro
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