linux-kernel - Re: [PATCH v2] staging: comedi: rewrite macro function with GNU extension typeof

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Date:   Mon, 4 Nov 2019 21:46:36 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Ian Abbott <abbotti@....co.uk>
Cc:     Jules Irenge <jbi.octave@...il.com>,
        outreachy-kernel@...glegroups.com, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] staging: comedi: rewrite macro function with GNU
 extension typeof

On Mon, Nov 04, 2019 at 05:03:04PM +0000, Ian Abbott wrote:
> On 04/11/2019 16:33, Jules Irenge wrote:
> > Rewrite macro function with the GNU extension typeof
> > to remove a possible side-effects of MACRO argument reuse "x".
> >   - Problem could rise if arguments have different types
> > and different use though.
> > 
> > Signed-off-by: Jules Irenge <jbi.octave@...il.com>
> > ---
> > v1 - had no full commit log message, with changes not intended to be in the patch
> > v2 - remove some changes not intended to be in this driver
> >       include note of a potential problem
> >   drivers/staging/comedi/comedi.h | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/staging/comedi/comedi.h b/drivers/staging/comedi/comedi.h
> > index 09a940066c0e..a57691a2e8d8 100644
> > --- a/drivers/staging/comedi/comedi.h
> > +++ b/drivers/staging/comedi/comedi.h
> > @@ -1103,8 +1103,10 @@ enum ni_common_signal_names {
> >   /* *** END GLOBALLY-NAMED NI TERMINALS/SIGNALS *** */
> > -#define NI_USUAL_PFI_SELECT(x)	(((x) < 10) ? (0x1 + (x)) : (0xb + (x)))
> > -#define NI_USUAL_RTSI_SELECT(x)	(((x) < 7) ? (0xb + (x)) : 0x1b)
> > +#define NI_USUAL_PFI_SELECT(x)\
> > +	({typeof(x) x_ = (x); (x_ < 10) ? (0x1 + x_) : (0xb + x_); })
> > +#define NI_USUAL_RTSI_SELECT(x)\
> > +	({typeof(x) x_ = (x); (x_ < 7) ? (0xb + x_) : 0x1b; })
> >   /*
> >    * mode bits for NI general-purpose counters, set with
> > 
> 
> I wasn't sure about this the first time around due to the use of GNU
> extensions in uapi header files, but I see there are a few, rare instances
> of this GNU extension elsewhere in other uapi headers (mainly in netfilter
> stuff), so I guess it's OK.  However, it  does mean that user code that uses
> these macros will no longer compile unless GNU extensions are enabled.
> 
> Does anyone know any "best practices" regarding use of GNU extensions in
> user header files under Linux?

We try to never do it if at all possible :)

thanks,

greg k-h