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

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Date:   Mon, 4 Nov 2019 17:03:04 +0000
From:   Ian Abbott <abbotti@....co.uk>
To:     Jules Irenge <jbi.octave@...il.com>,
        outreachy-kernel@...glegroups.com
Cc:     gregkh@...uxfoundation.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org, hsweeten@...ionengravers.com
Subject: Re: [PATCH v2] staging: comedi: rewrite macro function with GNU
 extension typeof

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?

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