linux-kernel - Re: [PATCH] staging: gdm724x: Add parenthesis to Macro arguments

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Message-ID: <0d8cd5e0-71f0-6748-08a6-84c3477c874d@gmail.com>
Date:   Mon, 8 Apr 2019 14:37:58 -0300
From:   Andre <andredainez@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     lkcamp@...ts.libreplanetbr.org, realwakka@...il.com,
        straube.linux@...il.com, hle@....eu.com, rico.schrage@...il.com,
        sophie.matter@....de, Valentin.Vidic@...net.hr, simon@...anor.nu,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: gdm724x: Add parenthesis to Macro arguments

Hi Greg, thanks for replying.

On 03/04/2019 01:26, Greg KH wrote:
> On Tue, Apr 02, 2019 at 10:04:05PM -0300, Andre Dainez wrote:
>> Fix checkpatch errors:
>>
>> CHECK: Macro argument 'len' may be better as '(len)' to avoid precedence issues
>> CHECK: Macro argument 'nlh' may be better as '(nlh)' to avoid precedence issues
>>
>> Signed-off-by: Andre Dainez <andredainez@...il.com>
>> ---
>>  drivers/staging/gdm724x/netlink_k.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/staging/gdm724x/netlink_k.c b/drivers/staging/gdm724x/netlink_k.c
>> index 92440c3..36d88f4 100644
>> --- a/drivers/staging/gdm724x/netlink_k.c
>> +++ b/drivers/staging/gdm724x/netlink_k.c
>> @@ -19,8 +19,8 @@ static DEFINE_MUTEX(netlink_mutex);
>>  #define ND_NLMSG_SPACE(len)	(NLMSG_SPACE(len) + ND_IFINDEX_LEN)
>>  #define ND_NLMSG_DATA(nlh)	((void *)((char *)NLMSG_DATA(nlh) + \
>>  						  ND_IFINDEX_LEN))
>> -#define ND_NLMSG_S_LEN(len)	(len + ND_IFINDEX_LEN)
>> -#define ND_NLMSG_R_LEN(nlh)	(nlh->nlmsg_len - ND_IFINDEX_LEN)
>> +#define ND_NLMSG_S_LEN(len)	((len) + ND_IFINDEX_LEN)
> 
> This makes sense, but:
> 
>> +#define ND_NLMSG_R_LEN(nlh)	((nlh)->nlmsg_len - ND_IFINDEX_LEN)
> 
> That does not, correct?
> 
Could you please clarify why this doesn't make sense?
If, for some reason I calculate by hand the pointer address and call this macro like: 
ND_NLMSG_R_LEN(nlh + sizeof(*nlh)), 
then it would expand like nlh + sizeof(*nlh)->nlmsg_len - ND_IFINDEX_LEN
which looks wrong in my pov, no?

Please, let me know if I misunderstood anything.
Thanks