From: David Laight <David.Laight@ACULAB.COM>
To: 'Linus Torvalds' <torvalds@linux-foundation.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Jens Axboe <axboe@kernel.dk>,
"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>
Subject: RE: [RFC PATCH 12/13] blk-mq.h: Fix parentheses around macro parameter use
Date: Sat, 6 May 2023 15:45:54 +0000 [thread overview]
Message-ID: <71d4279ca9524b2382059cd2f723e20e@AcuMS.aculab.com> (raw)
In-Reply-To: <CAHk-=wh-x1PL=UUGD__Dv6kd+kyCHjNF-TCHGG9ayLnysf-PdQ@mail.gmail.com>
From: Linus Torvalds
> Sent: 05 May 2023 20:55
....
> > The other exception I would add is when a parameter is used as an
> > lvalue, as:
> >
> > #define m(x) do { x = 2; } while (0)
>
> I really don't understand why you think '=' is so special. It's very
> much not special.
>
> It happens to have the lowest precedence, sure, but the keyword is "happens".
And consider what happens if you try:
m(a ? b : c)
Personally I'd avoid using parameters as lvalues if at all possible.
It is much better to have:
#define m(x) do { *(x) = 2; } while (0)
and require the caller do m(&foo) to make it obvious the value is changed.
(Apart from loop definitions...)
Things like the C++ 'int &arg' make it hard to scan read/search
code for places where variables get changed.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2023-05-06 15:46 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-04 20:05 [RFC PATCH 00/13] Fix parentheses around macro parameter use in headers Mathieu Desnoyers
2023-05-04 20:05 ` [RFC PATCH 01/13] rcu: rcupdate.h: Fix parentheses around macro parameter use Mathieu Desnoyers
2023-05-04 20:05 ` [RFC PATCH 02/13] rculist.h: Fix parentheses around macro pointer " Mathieu Desnoyers
2023-05-04 20:05 ` [RFC PATCH 03/13] rculist_nulls.h: Add " Mathieu Desnoyers
2023-05-04 20:05 ` [RFC PATCH 04/13] rculist_bl.h: Fix " Mathieu Desnoyers
2023-05-04 20:05 ` [RFC PATCH 05/13] list.h: " Mathieu Desnoyers
2023-05-08 12:16 ` Andy Shevchenko
2023-05-08 13:46 ` Mathieu Desnoyers
2023-05-12 11:02 ` Andy Shevchenko
2023-05-12 14:32 ` Mathieu Desnoyers
2023-05-04 20:05 ` [RFC PATCH 06/13] list_nulls.h: " Mathieu Desnoyers
2023-05-04 20:05 ` [RFC PATCH 07/13] list_bl.h: " Mathieu Desnoyers
2023-05-04 20:05 ` [RFC PATCH 08/13] llist.h: " Mathieu Desnoyers
2023-05-04 20:05 ` [RFC PATCH 09/13] klist.h: Fix parentheses around macro " Mathieu Desnoyers
2023-05-04 20:05 ` [RFC PATCH 10/13] resource_ext.h: Remove useless parentheses around macro parameters Mathieu Desnoyers
2023-05-04 20:05 ` [RFC PATCH 11/13] netdevice.h: Fix parentheses around macro parameter use Mathieu Desnoyers
2023-05-05 18:44 ` Jakub Kicinski
2023-05-05 18:54 ` Mathieu Desnoyers
2023-05-04 20:05 ` [RFC PATCH 12/13] blk-mq.h: " Mathieu Desnoyers
2023-05-05 13:56 ` Mathieu Desnoyers
2023-05-05 18:40 ` Linus Torvalds
2023-05-05 18:49 ` Mathieu Desnoyers
2023-05-05 19:54 ` Linus Torvalds
2023-05-05 20:08 ` Mathieu Desnoyers
2023-05-05 20:22 ` Linus Torvalds
2023-05-05 20:28 ` Mathieu Desnoyers
2023-05-08 14:28 ` Mathieu Desnoyers
2023-05-06 15:45 ` David Laight [this message]
2023-05-04 20:05 ` [RFC PATCH 13/13] bio.h: " Mathieu Desnoyers
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