Date: Wed, 6 Jun 2018 09:53:21 -0700 From: Conrad Meyer <cem@freebsd.org> To: Benjamin Kaduk <bjkfbsd@gmail.com>, Ravi Pokala <rpokala@freebsd.org> Cc: src-committers <src-committers@freebsd.org>, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r334702 - head/sys/sys Message-ID: <CAG6CVpWBwK-6ALhhcDpe4jrr52j9oMWjdheDRABHauiRFDE13A@mail.gmail.com> In-Reply-To: <CAJ5_RoALt_%2BASNN5E2wBch-CEn0sesJO=1Z2aJsz1EwsesdFiQ@mail.gmail.com> References: <201806060508.w56586c9053686@repo.freebsd.org> <6E6E92B2-7536-4281-8EAF-72823E84902E@panasas.com> <CAJ5_RoALt_%2BASNN5E2wBch-CEn0sesJO=1Z2aJsz1EwsesdFiQ@mail.gmail.com>
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Hi Ben, Ravi,
GCC/Clang are fine with the just curly braces, or just do/while(0).
The core benefit of the ({ }) syntax is to allow multiple-statement
macros to have expression syntax externally (i.e., yield a value) =E2=80=94
which is not possible in standard C due to concession to simplistic
parsers. Ben's link is a good one and does cover this in more detail.
The URL ("Statement-Exprs") is a hint ;-).
Best,
Conrad
On Wed, Jun 6, 2018 at 4:40 AM, Benjamin Kaduk <bjkfbsd@gmail.com> wrote:
> On Wed, Jun 6, 2018 at 6:35 AM, Ravi Pokala <rpokala@freebsd.org> wrote:
>>
>> Hi Mateusz,
>>
>> =EF=BB=BF-----Original Message-----
>> From: <owner-src-committers@freebsd.org> on behalf of Mateusz Guzik
>> <mjg@FreeBSD.org>
>> Date: 2018-06-06, Wednesday at 01:08
>> To: <src-committers@freebsd.org>, <svn-src-all@freebsd.org>,
>> <svn-src-head@freebsd.org>
>> Subject: svn commit: r334702 - head/sys/sys
>>
>> > ...
>> > #ifdef _KERNEL
>> > #define malloc(size, type, flags) ({
>> > \
>>
>> Now that I'm taking another look at this, I'm confused as to why the
>> entire macro expansion is inside parentheses? (The braces make sense, si=
nce
>> this is a block with local variables which need to be contained.)
>
>
> This is a gcc (and clang) extension to allow the macro body to be a code
> block -- standard C gets unhappy with just the curly braces.
> https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html is a maybe-releva=
nt
> page that google found me.
>
> -Ben
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