svn commit: r334702 - head/sys/sys

[Conrad Meyer]

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) —
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 at gmail.com> wrote:
> On Wed, Jun 6, 2018 at 6:35 AM, Ravi Pokala <rpokala at freebsd.org> wrote:
>>
>> Hi Mateusz,
>>
>> -----Original Message-----
>> From: <owner-src-committers at freebsd.org> on behalf of Mateusz Guzik
>> <mjg at FreeBSD.org>
>> Date: 2018-06-06, Wednesday at 01:08
>> To: <src-committers at freebsd.org>, <svn-src-all at freebsd.org>,
>> <svn-src-head at 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, since
>> 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-relevant
> page that google found me.
>
> -Ben