Re: curly braces inside for loop parameter

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: curly braces inside for loop parameter
Date	2015-05-17 12:28 -0700
Organization	None to speak of
Message-ID	<87d21z9f6u.fsf@kst-u.example.com> (permalink)
References	<a6f4eb2e-e20e-41da-b007-db42b48b0421@googlegroups.com>

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Daniel Doron <danielmeirdoron@gmail.com> writes:
> I am working on some stuff involving hlists in kernel. I have seen
> this in lists.h:
>
> #define hlist_for_each_entry(tpos, pos, head, member)			 \
> 	for (pos = (head)->first;					 \
> 	     pos &&							 \
> 		({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \
> 	     pos = pos->next)
>
>
> I have never encountered this syntax in C before, I am referring to
> the curly braces and dangling "1" inside the for loop second
> parameter.  Can someone explain this?

This:

    ( { ... } )

is called a "statement expression".  It doesn't exist in standard
C, so any code that uses it is not portable.  It's a gcc-specific
extension (presumably also implemented by other compilers that
emulate gcc).

The feature is documented here:
    https://gcc.gnu.org/onlinedocs/gcc-5.1.0/gcc/Statement-Exprs.html

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Thread

curly braces inside for loop parameter Daniel Doron <danielmeirdoron@gmail.com> - 2015-05-17 06:56 -0700
  Re: curly braces inside for loop parameter asetofsymbols@gmail.com - 2015-05-17 07:49 -0700
  Re: curly braces inside for loop parameter Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-05-17 15:52 +0100
  Re: curly braces inside for loop parameter Bartc <bc@freeuk.com> - 2015-05-17 16:01 +0100
    Re: curly braces inside for loop parameter David Kleinecke <dkleinecke@gmail.com> - 2015-05-17 09:14 -0700
      Re: curly braces inside for loop parameter Bartc <bc@freeuk.com> - 2015-05-17 17:43 +0100
    Re: curly braces inside for loop parameter raltbos@xs4all.nl (Richard Bos) - 2015-05-23 10:33 +0000
  Re: curly braces inside for loop parameter Keith Thompson <kst-u@mib.org> - 2015-05-17 12:28 -0700
  curly braces inside for loop parameter Chad <cdalten@gmail.com> - 2015-05-17 13:53 -0700
    Re: curly braces inside for loop parameter gazelle@shell.xmission.com (Kenny McCormack) - 2015-05-17 21:36 +0000

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