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To: root@chaos.analogic.com
Cc: "Hefty, Sean" <sean.hefty@intel.com>, Troy Benjegerdes <hozer@hozed.org>,
       infiniband-general@lists.sourceforge.net, linux-kernel@vger.kernel.org
Subject: Re: [Infiniband-general] Getting an Infiniband access layer in theLinux kernel
References: <C1B7430B33A4B14F80D29B5126C5E9470326258C@orsmsx401.jf.intel.com>
	<Pine.LNX.4.53.0402061150100.3862@chaos>
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From: Roland Dreier <roland@topspin.com>
Date: 06 Feb 2004 09:23:00 -0800
In-Reply-To: <Pine.LNX.4.53.0402061150100.3862@chaos>
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    Richard> If some major changes are being considered, I think it's
    Richard> time to get rid of the:

    Richard> do { } while(0) stuff that permiates a lot of MACROS and
    Richard> just use the { } as they were designed.

    Richard> Before everybody screams, think. It's perfectly correct
    Richard> to start a new "program unit" without a conditional
    Richard> expression.  You just add a curley-brace, then close the
    Richard> brace when you are though.

This is totally, totally wrong.  If you get rid of do { } while (0),
then you can't use the macro in an if statement.  Read any C FAQ for
details, or try the following:

    #define MAC(x) { x = x + 1; }

    int main() {
      int x = 0;

      if (1)
        MAC(x);
      else
        x = x - 1;
    }

I get the following (correct) error:

    $ gcc a.c
    a.c: In function `main':
    a.c:8: syntax error before "else"
    $ gcc --version
    gcc (GCC) 3.2.3 20030502 (Red Hat Linux 3.2.3-20)

because

    if (1)
        { x = x + 1 } ; /* <-- note semicolon
    else
        x = x - 1;

is not correct C.

By the way, it is possible to use parentheses and commas for some
simple macros, so for example the following is OK:

    #define MAC(x) ( x = x + 1, x = x * 2 )

    int main() {
      int x = 0;

      if (1)
        MAC(x);
      else
        x = x - 1;
    }

However I don't see anything wrong with the perfectly standard "do { }
while (0)" idiom.  Certainly if some compiler generates worse code for
that construct that just a plain { }, _that_ is a compiler bug that we
shouldn't have to work around.

 - Roland