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Date: Thu, 30 Nov 2017 16:43:03 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 3/3] maint: Fix macros with broken
 'do/while(0); ' usage
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To: Eric Blake <eblake@redhat.com>
Cc: qemu-devel@nongnu.org, qemu-trivial@nongnu.org, Gerd Hoffmann <kraxel@redhat.com>, Alistair Francis <alistair@alistair23.me>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Kevin Wolf <kwolf@redhat.com>, Max Reitz <mreitz@redhat.com>, "Edgar E. Iglesias" <edgar.iglesias@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>, Peter Maydell <peter.maydell@linaro.org>, Alexander Graf <agraf@suse.de>, Jason Wang <jasowang@redhat.com>, Subbaraya Sundeep <sundeep.lkml@gmail.com>, Stefan Berger <stefanb@linux.vnet.ibm.com>, Juan Quintela <quintela@redhat.com>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>, Christian Borntraeger <borntraeger@de.ibm.com>, Cornelia Huck <cohuck@redhat.com>, Richard Henderson <rth@twiddle.net>, Igor Mammedov <imammedo@redhat.com>, "open list:Block layer core" <qemu-block@nongnu.org>, "open list:Xilinx Zynq" <qemu-arm@nongnu.org>, "open list:New World" <qemu-ppc@nongnu.org>, "open list:S390" <qemu-s390x@nongnu.org>

On Thu, Nov 30, 2017 at 07:41:59AM -0600, Eric Blake wrote:
> The point of writing a macro embedded in a 'do { ... } while (0)'
> loop is so that the macro can be used as a drop-in statement with
> the caller supplying the trailing ';'.  Although our coding style
> frowns on brace-less 'if':
>   if (cond)
>     statement;
>   else
>     something else;
> the use of do/while (0) in a macro is absolutely essential for the
> purpose of avoiding a syntax error on the 'else' - but it only works
> if there is no trailing ';' in the macro (as the ';' in the code
> calling the macro would then be a second statement and cause the
> 'else' to not pair to the 'if').

Shouldn't matter if everyone puts the statements in {}, right?

> Many of the places touched in this code are examples of the ugly
> bit-rotting debug print statements; cleaning those up is left as
> a bite-sized task for another day.
> 
> Found mechanically via: $ git grep -B1 'while (0);' | grep -A1 \\\\
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>

We can't really rely on code still building for this to do the right
thing, can we?  I did my best to look for uses and I think it's OK, so

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

but I'm not merging this.

> -- 
> 2.14.3