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Re: cpp behaviour change
- To: Falk Hueffner <falk dot hueffner at student dot uni-tuebingen dot de>
- Subject: Re: cpp behaviour change
- From: Zack Weinberg <zack at wolery dot cumb dot org>
- Date: Wed, 21 Jun 2000 17:01:35 -0700
- Cc: gcc-bugs at gcc dot gnu dot org
- References: <87ya3yppob.fsf@zxmjz18.extern.uni-tuebingen.de>
On Thu, Jun 22, 2000 at 01:41:56AM +0200, Falk Hueffner wrote:
> Hi,
>
> I found out that this:
>
> #define FOO(V) printf("baz " ## #V);
> FOO(bar);
>
> will be converted by cpp 2.96 to
>
> printf("baz "#bar);;
>
> while 2.95.2 produced
>
> printf("baz ""bar"); ;
>
> Can anybody tell me which behaviour is correct?
C99 section 6.10.3.2 paragraph 2: ... The order of evaluation of # and
## operators is unspecified.
The current cpp chooses to go strictly left-to-right; the older one
did all the # operators first. Either choice is allowed.
If this fragment is intended to be fed to the C compiler, then the ##
is unnecessary. You can put as much whitespace as you want (including
comments) between consecutive string constants, and they will still be
pasted.
If you are using cpp to process something other than C source, please
use M4 instead.
zw