[cfe-commits] [Patch] Fix for comma-removal ahead of __VA_ARGS__ in macros

[Jean-Daniel Dupas]

Le 5 nov. 2012 à 12:07, Andy Gibbs <andyg1001 at hotmail.co.uk> a écrit :

> On Monday, November 05, 2012 9:50 AM, Jean-Daniel Dupas wrote:
>> Le 5 nov. 2012 à 09:08, Eli Friedman a écrit :
>>> On Sun, Nov 4, 2012 at 3:10 PM, Andy Gibbs wrote:
>>>> Hi,
>>>> 
>>>> Please could someone review the attached patch for me before I commit it.
>>>> 
>>>> This patch fixes the rules surrounding the removal of a leading comma where
>>>> a __VA_ARGS__ expansion is empty.
>>>> 
>>>> GCC has the extension, implemented already in clang for...
>>>> 
>>>> #define M(...) call(a, ## __VA_ARGS__)
>>>> 
>>>> This will expand to "call(a)" for "M()", removing the comma.  However, under
>>>> -std=c99, this behaviour changes so that the comma is not removed. The
>>>> attached patch follows this behaviour when in c99 mode in clang.  In gnu99
>>>> mode, it behaves as before, and similarly for pre-c99 dialects and for c++
>>>> (all checked against gcc's behaviour).
>>> 
>>> Why do we need to change the behavior in C99 mode?  As far as I can
>>> tell, it's impossible to construct a strictly conforming program for
>>> which the C99 standard specifies behavior different from the gcc
>>> extension.
>> 
>> It's going to break a lot of code. clang defaults to C99 (as does Xcode
>> projects) and a lot of code rely on this behavior.
> 
> Actually, clang defaults to C99 with GNU extensions turned on (i.e.
> -std=gnu99), not to strict C99 mode.  So unless -std=c99 is explicity
> set, this will not affect existing code.
> 

My bad. I though clang was defaulting to __STRICT_ANSI__, so you're right, it should not affect existing code.

-- Jean-Daniel