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

[Eli Friedman]

On Mon, Nov 5, 2012 at 3:07 AM, Andy Gibbs <andyg1001 at hotmail.co.uk> wrote:
> 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.
>
> Of course, code broken by this change would also be broken by compiling with
> gcc instead...
>
> Here is the line from the gcc help: "... In this case the C99 standard is
> clear that the comma must remain, however the existing GCC extension used to
> swallow the comma. So CPP retains the comma when conforming to a specific C
> standard, and drops it otherwise."

Ah, yes... but that text only applies in the case where the macro has
zero named arguments.

-Eli