Re: [Qemu-devel] [PATCH V2 2/6] libqblock type and structure defines

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

于 2012-9-12 6:52, Eric Blake 写道:
> On 09/11/2012 02:31 PM, Blue Swirl wrote:
>> On Mon, Sep 10, 2012 at 8:26 AM, Wenchao Xia <xiawenc@linux.vnet.ibm.com> wrote:
>>>    This patch contains type and defines used in APIs, one file for public usage
>>> by user, one for libqblock internal usage.
>>>
>
>>> +
>>> +#if __GNUC__ >= 4
>>
>> #if defined(__GNUC__) && __GNUC__ ...
>
> Seriously?  We require a C99-compliant compiler, which is required to
> treat the more compact version identically (all undefined names evaluate
> to 0 in the preprocessor), and HACKING doesn't mandate that we spell out
> a defined-ness check first.  Okay, so configure adds -Wundef to the set
> of CFLAGS, but I fail to see why we want -Wundef (that's an
> anachronistic warning, only there to help you if you are writing code
> portable to K&R).
>
   So if the preprocessor replaced __GNUC__ to 0, is there difference
between these two kinds of macoros?
#if __GNUC__ >= 4
#if defined(__GNUC__) && __GNUC__ >=4

   I guess they are the same.

-- 
Best Regards

Wenchao Xia