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Re: [Qemu-devel] [PATCH] Fixes a bug that tries to use the unimplemented


From: Pierre Riteau
Subject: Re: [Qemu-devel] [PATCH] Fixes a bug that tries to use the unimplemented function fdatasync on Mac OS X.
Date: Thu, 10 Dec 2009 17:49:41 +0100

On 9 déc. 2009, at 00:01, Alexander Graf wrote:

> 
> Am 08.12.2009 um 19:35 schrieb Blue Swirl <address@hidden>:
> 
>> On Tue, Dec 8, 2009 at 4:35 PM, Pierre Riteau <address@hidden> wrote:
>>> On 1 oct. 2009, at 18:13, Blue Swirl wrote:
>>> 
>>>> On Thu, Oct 1, 2009 at 6:30 PM, G 3 <address@hidden> wrote:
>>>>> On Sep 30, 2009, at 3:16 PM, Anthony Liguori wrote:
>>>>> 
>>>>> G 3 wrote:
>>>>> 
>>>>> This patch fixes a problem in the file cutils.c that prevents qemu from
>>>>> being built on Mac OS X. This patch makes sure fsync is used instead.
>>>>> 
>>>>> What is this against?  It doesn't apply to git.
>>>>> 
>>>>> This is against the savannah
>>>>> repository: git://git.savannah.nongnu.org/qemu.git. Is this the wrong
>>>>> repository?
>>>> 
>>>> The patch should not be needed since
>>>> 5f6b9e8fd5b9516170e582d9b6c27c98519a8031. Do you still have a problem?
>>> 
>>> Sorry to dig up this old thread, but I just tried compiling qemu on my Mac 
>>> and I see a warning when it compiles:
>>> cutils.c: In function ‘qemu_fdatasync’:
>>> cutils.c:128: warning: implicit declaration of function ‘fdatasync’
>>> 
>>> I'm running OS X 10.6.2 with Xcode 3.2.1.
>>> 
>>> Why this behavior? Because the configure check introduced in 5f6b9e finds 
>>> that fdatasync is available.
>>> Although the fdatasync function is not referenced in the standard includes 
>>> (hence the warning), it is present as a syscall:
>>> /usr/include/sys/syscall.h:#define      SYS_fdatasync      187
>>> Moreover, a fdatasync symbol is available through libSystem (which is why 
>>> the linking done by the configure check works).
>>> 
>>> It's not clear what is the best solution here. Googling the issue, some 
>>> projects add the fdatasync prototype to their headers to avoid warnings.
>>> Other assume that fdatasync could be a noop and fall back to fsync.
>>> 
>>> I would go with the second second: the function is not defined, there is no 
>>> man page, let's assume it doesn't work.
>>> To fix the configure check we can simply add -Werror to the compile_prog 
>>> cflags (I will send a patch if you agree with this).
>> 
>> I'd rather add something like
>> #ifdef __APPLE__
>> exit(1);
>> #endif
>> (or the shell equivalent for configure) and a comment with your explanation.
>> 
>> 
> 
> 
> I guess that's wrong. With --disable-werror it compiles just fine. So I think 
> we're rather missing a header include.
> 
> Alex


Compiling just fine doesn't necessarily mean thatt qemu will run fine and do 
what we want it to do.
But in this case it may: i took a look at the XNU sources (I didn't think about 
this solution first, I forgot it was open source...) and fdatasync appears to 
be implemented.
So I guess that we can probably add the missing prototype in one of our header?

-- 
Pierre Riteau -- http://perso.univ-rennes1.fr/pierre.riteau/





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