Re: [Qemu-devel] [PATCH] Fixes a bug that tries to use the unimplemented function fdatasync on Mac OS X.

[Blue Swirl]

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.