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

[Pierre Riteau]

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).

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