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From: G 3 <programmingkidx@gmail.com>
Subject: Re: [Qemu-devel] [PATCH] replace O_SYNC with O_FSYNC
Date: Sat, 20 Jun 2009 20:46:54 -0400
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To: Jamie Lokier <jamie@shareable.org>
Cc: qemu-devel@nongnu.org


On Jun 20, 2009, at 7:30 PM, Jamie Lokier wrote:

> Christoph Hellwig wrote:
>> On Fri, Jun 19, 2009 at 10:22:07PM -0400, m a wrote:
>>> This patch replaces O_SYNC with O_FSYNC. These two flags do the same
>>> thing, but only O_FSYNC is available in Mac OS 10.3 and under. It 
>>> only
>>> replaces O_SYNC if it doesn't exist. This patch allows the file
>>> block-raw-posix.c to compile on Mac OS 10.3. This is my first time
>>> submitting a patch, so there might have been a few mistakes made.
>>
>> But O_SYNC is a standard posix flag, while O_FSYNC appears to be
>> a BSD extension.  Also the actual code uses O_DSYNC anyway, which
>> also is in Posix but not actually natively supported by some OSes,
>> e.g. Linux (but still provided in libc there).
>
> If O_FSYNC and O_SYNC do the same thing, and O_SYNC is used anywhere,
> there's no harm in this for portability:
>
>     #if !defined(O_SYNC) && defined(O_FSYNC)
>     #define O_SYNC O_FSYNC
>     #endif
>
> The patch assumes O_FSYNC is defined if O_SYNC isn't, which is wrong.
>
>>>  /* OS X does not have O_DSYNC */
>>>  #ifndef O_DSYNC
>>>  #define O_DSYNC O_SYNC
>>
>> So if the code here is correct and Darwin is the only supported OS 
>> where
>> O_DSYNC is missing we could just replace the O_SYNC in the last line
>> with O_FSYNC.
>
> I agree, though the comment might be misleading, if there's another
> supported OS without O_DSYNC.
>
> -- Jamie
>
>

Your logic does make sense. I guess I should have done this instead:

#ifndef O_SYNC
	#ifdef O_FSYNC
		#define O_SYNC 	O_FSYNC
	
	#else
		#define O_SYNC 	0
	
	#endif
#endif

Defining O_SYNC as zero is ok because this flag is 'ORed' with other 
flags.