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Re: [Qemu-devel] [PATCH] replace O_SYNC with O_FSYNC


From: G 3
Subject: Re: [Qemu-devel] [PATCH] replace O_SYNC with O_FSYNC
Date: Sat, 20 Jun 2009 20:46:54 -0400


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.




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