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From "Brian Havard" <bri...@kheldar.apana.org.au>
Subject Re: [PATCH] Win32: Why explicitly futz with the file pointer?
Date Sun, 20 Oct 2002 22:57:33 GMT
On 20 Oct 2002 11:30:35 -0400, Jeff Trawick wrote:

>"William A. Rowe, Jr." <wrowe@apache.org> writes:
>
>> At 07:08 AM 10/20/2002, Bill Stoddard wrote:
>> >Why do we need to call SetFilePointer to each call of apr_file_write()? In
>> >the common case where only threads in a single process write to a file,
>> >calling SetFilePointer is a waste of cycles. If threads from multiple
>> >processes are writing to a file, then we are broken unless the application
>> >explicitly serializes access to apr_file_write()
>> 
>> I'll take a wild guess...
>> 
>> Unix append mode -always- writes to the end of the file, even when it is
>> in use by multiple processes.  Win32 has no absolute, atomic append
>> mode for it's file open.
>> 
>> But you are correct, there is a race here that even the 'emulation' falls
>> down.
>
>Maybe APR_APPEND needs to be cheap/simple append a la stdio append: we
>seek to the end of the file at open time and forget about it after
>that.
>
>Then we need new APR_WRITE_AT_END or something better named which is
>the expensive atomic append.  For Unix, this enables O_APPEND on the
>file and the kernel handles the details.  For Win32, this enables
>acquire-global-mutex + setfileptr + release-global-mutex prior to
>every write.  But then that has issues with non-related processes
>sharing the mutex.

Or you can write-lock the file around the seek/write pair. That's what I
ended up doing for OS/2 which has the same behaviour.

The lock & seek could be avoided if the file was opened in "deny write"
sharing mode but apr_file_open() doesn't have any way to specify that at
the moment & it wouldn't help with logs shared between processes.

-- 
 ______________________________________________________________________________
 |  Brian Havard                 |  "He is not the messiah!                   |
 |  brianh@kheldar.apana.org.au  |  He's a very naughty boy!" - Life of Brian |
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