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From Jeff Trawick <traw...@attglobal.net>
Subject Re: [PATCH] Win32: Why explicitly futz with the file pointer?
Date Sun, 20 Oct 2002 15:30:35 GMT
"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.

-- 
Jeff Trawick | trawick@attglobal.net
Born in Roswell... married an alien...

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