[Webdev] revisiting flock() for an example

[Lee Hosty]

At 01:05 01/05/01 +0100, Gabe Petrie wrote:

>[4] situations when using flock() is not necessary ?


 From c.l.p.m.

FWIW, here is an excerpt from open(2) on my RedHat 7.0 system:

        O_APPEND
               The  file  is  opened  in  append mode. Before each
               write, the file pointer is positioned at the end of
               the  file,  as if with lseek.  O_APPEND may lead to
               corrupted files on NFS file systems  if  more  than
               one  process  appends data to a file at once.  This
               is because NFS does  not  support  appending  to  a
               file,  so  the  client  kernel  has to simulate it,
               which can't be done without a race condition.

The key point is that the append mode isn't just "open the file and
start writing at the end".  Instead it is, "open the file and start
*every* write at the end".  This difference is what makes it safe to
append from multiple processes at the same time.

In addition to the NFS problem mentioned in the man page, there are at
least two additional problems:

1. If the write is larger than the buffer size, then it will possibly
be broken into multiple writes that could be interleaved with other
writes.

Now, this isn't a problem if you're writing single lines to a log
file, for example.  You can feel confident that each such write will
occur atomically and no corruption will occur.

2. It is OS-specific.  It may be the case that some OSes do not
provide this semantic for O_APPEND.  I'm honestly not sure how
prolific this functionality is.

Of course, flock isn't portable either, so using flock because you
can't trust the semantics of append mode is pretty silly.


If you want to use flock, use flock.  But don't think it is bringing
much additional robustness to the situation.

-- 
Ren Maddox
ren at tivoli.com


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ie. for appending (open FILEHANDLE, ">>$filename") small buffers on most 
OSes it doesnt cause corruption and therefore flock() is not necessary ...


Lee Hosty -x- hostyle at oscail.com -x- +353 (0)86 8768780
-- 
welcome to the Geophysical Inversion Facility ...