fs exclusive mode

[Mark Volkmann]

The docs say ...

" 'ax' - Like 'a' but fails if path exists."

and a bit later ...

"The exclusive flag 'x' ensures that path is newly created."

What is the point is using the 'ax' mode if it can't append to an existing file?

It seems that the 'wx' mode would do the same thing.

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R. Mark Volkmann
Object Computing, Inc.

[Ben Noordhuis]

Mode 'ax' (which is a shorthand for `O_APPEND | O_CREAT | O_EXCL |
O_WRONLY`, see `man 2 open`) is useful when there will be multiple
concurrent writers, like, say, different processes that write to a
single log file. Without the append flag, the processes would
overwrite each other's output.

[Mark Volkmann]

Thanks for explaining that! Doesn't your explanation mean that the documentation is wrong? From what you said, "ax" does NOT fail if path exists and does NOT ensure that path is newly created.

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[Ben Noordhuis]

On Wed, Oct 22, 2014 at 5:22 PM, Mark Volkmann
<r.mark....@gmail.com> wrote:
> Thanks for explaining that! Doesn't your explanation mean that the
> documentation is wrong? From what you said, "ax" does NOT fail if path
> exists and does NOT ensure that path is newly created.

No, the documentation is correct. If what you said was true, then
'ax' would be a synonym for 'a'.

'ax' creates and opens the file in append-only mode unless the file
exists. You use 'ax' when you would use 'wx' but also want
append-only semantics.

A typical use case is where a master process opens the log file with
'ax' and, if successful, starts workers that open the file with 'a'.
The 'ax' open mode ensures that the master can't race with other
master processes.