> I have a need to lock a file and/or a whole directory on a shared
> NFS file system from a perl script. Can anyone refer me to a good,
> generic method that is reliable enough ? Speed is not of any real
> need, but consistency is. My guess is that exim-folks are likely
> to have "been there, done that" already - so I would appreciate
> pointers.
Quoting from open(2) on Linux:
O_EXCL When used with O_CREAT, if the file already exists
it is an error and the open will fail. O_EXCL is
broken on NFS file systems, programs which rely on
it for performing locking tasks will contain a
race condition. The solution for performing
atomic file locking using a lockfile is to create
a unique file on the same fs (e.g., incorporating
hostname and pid), use link(2) to make a link to
the lockfile and use stat(2) on the unique file to
check if its link count has increased to 2. Do
not use the return value of the link() call.
This algorithm is used e.g. by procmail.
Michael
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