Re: Does mandatory locking need to be set when the file is locked?

[Trond Myklebust <trond.myklebust@fys.uio.no>]

>>>>> " " == Joseph D Wagner <theman@josephdwagner.info> writes:

     > My program DEPENDS upon mandatory locking, so I'd like a little
     > more info before I abandon that route entirely.

Then don't be surprised if it cannot work on networked filesystems.

NFS is, for instance, incapable of supporting mandatory locks in v2
and v3 (v4 will support it for ordinary files).

What's more, mmap() is forbidden on on any file with mandatory
locks (this is true for all filesystems).

Finally, the Linux mandatory locking implementation does not eliminate
all possible races: currently there is no attempt made to exclude one
thread from taking a lock while another is still in the process of
writing to the file, or if it already has a section of that file
mmapped. In that case, the lock call may well succeed, but the
other thread that is inside read/write/mmap will not see it.

So if you want to DEPEND on mandatory locking to work, then you
probably have some kernel debugging to do first.

Cheers,
  Trond