Re: pidfile_lock(3)

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Sun, Mar 20, 2016 at 08:53:03PM +0000, Roy Marples wrote:
> On Sunday 20 March 2016 14:53:46 you wrote:
> > On Sat, 19 Mar 2016 23:29:37 +0000
> > 
> > Roy Marples <roy%marples.name@localhost> wrote:
> > > pidfile(3) is pretty crap - it just writes to the file without any
> > > locking.
> > 
> > I don't understand why you think any of that matters.  Locks only
> > advisory anyway.  Any noncooperating process can (with sufficient
> > privilege) overwrite the file.  What cooperating process do you imagine
> > would honor a lock on a pidfile?
> 
> So it honors it's own lock to avoid user stupidity of starting the same daemon 
> more than once. This is the primary goal of the function.

That's the wrong way to do it, because it does not work reliably on NFS.
You can take advantage of O_CREAT|O_EXCL or rename() semantics to do the
same thing but in a much more robust way.

Thor