On Tue, 20 Mar 2001, Perrin Harkins wrote:
> On Wed, 21 Mar 2001, Stas Bekman wrote:
> > > You mean with DB_File? There's a big warning in the current version
> > > saying not to do that, because there is some initial buffering that
> > > happens when opening a database.
> >
> > The warning says not to lock on dbm fd but an external file!
>
> I think you'll still have problems with this technique, unless you
> tie/untie every time. I'm looking at the perldoc for DB_File version
> 1.76, at the section titled "Locking: the trouble with fd". At the very
> least, you'd have to call sync after acquiring a write lock but before
> writing anything.
Of course, you always call sync. The sync was always there, even with the
flawed locking scheme. The DB_File doc was updated as a follow up of the
research done by David Harris. This should work:
flock SH
tie()
read...
flock EX <===== start critical section
write...
sync()
flock SH <===== end critical section
read...
untie()
flock UN
notice that the locking is done on the *external* file (or fd).
The only problem in this approach is a possible writing starvation as
explained:
http://perl.apache.org/guide/dbm.html#Locking_dbm_Handlers_and_Write_L
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