Jay Sprenkle uttered:
On 1/21/07, Andrew Teirney <[EMAIL PROTECTED]> wrote:
>
> > What is a 'shared' lock? You can't share a lock, only one process
> > can own it at a time.
> >
>
> As per this document
>
> http://www.sqlite.org/lockingv3.html
>
> SHARED The database may be read but not written. Any number of
> processes can hold SHARED locks at the same time, hence there can be
> many simultaneous readers. But no other thread or process is allowed
> to write to the database file while one or more SHARED locks are
> active.
>
A file lock is used to serialize write access. When reading the file
is not locked. This makes little sense. There are no 'shared' locks.
Sorry, Accidently hit send before I was finished. This documentation
needs to be updated so it makes sense. Making up this conceptual 'shared
lock' thing instead of explaining how the locking mechanism really works
is not helpful. A 'shared lock' in reality is just opening the disk file
that makes up the database without locking. Any number of other
processes can read the file. You can't acquire a shared lock, you just
don't acquire a write lock.
No, you do acquire a lock. You must lock out any writer, but you do not
require locking out other readers, hence you can share the lock with other
readers, but it is a lock. It is a conceptual lock, and an OS level lock.
Without the shared lock, the writer would have no way of knowing whether
any other process is reading the file, thus making it unsafe to change the
file.
Christian
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