Record Locking Help ....

[Ben Duncan]

I thought about semaphores, but the risk of a deadly embrace seems
to increase with them (just research on their usage seems to indicate that).

I am thinking, at least with the mmap and LOCKBYTE, I have accomplished the same
thing.  The LOCKS will be atomic, any READ with a lock will be done by a this process:

A: Position pointer in file
B: fcntl lock  the LOCKBYTE
    B1: Failed, return failed status
C: READ record
    C1: test LOCKBYTE
        TRUE: return <record is lock> status
        FALSE:
           Set UID, PID and LOCKBYTE
           Force WRITE immediate of header
           Release fcntl Lock
D: return DATA

This way I have non-blocking I/O with self advisory locks.


fcntl is already in tdb, so I thought I would just continue using the same logic.

Roger Oberholtzer wrote:
<SNIP>

> As long as your locks are atomic. This will eliminate race conditions.
> As was mentioned by Kurt, semaphores are good for this. If you use
> semaphores on a record, you should not also need the fcntl locking as
> well. Start with 'man semctl', 'man semopt' and 'man semget'.
> 
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-- 
Ben Duncan   - VersAccounting Software LLC 336 Elton Road  Jackson MS, 39212
"Never attribute to malice, that which can be adequately explained by stupidity"
        - Hanlon's Razor