Buggy SELEC ... FROM UPDATE ?

Hello, (bis)

I try to use SELECT FOR UPDATE, but it seems to be buggy, hum ? Perhaps it's
my algorythm...

My syslock table:
---------------
id   integer
used boolean
---------------

Here is the algo:
---------------
boolean Got = false
while Got == false
    Start transaction
    SELECT * FROM syslock WHERE id=%d AND used='f' FOR UPDATE
    if SELECT returns a row
    then
        UPDATE syslock SET used='t' WHERE id=%d
        Commit transaction
        Got = true
    else
        Rollback transaction
        continue // yes, active wait ... I'm testing !
    end if
end while
Trace "I get the syslock number %d !!"
Do something with the syslock %d which is now reserved
UPDATE syslock SET used='f' WHERE id=%d
---------------

When a lot of threads (well, I tried 20, 5 and 2) used this algo, more than
one thread get the syslock %d at the same time.

What is the problem ?

Thanks
Patrice Espié
--
Le Monde, c'est comme un éléphant
qui va à la pêche