concurrency Access on a table

Hi.

I have to write a SQL-Script under PL/SQL 2.3.2.3.0

In this script I have to get a Primary Key (PK) for a table A from a  table ID_MANAGEMENT (IDM).

That means, every row in A has a PK which is taken out  from IDM.next_id. IDM.next_id will be updated with next_id+1.

Since several skripts may run, which want to write something in A, it could be that two Skripts read same IDM.next_id, so that they both will write something in A with same PK  (Producer-Consumer-Problem).
Is there something like a semaphore in Oracle, so that always after reading IDM.next_id it will be increased before some other instance of a script could read the same value in IDM.next_id?

Thank you very much,

bye Frank.

-- 

Frank Braun              fbraun_at_rz.uni-sb.de