iX V6.5 (30391-90011)

Chapter 4 119

Using the Database

Using the Locking Facility

Figure 4-3. Lock Descriptor List

Conditional and Unconditional Locking

You can request conditional or unconditional locking. If you request unconditional locking,

TurboIMAGE/XL returns control to your calling program only after the speciﬁed entity has

been locked. If you request conditional locking, TurboIMAGE/XL returns immediately. In

this case, the condition code must be examined to determine whether or not the requested

locks have been applied. If multiple lock descriptors are speciﬁed, the status area indicates

the numbers that have been applied. The calling program should call DBUNLOCK if only a

subset of the requested locks succeeded.

Access Modes and Locking

It is anticipated that access mode 1 will typically be used by applications implementing a

locking scheme. In this mode, TurboIMAGE/XL enforces the following rules:

• To modify (DBPUT, DBDELETE, or DBUPDATE) a data entry, you must ﬁrst issue a

successful lock covering the affected data entry. It can be a data entry, data set, or

database lock.

• To add to or delete from (DBPUT or DBDELETE) a manual master data set, you must ﬁrst

successfully lock the data set or database. To update (DBUPDATE) a master data set,

data entry level locks are sufﬁcient.

If your application opens the database in access mode 2, it is recommended that you use

locking to coordinate updates with other users.

TurboIMAGE/XL does not prevent any process from reading data even though another

process holds a lock on it. If you want to ensure that no modiﬁcations are in progress while

you are reading from the database, you should place an appropriate lock on the data before

starting. Therefore, you may want to use locking in access modes 2, 4, 5, and 6 to

coordinate the reading and modifying sequences and ensure that they do not occur

concurrently.

Because access mode 3 and 7 users have exclusive control of the database and access mode

8 users allow concurrent reading only, locking need not be used in these modes.