The LOCK command reserves and releases resource names, and provides a semaphore capability for GT.M processes. This capability can be used for interprocess synchronization and signaling.

Assigning a LOCK does not specify any explicit control over variables and does not directly effect either read or write access to global (or local) data. However, an application that adheres to clearly defined conventions of LOCKing before any access can indirectly achieve such an effect.

FIS recommends implementing database Consistency using transaction processing rather than LOCKs. If you wish to avoid GT.M's use of optimistic concurrency for TP, place the LOCK just before the original TSTART and release it after the final TCOMMIT.

The format of the LOCK command is:

L[OCK][:tvexpr] [[-|+]nref|(nref[,...])[:numexpr] [,...]]

GT.M records LOCK and ZALLOCATE information in the "lock database." GT.M distributes the lock database in space associated with the database identified by the current Global Directory. However, the lock database does not overlap or coincide with the body of the database files holding the global data. Only the LOCK, ZALLOCATE and ZDEALLOCATE commands, and the LKE utility program access the lock database.

GT.M maps reservations of names starting with ^ to the database file used to map global variables of the same name. If the Global Directory maps the name A to file A.DAT, GT.M maps all reservations on ^A to file space associated with A.DAT.

GT.M maps reservations on names not starting with ^ to the region of the database specified with the GDE command LOCK -REGION=. By default, when GDE creates a Global Directory any reservations of local names are mapped to the region DEFAULT.

These two factors effect the following result in the programming environment:

Since GT.M uses resource names as semaphores for signaling among multiple processes in a database environment, they interlock in a tree structured fashion. When LOCK or ZALLOCATE reserves a subscripted resource name such as ^D(1), other users of the database mapped by the LOCKing (or ZALLOCATEing) process cannot reserve ancestors of that name, such as ^D, or descendants, such as ^D(1,2), until LOCK or ZDEALLOCATE releases that name.

Execution of the LOCK command does not affect the value or the state of a variable. LOCK tests each argument to determine whether the process can claim the name space. If another GT.M process has a LOCK on that name space, GT.M suspends the current process until the other process releases the name space. To prevent the potential "indefinite" suspension of a routine execution, specify a timeout for the LOCK command.

LOCK with a leading plus (+) or minus (-) sign (incremental LOCKing) allows the acquisition and release of locks without releasing all currently held locks. This can lead to deadlocks. For example, a deadlock occurs if two users LOCK resources named A and B in the following sequence.

Deadlock Situation



L +A

L +B

L +B

L +A

To avoid deadlocks, use LOCK without a leading + or - sign on its arguments because such a command releases all previously LOCKed resources; uniformly implement well designed LOCK accumulation orders and/or use a timeout with the LOCK command.

If a LOCK command specifies a timeout, and GT.M acquires ownership of the named resource before the timeout elapses, LOCK sets $TEST to TRUE (1). If GT.M cannot acquire ownership of the named resource within the specified timeout, LOCK sets $TEST to FALSE (0). If a LOCK command does not specify a timeout, the execution of the command does not affect $TEST. If a LOCK with an argument having a leading minus sign (-) specifies a timeout, the command always sets $TEST to TRUE (1).

If a process issues a LOCK command for a named resource already ZALLOCATEd by that process, the resource is both ZALLOCATEd and LOCKed. LOCK does not release ZALLOCATEd resources. To release such a named resource, the process must both ZDEALLOCATE and unLOCK the resource. For more information, refer to “ZAllocate”.

For more information on troubleshooting locks with the GT.M Lock Utility (LKE), refer to the chapter on that utility in the GT.M Administration and Operations Guide.