JetGetLock Function

Article
08/26/2021

Applies to: Windows | Windows Server

JetGetLock Function

The JetGetLock function provides a means to explicitly reserve the ability to update a row, write lock, or to explicitly prevent a row from being updated by any other session, read lock. Normally, row write locks are acquired implicitly as a result of updating rows. Read locks are usually not required because of record versioning. However, in some cases a transaction may desire to explicitly lock a row to enforce serialization, or to ensure that a subsequent operation will succeed by virtue that required locks have already been taken.

JET_ERR JET_API JetGetLock(
  __in          JET_SESID sesid,
  __in          JET_TABLEID tableid,
  __in          JET_GRBIT grbit
);

Parameters

sesid

The session that will be used for this call.

tableid

The cursor that will be used for this call.

grbit

A group of bits that contain the options to be used for this call, which include zero or more of the following:

Value	Meaning
JET_bitReadLock	This flag causes a read lock to be acquired on the current record. Read locks are incompatible with write locks already held by other sessions but are compatible with read locks held by other sessions.
JET_bitWriteLock	This flag causes a write lock to be acquired on the current record. Write locks are not compatible with write or read locks held by other sessions but are compatible with read locks held by the same session.

Return Value

This function returns the JET_ERR datatype with one of the following return codes. For more information about the possible ESE errors, see Extensible Storage Engine Errors and Error Handling Parameters.

Return code	Description
JET_errSuccess	The operation completed successfully.
JET_errClientRequestToStopJetService	It is not possible to complete the operation because all activity on the instance associated with the session has ceased as a result of a call to JetStopService.
JET_errInstanceUnavailable	It is not possible to complete the operation because the instance associated with the session has encountered a fatal error that requires that access to all data be revoked to protect the integrity of that data. This error will only be returned by Windows XP and later releases.
JET_errInvalidgrbit	The given grbit is neither JET_bitReadLock or JET_bitWriteLock. It must be one of these two flags.
JET_errNoCurrentRecord	Cursor must be on a record in order to acquire a lock. Locks are always on records.
JET_errNotInitialized	It is not possible to complete the operation because the instance associated with the session has not been initialized yet.
JET_errNotInTransaction	Locks can only be acquired by sessions in a transaction.
JET_errPermissionDenied	Cursor cannot be read only and acquire a write lock.
JET_errRestoreInProgress	It is not possible to complete the operation because a restore operation is in progress on the instance associated with the session.
JET_errSessionSharingViolation	The same session cannot be used for more than one thread at the same time. This error will only be returned by Windows XP and later releases.
JET_errTermInProgress	It is not possible to complete the operation because the instance associated with the session is being shut down.
JET_errTransReadOnly	Session must have write permissions to acquire write lock.
JET_errWriteConflict	The error returned when a conflicting lock is requested.

On success, session has acquired requested lock.

On failure, session has not acquired requested lock.

Remarks

Write locks cannot be acquired with sessions or cursors that have read-only permissions, even if the session and cursor ultimately do not perform an update operation. Both the session and the cursor must have write privileges in order to acquire a write lock.

Read and write locks are a means of pessimistic locking. Pessimistic locking expects multiple concurrent sessions to conflict and acquire locks in advance to ensure that their operations succeed.

Most operations will be serializable by virtue of locks implicitly taken. However, some operations will not. To illustrate this, consider the two transactions,

T1 : R(A), U(B)

T2 : R(B), U(A)

Record level versioning will ensure that each transaction when executed concurrently will see the original values for A and B. There is no serial order of execution that could produce the same results for A and B in the case that the results are dependent upon the data read. In order for the application to make this transaction serializable, it should acquire an explicit read lock on A and B in each transaction when the value is read.

Requirements

Requirement	Value
Client	Requires Windows Vista, Windows XP, or Windows 2000 Professional.
Server	Requires Windows Server 2008, Windows Server 2003, or Windows 2000 Server.
Header	Declared in Esent.h.
Library	Use ESENT.lib.
DLL	Requires ESENT.dll.

JetGetLock Function