Locking of a record in SQL Server SP

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Hi,

IF user1 is accessing a row ,this particular row should not be accessed by other users.Only when the lock is released (After user1 completes his processing with that row) that row should be accessible by others.The locking of a row has to be done at the selection of a particular row(Has to be done in select statement).

This is an SP level coding where commands between the begin trans and commit trans will only appear .The begin trans and commit trans functionalities are done by DLL's which cannot be changed.