LOCK.tag(figuring out granularity of lock)

following is taken from postgresql-7.3.2/src/backend/storage/lmgr/readme:

"If we are setting a table level lock
both the blockId and tupleId (in an item pointer this is called
the position) are set to invalid, if it is a page level lock the
blockId is valid, while the tupleId is still invalid. Finally if
this is a tuple level lock (we currently never do this) then both
the blockId and tupleId are set to valid specifications. "

how do we check whether blockId and tupleId of LOCK.tag are valid or
invalid?
is blockId same as tag.objId.blkno? what field in LOCK.tag is tupleId?
thanks
jenny

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