m4_comment([$Id: stdmode.so,v 10.26 2001/05/22 19:39:32 bostic Exp $])

m4_ref_title(Locking Subsystem,
    Standard lock modes, standard @lock modes, lock/max, lock/dead)

m4_p([dnl
The m4_db locking protocol is described by a conflict matrix.  A
conflict matrix is an NxN array in which N is the number of different
lock modes supported, and the (i, j)th entry of the array indicates
whether a lock of mode i conflicts with a lock of mode j.  In addition,
m4_db defines the type m4_arg(db_lockmode_t), which is the type of a
lock mode within a conflict matrix.])

m4_p([dnl
The following is an example of a conflict matrix.  The actual conflict
matrix used by m4_db to support the underlying access methods is more
complicated, but this matrix shows the lock mode relationships available
to applications using the m4_db Locking subsystem interfaces directly.])

m4_tagbegin
m4_tag(DB_LOCK_NG, [not granted (always 0)])
m4_tagns(DB_LOCK_READ, [read (shared)])
m4_tagns(DB_LOCK_WRITE, [write (exclusive)])
m4_tagns(DB_LOCK_IWRITE, [intention to write (shared)])
m4_tagns(DB_LOCK_IREAD, [intention to read (shared)])
m4_tagns(DB_LOCK_IWR, [intention to read and write (shared)])
m4_tagend

m4_p([dnl
In a conflict matrix, the rows indicate the lock that is held, and the
columns indicate the lock that is requested.  A 1 represents a conflict
(that is, do not grant the lock if the indicated lock is held), and a
0 indicates that it is OK to grant the lock.])

m4_indent([dnl
		Notheld	Read    Write	IWrite	IRead	IRW
Notheld		0	0	0	0	0	0
Read*		0	0	1	1	0	1
Write**		0	1	1	1	1	1
Intent Write	0	1	1	0	0	0
Intent Read	0	0	1	0	0	0
Intent RW	0	1	1	0	0	0])

m4_tagbegin
m4_tag(*, [dnl
In this case, suppose that there is a read lock held on an object.  A new
request for a read lock would be granted, but a request for a write lock
would not.])

m4_tag(**, [dnl
In this case, suppose that there is a write lock held on an object.  A
new request for either a read or write lock would be denied.])
m4_tagend

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