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177 lines
6.2 KiB
Plaintext
177 lines
6.2 KiB
Plaintext
Locking schemes used with DBF files.
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Przemyslaw Czerpak (druzus/at/poczta.onet.pl)
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Exclusive/shared modes (DENY READ, DENY WRITE) in POSIX systems.
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================================================================
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Exclusive modes (DENY READ, DENY WRITE) do not exist in POSIX systems.
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They can be emulated by fcntl() or flock() locks. Harbour uses flock()
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in systems where it does not create conflicts with fcntl() locks used
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to synchronize file access by different processes in shared mode.
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Currently it's enabled by default only in Linux (tests with *BSD systems
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and MacOSX showed that it interacts with fcntl locks causing deadlocks).
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On other POSIX systems Harbour uses fcntl() read and write locks at
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address 0x7fffffff (size 1) to simulate shared and exclusive mode.
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CLIP and FlagShip use fcntl() locks to simulate shared/exclusive modes.
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CLIP shared/exclusive flag: 0x7fffffff : 1 (the same as Harbour)
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FlagShip shared/exclusive flag: 0x7ffffbfc : 1
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File access synchronization (record and file locking) in shared mode.
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=====================================================================
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FlagShip uses different record and index locking schemes
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(i.e. for RLOCK it locks record area) and it is not described
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here.
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In documentation CLIP can use Clipper like or FoxPro like locking
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schemes and it can be controlled by SET LOCKSTYLE [TO] ... command
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or Set( _SET_LOCKSTYLE, 0 | 1 ) function.
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This switch only changes the RLOCK/FLOCK offset.
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For:
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SET LOCKSTYLE [TO] CLIPPER | CLIPPER50 | CLIPPER53
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or:
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Set( _SET_LOCKSTYLE, 0 )
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this offset is set to 1'000'000'000. This is header lock address
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and records are locked at above address + record number.
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This is compatible with standard Clipper locking.
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Commands:
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SET LOCKSTYLE [TO] CLIPPER52 | FOXPRO | SIX
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or:
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Set( _SET_LOCKSTYLE, 1 )
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set this offset to 0x10000000 keeping the same algorithm
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for record locking what is incompatible with any other locking
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schemes known by me. For sure it's not compatible with [V]FP
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locking (see details about this locking scheme below).
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For more information look at CLIP source code.
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(*) In Harbour index read locks are shared on platforms which support
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shared (read) locks.
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Clipper DBFNTX locking:
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=======================
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all locks are exclusive (*)
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DBF HEADER LOCK: @1000000000 : 1 (exclusive)
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DBF RECORD LOCK: @1000000000 + recNO : 1 (exclusive)
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DBF FLOCK SIZE: 1000000000
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=> maximum records: 3'294'967'295
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maximum file size in non POSIX systems: 1'000'000'000
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NTX READ LOCK: @1000000000 : 1 (exclusive)
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NTX WRITE LOCK: @1000000000 : 1 (exclusive)
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In Harbour it's DB_DBFLOCK_CLIPPER.
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Clipper new DBFNTX locking (ntxlock2.obj):
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==========================================
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all locks are exclusive (*)
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DBF HEADER LOCK: @4000000000 : 1 (exclusive)
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DBF RECORD LOCK: @4000000000 + recNO : 1 (exclusive)
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DBF FLOCK SIZE: 294967295
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=> maximum records: 294'967'295
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maximum file size in non POSIX systems: 4'000'000'000
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NTX READ LOCK: @1000000000 : 1 (exclusive)
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NTX WRITE LOCK: @1000000000 : 1 (exclusive)
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In Harbour it's DB_DBFLOCK_CLIPPER2.
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CL52 DBFCDX, SIX3 SIXCDX, SIX3 SIXNSX, [V]FP CDX
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================================================
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all locks are exclusive (*)
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when there is no structural index:
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DBF HEADER LOCK: @0x40000000 : 1
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DBF RECORD LOCK: @0x40000000 + record offset : 1 (some implementations
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locks whole record)
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DBF FLOCK SIZE: 0x3ffffffd
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=> maximum file size: 1'073'741'823
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when structural index is open:
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DBF HEADER LOCK: @0x7ffffffe : 1
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DBF RECORD LOCK: @0x7ffffffe - recNO : 1
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DBF FLOCK SIZE: 0x07ffffff
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=> maximum records: 134'217'727
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maximum file size in non POSIX systems: 2'013'265'919
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index file locking is the same for production and normal indexes:
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CDX READ LOCK: @0x7ffffffe : 1
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CDX WRITE LOCK: @0x7ffffffe : 1
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In Harbour it's DB_DBFLOCK_VFP.
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CL53 DBFCDX, COMIX (hyper locking)
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==================================
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all locks are exclusive (*), in index shared locks are emulated
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DBF HEADER LOCK: @1000000000 : 1
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DBF RECORD LOCK: @1000000000 + recNO : 1
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DBF FLOCK SIZE: 1000000000
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=> maximum records: 1'000'000'000
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maximum file size in non POSIX systems: 1'000'000'000
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CDX READ LOCK: @0xffff0000 + random value from 0x0 to 0xffff : 1
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CDX WRITE LOCK: @0xfffeffff : 0x10001
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if fail then @0xfffeffff : 1
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prepare the index modification in memory and before writing to
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index file lock @0xffff0000 : 0x10000
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to eliminate starvation effect caused by many readers
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on each 16-th read lock reading process tries to lock write
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area @0xfffeffff : 1 instead and then sets normal read lock
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In Harbour it's DB_DBFLOCK_COMIX.
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HARBOUR 32-bit locking
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======================
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all locks are exclusive (*), in index shared locks are emulated
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DBF HEADER LOCK: @4000000000 : 1 (exclusive)
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DBF RECORD LOCK: @4000000000 + recNO : 1 (exclusive)
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DBF FLOCK SIZE: 294967295
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=> maximum records: 294'967'295
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maximum file size in non POSIX systems: 4'000'000'000
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CDX READ LOCK: @0xffff0000 + random value from 0x0 to 0xffff : 1
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CDX WRITE LOCK: @0xfffeffff : 0x10001
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if fail then @0xfffeffff : 1
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prepare the index modification in memory and before writing to
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index file lock @0xffff0000 : 0x10000
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to eliminate starvation effect caused by many readers
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on each 16-th read lock reading process tries to lock write
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area @0xfffeffff : 1 instead and then sets normal read lock
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In Harbour it's DB_DBFLOCK_HB32.
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HARBOUR 64-bit locking
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======================
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all locks are exclusive (*), in index shared locks are emulated
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DBF HEADER LOCK: @0x7fffffff00000001 : 1
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DBF RECORD LOCK: @0x7fffffff00000001 + recNO : 1
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DBF FLOCK SIZE: 0xfffffffeUL
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=> maximum records: 4'294'967'294
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maximum file size: no limits created by locks
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CDX READ LOCK: @0x7fffffff00000000 + random value from 0x1 to 0xffff : 1
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CDX WRITE LOCK: @0x7fffffff00000000 : 0x10001
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if fail then @0x7fffffff00000000 : 1
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prepare the index modification in memory and before writing to
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index file lock @0x7fffffff00000001 : 0x10000
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to eliminate starvation effect caused by many readers
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on each 16-th read lock reading process tries to lock write
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area @0x7fffffff00000000 : 1 instead and then sets normal read lock
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In Harbour it's DB_DBFLOCK_HB64.
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