dba_open

参数

path

Commonly a regular path in your filesystem.

mode

It is r for read access, w for read/write access to an already existing database, c for read/write access and database creation if it doesn't currently exist, and n for create, truncate and read/write access. The database is created in BTree mode, other modes (like Hash or Queue) are not supported.

Additionally you can set the database lock method with the next char. Use l to lock the database with a .lck file or d to lock the databasefile itself. It is important that all of your applications do this consistently.

If you want to test the access and do not want to wait for the lock you can add t as third character. When you are absolutely sure that you do not require database locking you can do so by using - instead of l or d. When none of d, l or - is used, dba will lock on the database file as it would with d.

注意:

There can only be one writer for one database file. When you use dba on a web server and more than one request requires write operations they can only be done one after another. Also read during write is not allowed. The dba extension uses locks to prevent this. See the following table:

DBA locking

already open	mode = "rl"	mode = "rlt"	mode = "wl"	mode = "wlt"	mode = "rd"	mode = "rdt"	mode = "wd"	mode = "wdt"
not open	ok	ok	ok	ok	ok	ok	ok	ok
mode = "rl"	ok	ok	wait	false	illegal	illegal	illegal	illegal
mode = "wl"	wait	false	wait	false	illegal	illegal	illegal	illegal
mode = "rd"	illegal	illegal	illegal	illegal	ok	ok	wait	false
mode = "wd"	illegal	illegal	illegal	illegal	wait	false	wait	false

• ok: the second call will be successful.
• wait: the second call waits until dba_close() is called for the first.
• false: the second call returns false.
• illegal: you must not mix "l" and "d" modifiers for mode parameter.

handler

The name of the handler which shall be used for accessing path. It is passed all optional parameters given to dba_open() and can act on behalf of them. If handler is null, then the default handler is invoked.

permission

Optional int parameter which is passed to the driver. It has the same meaning as the permissions parameter of chmod(), and defaults to 0644.

The db1, db2, db3, db4, dbm, gdbm, ndbm, and lmdb drivers support the permission parameter.

map_size

Optional int parameter which is passed to the driver. Its value should be a multiple of the page size of the OS, or zero, to use the default map size.

Only the lmdb driver accepts the map_size parameter.

flags

Flags to pass to the database drivers. If null the default flags will be provided. Currently, only the LMDB driver supports the following flags DBA_LMDB_USE_SUB_DIR and DBA_LMDB_NO_SUB_DIR.