dba_open

(PHP 4, PHP 5, PHP 7, PHP 8)

dba_open — Open database

说明

dba_open(
    string $path,
    string $mode,
    ?string $handler = null,
    int $permission = 0644,
    int $map_size = 0,
    ?int $flags = null
): resource|false

dba_open() establishes a database instance for path with mode using handler.

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

**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

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.

返回值

Returns a positive handle on success 或者在失败时返回 false.

错误／异常

false is returned and an E_WARNING level error is issued when handler is null, but there is no default handler.

更新日志

版本	说明
8.2.0	`flags` is added.
8.2.0	`handler` is now nullable.
7.3.14, 7.4.2	The `lmdb` driver now supports an additional `map_size` parameter.

参见

dba_popen() - Open database persistently
dba_close() - Close a DBA database

add a note

User Contributed Notes 6 notes

down

doppelbauer at gmail dot com ¶

18 years ago


Windows does not support locking the database. You may use $_ENV to determine the OS:

$locking = (stripos($_ENV['OS'],'windows') === false ? 'd' : 'l');

down

mskala at ansuz dot sooke dot bc dot ca ¶

16 years ago


As of GDBM version 1.8.3, GDBM's underlying open call uses non-blocking calls to flock() on systems that have flock().  As a result, calls with "rd" or "wd" locking modes will return error ("Can't be reader" or "Can't be writer") instead of waiting.  Use "rl" or "wl" instead, to make PHP do its own locking external to GDBM.

down

trohit at blue bottle dot com ¶

17 years ago


Here's a simple example to use the dba_open function

<?php

$id = dba_open("/tmp/test.db", "n", "gdbm");

if (!$id) {
        echo "dba_open failed\n";
        exit;
}

dba_replace("key", "This is an example!", $id);

if (dba_exists("key", $id)) {
        echo dba_fetch("key", $id);
        dba_delete("key", $id);
}

dba_close($id);
?>

down

xy ät affenkrieger.de ¶

18 years ago


If you get some strange errors like
dba_open(): myDbFilename.db : Permission denied
than you are propably using PHP on a Windoze machine. You have to make sure that the following conditions are met:

1) Use an absolute path to your db file. Relative paths will cause problems with locking
2) Specify a locking mode - that's the second character of the mode-argument, or else opening a dba-file will cause several notices/warnings etc.

And a final, general note:
3) Always use the english PHP doc on this site - the translations are often old as hell and miss important informations

HTH, Nils.

down

-1

dracoirs at gmail dot com ¶

14 years ago


Apache doesn't support Berkeley DB Btree, so you can't manipulate use db4 as the type of database if you want to do DBM authentication with Apache.

gdbm seemed to work fine though, even though it supposedly using Btree instead of hash.  It makes you wonder why Apache would use hash for one dbmtype versus btree for another.

So since Apache and PHP don't have options to choose the method for the Berkeley DBs, you are out of luck.

down

-2

cbemerine at gmail dot com ¶

15 years ago


Note the “c” create flag does not work if MySQL was built with the “cdb” DBA handler compile option which is common for many distros.  By definition the cdb DBA handler is optimized for reading/writing and “no updates are allowed.”  

<?php
$dbh = dba_open( "./data2/productz", "c", "cdb") or die( "Couldn't open Database" ); 
?>

instead use

<?php
$dbh = dba_open( "./data2/productz", "n", "cdb" ) or die( "Couldnt open Database" );
?>

generates this error message in the /var/log/apache2/error.log: 
[Sun Sep 06 04:18:15 2009] [error] [client 192.168.1.125] PHP Warning:  dba_open(./data2/productz,c) [<a href='function.dba-open'>function.dba-open</a>]: Driver initialization failed for handler: cdb: Update operations are not supported in /var/www/projects/testcdb-c.php on line 43

see user contributed comment under dba_handlers() to see which DBA handlers are supported by your build of MySQL and note about using “cdb” compiled DBA systems: 

also see user contributed comment under dba_replace() about incompatibilities with cdb DBA handler compiled MySQL systems.

add a note