PHP Unconference Europe 2015

dba_open

(PHP 4, PHP 5)

dba_openOpen database

Descrizione

resource dba_open ( string $path , string $mode [, string $handler [, mixed $... ]] )

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

Elenco dei parametri

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.

Nota:

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 successfull.
  • 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.

Valori restituiti

Returns a positive handle on success o FALSE in caso di fallimento.

Log delle modifiche

Versione Descrizione
4.3.0 It's possible to open database files over network connection. However in cases a socket connection will be used (as with http or ftp) the connection will be locked instead of the resource itself. This is important to know since in such cases locking is simply ignored on the resource and other solutions have to be found.

Vedere anche:

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User Contributed Notes 6 notes

up
1
cbemerine at gmail dot com
5 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.
up
1
trohit at blue bottle dot com
6 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);
?>
up
1
doppelbauer at gmail dot com
8 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');
up
0
dracoirs at gmail dot com
4 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.
up
0
mskala at ansuz dot sooke dot bc dot ca
6 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.
up
-1
xy ät affenkrieger.de
8 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.
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