dba_open
(PHP 4, PHP 5)
dba_open — Open database
Описание
dba_open() establishes a database instance for path with mode using handler .
Список параметров
- 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.
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.
Замечание: 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.
Возвращаемые значения
Returns a positive handle on success, or FALSE on failure.
Список изменений
| Версия | Описание |
|---|---|
| 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. |
| 4.3.0 | Locking and the mode modifiers "l", "d", "-" and "t" were added. In previous PHP versions, you must use semaphores to guard against simultaneous database access for any database handler with the exception of GDBM. See System V semaphore support. |
| before 4.3.5 | open mode 'c' is broken for several internal handlers and truncates the database instead of appending data to an existent database. Also dbm and ndbm fail on mode 'c' in typical configurations (this cannot be fixed). |
Коментарии
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.
Windows does not support locking the database. You may use $_ENV to determine the OS:
$locking = (stripos($_ENV['OS'],'windows') === false ? 'd' : 'l');
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);
?>
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.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.
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.