dba_open

mail {at} thorben [dash]heins(dot) de (2011-11-21 01:39:56)

The other day i was struggeling with some strange behaviour, of two concurrently running scripts, that i want to share here. I am omitting most of the error handling for brevity.
script1:
$h = dba_open("file_a", 'r', 'gdbm');
while (true)
{
dba_fetch("somekey", $h);
sleep(1);
}
dba_close($h);
______________________
script2:
// while script1 is running until eternity...
$ok = rename("file_a", "file_b");
if (! $ok)
echo "Could not move\n";
else
echo "I like to move it, move it!\n";
// GOAL: Manipulate the file only if it is not opened for reading...
RESULT:
I like to move it, move it!
So you can very well rename a file, while it is opened. The lookup in script1 is still working. Manipulating the file in script2 will fail, as there is still an gdbm lock on the file from script1.

dracoirs at gmail dot com (2010-05-03 15:10:43)

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.

cbemerine at gmail dot com (2009-09-06 04:59:33)

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.

mskala at ansuz dot sooke dot bc dot ca (2008-05-16 14:15:21)

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.

trohit at blue bottle dot com (2007-11-23 03:34:42)

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);
?>

doppelbauer at gmail dot com (2006-10-20 07:36:48)

Windows does not support locking the database. You may use $_ENV to determine the OS:
$locking = (stripos($_ENV['OS'],'windows') === false ? 'd' : 'l');

xy ?t affenkrieger.de (2006-03-30 06:13:26)

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.