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14.1. Making and Using a DBM File
14.1.1. Problem
You want to create, populate, inspect, or
delete values in a DBM database.
14.1.2. Solution
Use
tie to open the database and make it accessible
through a hash. Then use the hash as you normally would. When you're
done, call untie:use DB_File; # load database module
tie %HASH, "DB_File", $FILENAME # open database to be accessed
or die "Can't open $FILENAME:$!\n"; # through %HASH
$V = $HASH{$KEY}; # retrieve from database
$HASH{$KEY} = $VALUE; # put value into database
if (exists $HASH{$KEY}) { # check whether in database
# ...
}
delete $HASH{$KEY}; # delete from database
untie %HASH; # close the database
14.1.3. Discussion
Accessing a database as a hash is powerful but easy, giving you a
persistent hash that sticks around after the program using it has
finished running. It's also much faster than loading in a new hash
every time; even if the hash has a million entries, your program
starts up virtually instantaneously.The program in Example 14-1 treats the database as
though it were a normal hash. You can even call
keys or each on it. Likewise,
exists and defined are
implemented for tied DBM hashes. Unlike a normal hash, a DBM hash
does not distinguish between those two
functions.
Example 14-1. userstats
#!/usr/bin/perl -w
# userstats - generates statistics on who is logged in.
# call with an argument to display totals
use DB_File;
$db = "/tmp/userstats.db"; # where data is kept between runs
tie(%db, 'DB_File', $db) or die "Can't open DB_File $db : $!\n";
if (@ARGV) {
if ("@ARGV" eq "ALL") {
@ARGV = sort keys %db;
}
foreach $user (@ARGV) {
print "$user\t$db{$user}\n";
}
} else {
@who = `who`; # run who(1)
if ($?) {
die "Couldn't run who: $?\n"; # exited abnormally
}
# extract username (first thing on the line) and update
foreach $line (@who) {
$line =~ /^(\S+)/;
die "Bad line from who: $line\n" unless $1;
$db{$1}++;
}
}
untie %db;
We use who to get a list of users logged in.
This typically produces output like:gnat ttyp1 May 29 15:39 (coprolith.frii.com)
If the userstats program is called without any
arguments, it checks who's logged on and updates the database
appropriately.If the program is called with arguments, these are treated as
usernames whose information will be presented. The special argument
"ALL" sets @ARGV to a sorted
list of DBM keys. For large hashes with many keys, this is
prohibitively expensive—a better solution would be to use the
BTREE bindings of DB_File described in Recipe 14.5.The old dbmopen function still works. Here's the
solution rewritten to use dbmopen and
dbmclose:use DB_File; # optional; overrides default
dbmopen %HASH, $FILENAME, 0666 # open database, accessed through %HASH
or die "Can't open $FILENAME:$!\n";
$V = $HASH{$KEY}; # retrieve from database
$HASH{$KEY} = $VALUE; # put value into database
if (exists $HASH{$KEY}) { # check whether in database
# ...
}
delete $HASH{$KEY}; # remove from database
dbmclose %HASH; # close the database
14.1.4. See Also
The documentation for the standard modules GDBM_File, NDBM_File,
SDBM_File, and DB_File, some of which are in Chapter 32 of
Programming Perl;
perltie(1); Chapter 14 of Programming
Perl; the discussion on the effect of your
umask on file creation in Recipe 7.1; Recipe 13.15
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| 14. Database Access |  | 14.2. Emptying a DBM File |
Copyright © 2003 O'Reilly & Associates. All rights reserved.
