Programming Perl, 3rd Edition

DB_File

use DB_File;

Tie a hash to a DBM-style file:

tie(%hash, "DB_File", $filename)     # Open database.
    or die "Can't open $filename: $!";

$v = $hash{"key"};                   # Retrieve from database.
$hash{"key"} = "value";              # Put value into database.
untie %hash;

Tie a hash to a B-tree file, but still access as a regular DBM hash:

tie(%hash, "DB_File", "mytree", O_RDWR|O_CREAT, 0666, $DB_BTREE)
    or die "Cannot open file `mytree': $!";

while (($k, $v) = each %hash) {     # Do in-order traversal.
    print "$k => $v\n";
}

Tie an array to a plain text file:

tie(@lines, "DB_File", $textfile, O_RDWR|O_CREAT, 0666, $DB_RECNO)
    or die "Cannot open textfile $textfile: $!";

# Write a few lines to the file, overwriting any old contents.
$lines[0] = "first line";
$lines[1] = "second line";
$lines[2] = "third line";

push @lines, "penult", "last";  # Append two lines to the file.
$wc = scalar @lines;            # Count lines in file.
$last = pop @lines;             # Delete and retrieve last line.

The DB_File module provides tied access to Berkeley DB.^[1] The default tie function gives you a standard DBM-style database with some features that no other DBM library provides: there are no size limits on either keys or values, and your data is stored in a byte-order independent format.

The second tie mechanism uses B-trees to give you a true ISAM (indexed sequential access method) file, that is, a hash whose keys are automatically ordered—alphabetically by default, but configurable by the user.

The third tie mechanism binds an array to a file of records ...