Manpages - SDBM_File.3perl
Table of Contents
NAME
SDBM_File - Tied access to sdbm files
SYNOPSIS
use Fcntl; # For O_RDWR, O_CREAT, etc. use SDBM_File; tie(%h, SDBM_File, filename, O_RDWR|O_CREAT, 0666) or die “Couldnt tie SDBM file filename: $!; aborting”; # Now read and change the hash $h{newkey} = newvalue; print $h{oldkey}; … untie %h;
DESCRIPTION
SDBM_File establishes a connection between a Perl hash variable and a
file in SDBM_File format. You can manipulate the data in the file just
as if it were in a Perl hash, but when your program exits, the data will
remain in the file, to be used the next time your program runs.
Tie
Use SDBM_File with the Perl built-in tie function to establish the
connection between the variable and the file.
tie %hash, SDBM_File, $basename, $modeflags, $perms; tie %hash, SDBM_File, $dirfile, $modeflags, $perms, $pagfilename;
$basename is the base filename for the database. The database is two
files with .dir and .pag extensions appended to $basename,
$basename.dir (or .sdbm_dir on VMS, per DIRFEXT constant) $basename.pag
The two filenames can also be given separately in full as $dirfile and
$pagfilename. This suits for two files without .dir and .pag
extensions, perhaps for example two files from :Temp.
$modeflags can be the following constants from the Fcntl module (in
the style of the open (2) system call),
O_RDONLY read-only access O_WRONLY write-only access O_RDWR read and write access
If you want to create the file if it does not already exist then
bitwise-OR (|) O_CREAT too. If you omit O_CREAT and the database
does not already exist then the tie call will fail.
O_CREAT create database if doesnt already exist
$perms is the file permissions bits to use if new database files are
created. This parameter is mandatory even when not creating a new
database. The permissions will be reduced by the user’s umask so the
usual value here would be 0666, or if some very private data then 0600.
(See umask in perlfunc.)
EXPORTS
SDBM_File optionally exports the following constants:
PAGFEXT- the extension used for the page file, usually.pag.DIRFEXT- the extension used for the directory file,.direverywhere but VMS, where it is.sdbm_dir.PAIRMAX- the maximum size of a stored hash entry, including the length of both the key and value.
These constants can also be used with fully qualified names, eg.
SDBM_File::PAGFEXT.
DIAGNOSTICS
On failure, the tie call returns an undefined value and probably sets
$! to contain the reason the file could not be tied.
“sdbm store returned -1, errno 22, key ”…“ at …”
This warning is emitted when you try to store a key or a value that is too long. It means that the change was not recorded in the database. See BUGS AND WARNINGS below.
SECURITY WARNING
Do not accept SDBM files from untrusted sources!
The sdbm file format was designed for speed and convenience, not for portability or security. A maliciously crafted file might cause perl to crash or even expose a security vulnerability.
BUGS AND WARNINGS
There are a number of limits on the size of the data that you can store in the SDBM file. The most important is that the length of a key, plus the length of its associated value, may not exceed 1008 bytes.
See tie in perlfunc, perldbmfilter, Fcntl