SDBM_File - Tied access to sdbm files
use Fcntl; # For O_RDWR, O_CREAT, etc. use SDBM_File; tie(%h, 'SDBM_File', 'filename', O_RDWR|O_CREAT, 0666) or die "Couldn't tie SDBM file 'filename': $!; aborting"; # Now read and change the hash $h{newkey} = newvalue; print $h{oldkey}; ... untie %h;
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.
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 File::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 doesn't 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.)
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, .dir
everywhere 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
.
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.
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, perldbmfilter, Fcntl
© 1993–2016 Larry Wall and others
Licensed under the GNU General Public License version 1 or later, or the Artistic License.
The Perl logo is a trademark of the Perl Foundation.
https://perldoc.perl.org/5.26.0/SDBM_File.html