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 the File::Temp manpage.
$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
perlfunc/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.
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 perlfunc/tie, perldbmfilter, Fcntl
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