SDBM_File - Online Linux Manual PageSection : 3pm
Updated : 2023-01-19
Source : perl v5.36.0
Note : Perl Programmers Reference Guide
NAMESDBM_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 "Couldn't 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.
TieUse 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” in perlfunc.)
EXPORTSSDBM_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.
DIAGNOSTICSOn 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 WARNINGSThere 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 0
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