1SDBM_File(3pm)         Perl Programmers Reference Guide         SDBM_File(3pm)
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NAME

6       SDBM_File - Tied access to sdbm files
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SYNOPSIS

9        use Fcntl;   # For O_RDWR, O_CREAT, etc.
10        use SDBM_File;
11
12        tie(%h, 'SDBM_File', 'filename', O_RDWR|O_CREAT, 0666)
13          or die "Couldn't tie SDBM file 'filename': $!; aborting";
14
15        # Now read and change the hash
16        $h{newkey} = newvalue;
17        print $h{oldkey};
18        ...
19
20        untie %h;
21

DESCRIPTION

23       "SDBM_File" establishes a connection between a Perl hash variable and a
24       file in SDBM_File format.  You can manipulate the data in the file just
25       as if it were in a Perl hash, but when your program exits, the data
26       will remain in the file, to be used the next time your program runs.
27
28   Tie
29       Use "SDBM_File" with the Perl built-in "tie" function to establish the
30       connection between the variable and the file.
31
32           tie %hash, 'SDBM_File', $basename, $modeflags, $perms;
33
34           tie %hash, 'SDBM_File', $dirfile,  $modeflags, $perms, $pagfilename;
35
36       $basename is the base filename for the database.  The database is two
37       files with ".dir" and ".pag" extensions appended to $basename,
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39           $basename.dir     (or .sdbm_dir on VMS, per DIRFEXT constant)
40           $basename.pag
41
42       The two filenames can also be given separately in full as $dirfile and
43       $pagfilename.  This suits for two files without ".dir" and ".pag"
44       extensions, perhaps for example two files from File::Temp.
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46       $modeflags can be the following constants from the "Fcntl" module (in
47       the style of the open(2) system call),
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49           O_RDONLY          read-only access
50           O_WRONLY          write-only access
51           O_RDWR            read and write access
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53       If you want to create the file if it does not already exist then
54       bitwise-OR ("|") "O_CREAT" too.  If you omit "O_CREAT" and the database
55       does not already exist then the "tie" call will fail.
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57           O_CREAT           create database if doesn't already exist
58
59       $perms is the file permissions bits to use if new database files are
60       created.  This parameter is mandatory even when not creating a new
61       database.  The permissions will be reduced by the user's umask so the
62       usual value here would be 0666, or if some very private data then 0600.
63       (See "umask" in perlfunc.)
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EXPORTS

66       SDBM_File optionally exports the following constants:
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68       •   "PAGFEXT" - the extension used for the page file, usually ".pag".
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70       •   "DIRFEXT" - the extension used for the directory file, ".dir"
71           everywhere but VMS, where it is ".sdbm_dir".
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73       •   "PAIRMAX" - the maximum size of a stored hash entry, including the
74           length of both the key and value.
75
76       These constants can also be used with fully qualified names, eg.
77       "SDBM_File::PAGFEXT".
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DIAGNOSTICS

80       On failure, the "tie" call returns an undefined value and probably sets
81       $! to contain the reason the file could not be tied.
82
83   "sdbm store returned -1, errno 22, key "..." at ..."
84       This warning is emitted when you try to store a key or a value that is
85       too long.  It means that the change was not recorded in the database.
86       See BUGS AND WARNINGS below.
87

SECURITY WARNING

89       Do not accept SDBM files from untrusted sources!
90
91       The sdbm file format was designed for speed and convenience, not for
92       portability or security.  A maliciously crafted file might cause perl
93       to crash or even expose a security vulnerability.
94

BUGS AND WARNINGS

96       There are a number of limits on the size of the data that you can store
97       in the SDBM file.  The most important is that the length of a key, plus
98       the length of its associated value, may not exceed 1008 bytes.
99
100       See "tie" in perlfunc, perldbmfilter, Fcntl
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104perl v5.34.1                      2022-03-15                    SDBM_File(3pm)
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