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.\"
.IX Title "SDBM_File 3perl"
.TH SDBM_File 3perl 2024-05-30 "perl v5.38.2" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH NAME
SDBM_File \- Tied access to sdbm files
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 2
\& use Fcntl;   # For O_RDWR, O_CREAT, etc.
\& use SDBM_File;
\&
\& tie(%h, \*(AqSDBM_File\*(Aq, \*(Aqfilename\*(Aq, O_RDWR|O_CREAT, 0666)
\&   or die "Couldn\*(Aqt tie SDBM file \*(Aqfilename\*(Aq: $!; aborting";
\&
\& # Now read and change the hash
\& $h{newkey} = newvalue;
\& print $h{oldkey}; 
\& ...
\&
\& untie %h;
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\f(CW\*(C`SDBM_File\*(C'\fR 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.
.SS Tie
.IX Subsection "Tie"
Use \f(CW\*(C`SDBM_File\*(C'\fR with the Perl built-in \f(CW\*(C`tie\*(C'\fR function to establish
the connection between the variable and the file.
.PP
.Vb 1
\&    tie %hash, \*(AqSDBM_File\*(Aq, $basename, $modeflags, $perms;
\&
\&    tie %hash, \*(AqSDBM_File\*(Aq, $dirfile,  $modeflags, $perms, $pagfilename;
.Ve
.PP
\&\f(CW$basename\fR is the base filename for the database.  The database is two
files with ".dir" and ".pag" extensions appended to \f(CW$basename\fR,
.PP
.Vb 2
\&    $basename.dir     (or .sdbm_dir on VMS, per DIRFEXT constant)
\&    $basename.pag
.Ve
.PP
The two filenames can also be given separately in full as \f(CW$dirfile\fR
and \f(CW$pagfilename\fR.  This suits for two files without ".dir" and ".pag"
extensions, perhaps for example two files from File::Temp.
.PP
\&\f(CW$modeflags\fR can be the following constants from the \f(CW\*(C`Fcntl\*(C'\fR module (in
the style of the \fBopen\fR\|(2) system call),
.PP
.Vb 3
\&    O_RDONLY          read\-only access
\&    O_WRONLY          write\-only access
\&    O_RDWR            read and write access
.Ve
.PP
If you want to create the file if it does not already exist then bitwise-OR
(\f(CW\*(C`|\*(C'\fR) \f(CW\*(C`O_CREAT\*(C'\fR too.  If you omit \f(CW\*(C`O_CREAT\*(C'\fR and the database does not
already exist then the \f(CW\*(C`tie\*(C'\fR call will fail.
.PP
.Vb 1
\&    O_CREAT           create database if doesn\*(Aqt already exist
.Ve
.PP
\&\f(CW$perms\fR 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.)
.SH EXPORTS
.IX Header "EXPORTS"
SDBM_File optionally exports the following constants:
.IP \(bu 4
\&\f(CW\*(C`PAGFEXT\*(C'\fR \- the extension used for the page file, usually \f(CW\*(C`.pag\*(C'\fR.
.IP \(bu 4
\&\f(CW\*(C`DIRFEXT\*(C'\fR \- the extension used for the directory file, \f(CW\*(C`.dir\*(C'\fR
everywhere but VMS, where it is \f(CW\*(C`.sdbm_dir\*(C'\fR.
.IP \(bu 4
\&\f(CW\*(C`PAIRMAX\*(C'\fR \- the maximum size of a stored hash entry, including the
length of both the key and value.
.PP
These constants can also be used with fully qualified names,
eg. \f(CW\*(C`SDBM_File::PAGFEXT\*(C'\fR.
.SH DIAGNOSTICS
.IX Header "DIAGNOSTICS"
On failure, the \f(CW\*(C`tie\*(C'\fR call returns an undefined value and probably
sets \f(CW$!\fR to contain the reason the file could not be tied.
.ie n .SS """sdbm store returned \-1, errno 22, key ""..."" at ..."""
.el .SS "\f(CWsdbm store returned \-1, errno 22, key ""..."" at ...\fP"
.IX Subsection "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.
.SH "SECURITY WARNING"
.IX Header "SECURITY WARNING"
\&\fBDo not accept SDBM files from untrusted sources!\fR
.PP
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.
.SH "BUGS AND WARNINGS"
.IX Header "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.
.PP
See "tie" in perlfunc, perldbmfilter, Fcntl