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.\"
.IX Title "NDBM_File 3perl"
.TH NDBM_File 3perl 2023-11-30 "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
NDBM_File \- Tied access to ndbm files
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 2
\&  use Fcntl;   # For O_RDWR, O_CREAT, etc.
\&  use NDBM_File;
\&
\&  tie(%h, \*(AqNDBM_File\*(Aq, \*(Aqfilename\*(Aq, O_RDWR|O_CREAT, 0666)
\&    or die "Couldn\*(Aqt tie NDBM file \*(Aqfilename\*(Aq: $!; aborting";
\&
\&  # Now read and change the hash
\&  $h{newkey} = newvalue;
\&  print $h{oldkey};
\&  ...
\&
\&  untie %h;
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\f(CW\*(C`NDBM_File\*(C'\fR establishes a connection between a Perl hash variable and
a file in NDBM_File format;.  You can manipulate the data in the file
just as if it were in a Perl hash, but when your program exits, the
data will remain in the file, to be used the next time your program
runs.
.PP
Use \f(CW\*(C`NDBM_File\*(C'\fR with the Perl built-in \f(CW\*(C`tie\*(C'\fR function to establish
the connection between the variable and the file.  The arguments to
\&\f(CW\*(C`tie\*(C'\fR should be:
.IP 1. 4
The hash variable you want to tie.
.IP 2. 4
The string \f(CW"NDBM_File"\fR.  (Ths tells Perl to use the \f(CW\*(C`NDBM_File\*(C'\fR
package to perform the functions of the hash.)
.IP 3. 4
The name of the file you want to tie to the hash.
.IP 4. 4
Flags.  Use one of:
.RS 4
.ie n .IP """O_RDONLY""" 2
.el .IP \f(CWO_RDONLY\fR 2
.IX Item "O_RDONLY"
Read-only access to the data in the file.
.ie n .IP """O_WRONLY""" 2
.el .IP \f(CWO_WRONLY\fR 2
.IX Item "O_WRONLY"
Write-only access to the data in the file.
.ie n .IP """O_RDWR""" 2
.el .IP \f(CWO_RDWR\fR 2
.IX Item "O_RDWR"
Both read and write access.
.RE
.RS 4
.Sp
If you want to create the file if it does not exist, add \f(CW\*(C`O_CREAT\*(C'\fR to
any of these, as in the example.  If you omit \f(CW\*(C`O_CREAT\*(C'\fR and the file
does not already exist, the \f(CW\*(C`tie\*(C'\fR call will fail.
.RE
.IP 5. 4
The default permissions to use if a new file is created.  The actual
permissions will be modified by the user's umask, so you should
probably use 0666 here. (See "umask" in perlfunc.)
.SH DIAGNOSTICS
.IX Header "DIAGNOSTICS"
On failure, the \f(CW\*(C`tie\*(C'\fR call returns an undefined value and probably
sets \f(CW$!\fR to contain the reason the file could not be tied.
.ie n .SS """ndbm store returned \-1, errno 22, key ""..."" at ..."""
.el .SS "\f(CWndbm store returned \-1, errno 22, key ""..."" at ...\fP"
.IX Subsection "ndbm 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 AND PORTABILITY"
.IX Header "SECURITY AND PORTABILITY"
\&\fBDo not accept NDBM files from untrusted sources.\fR
.PP
On modern Linux systems these are typically GDBM files, which are not
portable across platforms.
.PP
The GDBM documentation doesn't imply that files from untrusted sources
can be safely used with \f(CW\*(C`libgdbm\*(C'\fR.
.PP
Systems that don't use GDBM compatibilty for ndbm support will be
using a platform specific library, possibly inherited from BSD
systems, where it may or may not be safe to use an untrusted file.
.PP
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 NDBM 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