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.\" ========================================================================
.\"
.IX Title "NDBM_File 3"
.TH NDBM_File 3 "2019-10-24" "perl v5.30.2" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
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.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 $h{oldkey};
\&  ...
\&
\&  untie %h;
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\f(CW\*(C`NDBM_File\*(C'\fR establishes a connection between a Perl hash variable and
a file in NDBM_File format;.  You can manipulate the data in the file
just as if it were in a Perl hash, but when your program exits, the
data will remain in the file, to be used the next time your program
runs.
.PP
Use \f(CW\*(C`NDBM_File\*(C'\fR with the Perl built-in \f(CW\*(C`tie\*(C'\fR function to establish
the connection between the variable and the file.  The arguments to
\&\f(CW\*(C`tie\*(C'\fR should be:
.IP "1." 4
The hash variable you want to tie.
.IP "2." 4
The string \f(CW"NDBM_File"\fR.  (Ths tells Perl to use the \f(CW\*(C`NDBM_File\*(C'\fR
package to perform the functions of the hash.)
.IP "3." 4
The name of the file you want to tie to the hash.
.IP "4." 4
Flags.  Use one of:
.RS 4
.ie n .IP """O_RDONLY""" 2
.el .IP "\f(CWO_RDONLY\fR" 2
.IX Item "O_RDONLY"
Read-only access to the data in the file.
.ie n .IP """O_WRONLY""" 2
.el .IP "\f(CWO_WRONLY\fR" 2
.IX Item "O_WRONLY"
Write-only access to the data in the file.
.ie n .IP """O_RDWR""" 2
.el .IP "\f(CWO_RDWR\fR" 2
.IX Item "O_RDWR"
Both read and write access.
.RE
.RS 4
.Sp
If you want to create the file if it does not exist, add \f(CW\*(C`O_CREAT\*(C'\fR to
any of these, as in the example.  If you omit \f(CW\*(C`O_CREAT\*(C'\fR and the file
does not already exist, the \f(CW\*(C`tie\*(C'\fR call will fail.
.RE
.IP "5." 4
The default permissions to use if a new file is created.  The actual
permissions will be modified by the user's umask, so you should
probably use 0666 here. (See \*(L"umask\*(R" in perlfunc.)
.SH "DIAGNOSTICS"
.IX Header "DIAGNOSTICS"
On failure, the \f(CW\*(C`tie\*(C'\fR call returns an undefined value and probably
sets \f(CW$!\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 \s-1BUGS AND WARNINGS\s0 below.
.SH "SECURITY AND PORTABILITY"
.IX Header "SECURITY AND PORTABILITY"
\&\fBDo not accept \s-1NDBM\s0 files from untrusted sources.\fR
.PP
On modern Linux systems these are typically \s-1GDBM\s0 files, which are not
portable across platforms.
.PP
The \s-1GDBM\s0 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 \s-1GDBM\s0 compatibilty for ndbm support will be
using a platform specific library, possibly inherited from \s-1BSD\s0
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 \s-1NDBM\s0 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 \*(L"tie\*(R" in perlfunc, perldbmfilter, Fcntl