NDBM_File

Section: Perl Programmers Reference Guide (3perl)
Updated: 2019-04-20
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NAME

NDBM_File - Tied access to ndbm files

SYNOPSIS

  use Fcntl;   # For O_RDWR, O_CREAT, etc.
  use NDBM_File;

  tie(%h, 'NDBM_File', 'filename', O_RDWR|O_CREAT, 0666)
    or die "Couldn't tie NDBM file 'filename': $!; aborting";

  # Now read and change the hash
  $h{newkey} = newvalue;
  print $h{oldkey};
  ...

  untie %h;

DESCRIPTION

"NDBM_File" 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.

Use "NDBM_File" with the Perl built-in "tie" function to establish the connection between the variable and the file. The arguments to "tie" should be:

1.

The hash variable you want to tie.

2.

The string "NDBM_File". (Ths tells Perl to use the "NDBM_File" package to perform the functions of the hash.)

3.

The name of the file you want to tie to the hash.

4.

Flags. Use one of:

"O_RDONLY": Read-only access to the data in the file.
"O_WRONLY": Write-only access to the data in the file.
"O_RDWR": Both read and write access.

: If you want to create the file if it does not exist, add "O_CREAT" to any of these, as in the example. If you omit "O_CREAT" and the file does not already exist, the "tie" call will fail.

5.

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.)

DIAGNOSTICS

On failure, the "tie" call returns an undefined value and probably sets $! to contain the reason the file could not be tied.

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.

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.

See ``tie'' in perlfunc, perldbmfilter, Fcntl