Manpages - NDBM_File.3perl

NDBM_File - Tied access to ndbm files

use Fcntl; # For O_RDWR, O_CREAT, etc. use NDBM_File; tie(%h, NDBM_File, filename, O_RDWR|O_CREAT, 0666) or die “Couldnt tie NDBM file filename: $!; aborting”; # Now read and change the hash $h{newkey} = newvalue; print $h{oldkey}; … untie %h;

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.)

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

Do not accept NDBM files from untrusted sources.

On modern Linux systems these are typically GDBM files, which are not portable across platforms.

The GDBM documentation doesn’t imply that files from untrusted sources can be safely used with libgdbm.

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.

A maliciously crafted file might cause perl to crash or even expose a security vulnerability.

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