The four filter_* methods shown above are available in all the DBM modules that ship with Perl, namely DB_File, GDBM_File, NDBM_File, ODBM_File and SDBM_File.
Each of the methods work identically, and are used to install (or uninstall) a single DBM Filter. The only difference between them is the place that the filter is installed.
If a filter has been installed with this method, it will be invoked every time you write a key to a DBM database.
If a filter has been installed with this method, it will be invoked every time you write a value to a DBM database.
If a filter has been installed with this method, it will be invoked every time you read a key from a DBM database.
If a filter has been installed with this method, it will be invoked every time you read a value from a DBM database.
You can use any combination of the methods from none to all four.
All filter methods return the existing filter, if present, or undef in not.
To delete a filter pass undef to it.
When each filter is called by Perl, a local copy of $_ will contain the key or value to be filtered. Filtering is achieved by modifying the contents of $_. The return code from the filter is ignored.
An Example -- the NULL termination problem.
DBM Filters are useful for a class of problems where you always want to make the same transformation to all keys, all values or both.
Similarly the NULL needs to be taken into account when you are considering the length of existing keys/values.
It would be much better if you could ignore the NULL terminations issue in the main application code and have a mechanism that automatically added the terminating NULL to all keys and values whenever you write to the database and have them removed when you read from the database. As I'm sure you have already guessed, this is a problem that DBM Filters can fix very easily.
The code above uses SDBM_File, but it will work with any of the DBM modules.
Hopefully the contents of each of the filters should be self-explanatory. Both ``fetch filters remove the terminating NULL , and both ``store filters add a terminating NULL .
Another Example -- Key is a C int.
the key 12345 will get stored in the DBM database as the 5 byte string ``12345''. If you actually want the key to be stored in the DBM database as a C int, you will have to use pack when writing, and unpack when reading.
The code above uses DB_File, but again it will work with any of the DBM modules.