maildropgdbm − GDBM/DB support in maildrop
The gdbm family of functions provides access to the GDBM library − a library of routines that manage simple database files. The library provides a way of quickly storing and looking up key/data pairs.
GDBM support in maildrop is optional, and may not be available to you.
GDBM support in maildrop can optionally be implemented using the DB library. This option is selected by the system administrator. If this is the case, these functions still work exactly as described below, except that they will operate on DB hash files, instead of GDBM files.
To see whether GDBM or DB support is used, run the command "maildrop −v".
GDBM support is minimal, and simplistic. A filter file may have only one gdbm file open at the same time. However, the filter file can close the current gdbm file, and open another one. If another filter file is included using the include statement, the included filter file may open its own, separate, gdbm file.
A GDBM file contains a list of key/value pairs. All keys in the GDBM file are unique. After storing an arbitrary key/value pair in the GDBM file, the value associated with the given key can be quickly located and retrieved.
gdbmclose − close gdbm file
This function closes the current GDBM file.
gdbmfetch − retrieve data
gdbmfetch (key [, options] [, default])
This function retrieves the data for the given key. key is the key to retrieve. The gdbmfetch function returns the data associated with this key. If the key does not exist in the GDBM file, gdbmfetch returns the default argument. If the default argument is not specified, gdbmfetch returns empty text. Please note that the default argument is not actually evaluated unless the key does not exist in the GDBM file.
The options argument specifies additional maildrop value−added features. The following functionality is not available in the GDBM library, but is rather provided by maildrop.
If the options argument is set to "D", and the key could not be found in the GDBM database, and the key is of the form "user@domain", maildrop will then attempt to look up the key "user@". If that key is also not found, maildrop finally looks up the key "domain".
If "domain" is also not found, and domain is of the form "a.b.c.d.tld" (with variable number of period−separated sections), maildrop then attempts to look up the key "b.c.d.tld". If that key is not found, maildrop tries "c.d.tld", and so on, until a key is found, or there are no more subdomains to remove, at which point gdbmfetch will return either the default argument, or empty text.
If the options argument is set to "D", and the key could not be found in the GDBM database, and the key is of the form "a.b.c.d.tld" (with variable number of period−separated sections), maildrop will also attempt to look up keys for successive higher−level domains in the GDBM database.
GDBM databases are case sensitive. Make sure that the GDBM database is created using lowercase letters only, and use the tolower  function to convert the key to lowercase.
If the options argument is "I", and the key is not in the GDBM database, and the key is of the form "w.x.y.z" (with variable number of period−separated sections), maildrop then tries to look up the key "w.x.y", then "w.x", until a key is found, or there are no more sections to remove. Use this feature to look up IP−address based GDBM lists.
These features are implemented by brute force: if the query doesn't succeed, try again. Take note of potential denial−of−service attacks where key is set to a long text string consisting mostly of periods, which will result in numerous GDBM queries that will take an excessive amount of time to complete.
gdbmopen − open gdbm file
gdbmopen (file [, mode])
gdbmopen opens the indicated GDBM file. The optional second argument specifies the following:
Open this GDBM file for reading.
Open this GDBM file for reading and writing.
Open this GDBM file for reading and writing. If the GBDM file doesn't exist, create it.
Create a new GDBM file. If the file exists, the existing file is deleted. The file is opened for reading and writing.
The mode argument defaults to "R" is used. In embedded mode, only "R" is allowed.
The GDBM library allows multiple processes to read the same GDBM file at the same time, but it does not allow multiple access when the GDBM file is open for writing. Using flock  or dotlock  is highly recommended.
In delivery mode, maildrop runs from the recipient's home directory. Keep that in mind while specifying the filename.
The gdbmopen function returns 0 if the GDBM file was successfully opened, non−zero otherwise.
gdbmstore − store data
key is the key value to store in the GDBM file. value is the value to store. If key already exists in the GDBM file, value replacest the old value. The gdbmstore function is only permitted if the GDBM file is opened for writing. If gdbmopen opened the GDBM file for reading only, gdbmstore will return −1. Otherwise, gdbmstore returns 0.