maildropgdbm(7)
NAME
maildropgdbm - GDBM/DB support in maildrop
SYNOPSIS
gdbmopen(filename, mode) gdbmclose gdbmfetch(key [,default]) gdbmstore(key,value)
DESCRIPTION
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
gdbmclose
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.
Note
GDBM databases are case sensitive. Make sure that the GDBM database
is created using lowercase letters only, and use the tolower[1]
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.
Note
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:
"R"
Open this GDBM file for reading.
"W"
Open this GDBM file for reading and writing.
"C"
Open this GDBM file for reading and writing. If the GBDM file
doesn't exist, create it.
"N"
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[2] or dotlock[3] 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
gdbmstore(key, value)
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.
AUTHOR
Sam Varshavchik
Author
NOTES
1. tolower
http://www.courier-mta.org/maildrop/maildropfilter.html#tolower
2. flock
http://www.courier-mta.org/maildrop/maildropfilter.html#flock
3. dotlock
http://www.courier-mta.org/maildrop/maildropfilter.html#dotlock
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
