1 DBM Libraries for use with Exim
2 -------------------------------
7 Exim uses direct-access (so-called "dbm") files for a number of different
8 purposes. These are files arranged so that the data they contain is indexed and
9 can quickly be looked up by quoting an appropriate key. They are used as
12 1. Exim keeps its "hints" databases in dbm files.
14 2. The configuration can specify that certain things (e.g. aliases) be looked
17 3. The configuration can contain expansion strings that involve lookups in dbm
20 4. The filter commands "mail" and "vacation" have a facility for replying only
21 once to each incoming address. The record of which addresses have already
22 received replies may be kept in a dbm file, depending on the configuration
23 option once_file_size.
25 The runtime configuration can be set up without specifying 2 or 3, but Exim
26 always requires the availability of a dbm library, for 1 (and 4 if configured
33 The original library that provided the dbm facility in Unix was called "dbm".
34 This seems to have been superseded quite some time ago by a new version called
35 "ndbm" which permits several dbm files to be open at once. Several operating
36 systems, including those from Sun, contain ndbm as standard.
38 A number of alternative libraries also exist, the most common of which seems to
39 be Berkeley DB (just called DB hereinafter). Release 1.85 was around for
40 some time, and various releases 2.x began to appear towards the end of 1997. In
41 November 1999, version 3.0 was released, and the ending of support for 2.7.7,
42 the last 2.x release, was announced for November 2000. (Support for 1.85 has
43 already ceased.) There were further 3.x releases, but by the end of 2001, the
44 current release was 4.0.14. In 2022 it was 5.3.28 on Linux (the then-owner
45 has developed it further but Exim does not support anything after 5.x).
47 There are major differences in implementation and interface between the DB 1.x
48 and 2.x/3.x/4.x releases, and they are best considered as two independent dbm
49 libraries. Changes to the API were made for 3.0 and again for 3.1.
51 Another DBM library is the GNU library, gdbm, though this does not seem to be
54 Yet another dbm library is tdb (Trivial Data Base) which has come out of the
55 Samba project. The first releases seem to have been in mid-2000.
57 Some older Linux releases contain gdbm as standard, while others contain no dbm
58 library. More recent releases contain DB 1.85 or 2.x or later, and presumably
59 will track the development of the DB library. Some BSD versions of Unix include
60 DB 1.85 or later. All of the non-ndbm libraries except tdb contain
61 compatibility interfaces so that programs written to call the ndbm functions
62 should, in theory, work with them, but there are some potential pitfalls which
63 have caught out Exim users in the past.
65 Exim has been tested with ndbm, gdbm, DB 1.85, DB 2.x, DB 3.1, DB 4.0.14, and
66 tdb 1.0.2, in various different modes in some cases, and is believed to work
67 with all of them if it and they are properly configured.
69 I have considered the possibility of calling different dbm libraries for
70 different functions from a single Exim binary. However, because all bar one of
71 the libraries provide ndbm compatibility interfaces (and therefore the same
72 function names) it would require a lot of complicated, error-prone trickery to
73 achieve this. Exim therefore uses a single library for all its dbm activities.
75 However, Exim does also support cdb (Constant Data Base), an efficient file
76 arrangement for indexed data that does not change incrementally (for example,
77 alias files). This is independent of any dbm library and can be used alongside
84 The configuration option EXIMDB_LOCK_TIMEOUT controls how long Exim waits to
85 get a lock on a hints database. From version 1.80 onwards, Exim does not
86 attempt to take out a lock on an actual database file (this caused problems in
87 the past). Instead, it takes out an fcntl() lock on a separate file whose name
88 ends in ".lockfile". This ensures that Exim has exclusive access to the
89 database before even attempting to open it. Exim creates the lock file the
90 first time it needs it. It should never be removed.
96 The OS-specific configuration files that are used to build Exim specify the use
97 of Berkeley DB on those systems where it is known to be standard. In the
98 absence of any special configuration options, Exim uses the ndbm set of
99 functions to control its dbm databases. This should work with any of the dbm
100 libraries because those that are not ndbm have compatibility interfaces.
101 However, there is one awful pitfall:
103 Exim #includes a header file called ndbm.h which defines the functions and the
104 interface data block; gdbm and DB 1.x provide their own versions of this header
105 file, later DB versions do not. If it should happen that the wrong version of
106 nbdm.h is seen by Exim, it may compile without error, but fail to operate
107 correctly at runtime.
109 This situation can easily arise when more than one dbm library is installed on
110 a single host. For example, if you decide to use DB 1.x on a system where gdbm
111 is the standard library, unless you are careful in setting up the include
112 directories for Exim, it may see gdbm's ndbm.h file instead of DB's. The
113 situation is even worse with later versions of DB, which do not provide an
116 One way out of this for gdbm and any of the versions of DB is to configure Exim
117 to call the DBM library in its native mode instead of via the ndbm
118 compatibility interface, thus avoiding the use of ndbm.h. This is done by
119 setting the USE_DB configuration option if you are using Berkeley DB, or
120 USE_GDBM if you are using gdbm. This is the recommended approach.
126 The ndbm library holds its data in two files, with extensions .dir and .pag.
127 This makes atomic updating of, for example, alias files, difficult, because
128 simple renaming cannot be used without some risk. However, if your system has
129 ndbm installed, Exim should compile and run without any problems.
135 The gdbm library, when called via the ndbm compatibility interface, makes two
136 hard links to a single file, with extensions .dir and .pag. As mentioned above,
137 gdbm provides its own version of the ndbm.h header, and you must ensure that
138 this is seen by Exim rather than any other version. This is not likely to be a
139 problem if gdbm is the only dbm library on your system.
141 If gdbm is called via the native interface (by setting USE_GDBM in your
142 Local/Makefile), it uses a single file, with no extension on the name, and the
143 ndbm.h header is not required.
145 The gdbm library does its own locking of the single file that it uses. From
146 version 1.80 onwards, Exim locks on an entirely separate file before accessing
147 a hints database, so gdbm's locking should always succeed.
153 1.85 was the most widespread DB 1.x release; there is also a 1.86 bug-fix
154 release, but the belief is that the bugs it fixes will not affect Exim.
155 However, maintenance for 1.x releases has been phased out.
156 Exim no loger supports these releases for current builds.
158 This dbm library can be called by Exim in one of two ways: via the ndbm
159 compatibility interface, or via its own native interface. There are two
160 advantages to doing the latter: (1) you don't run the risk of Exim's seeing the
161 "wrong" version of the ndbm.h header, as described above, and (2) the
162 performance is better. It is therefore recommended that you set USE_DB=yes in an
163 appropriate Local/Makefile-xxx file. (If you are compiling for just one OS, it
164 can go in Local/Makefile itself.)
166 When called via the compatibility interface, DB 1.x creates a single file with
167 a .db extension. When called via its native interface, no extension is added to
168 the file name handed to it.
170 DB 1.x does not do any locking of its own.
176 DB 2.x was released in 1997. It is a major re-implementation and its native
177 interface is incompatible with DB 1.x, though a compatibility interface was
178 introduced in DB 2.1.0, and there is also an ndbm.h compatibility interface.
180 Like 1.x, it can be called from Exim via the ndbm compatibility interface or
181 via its native interface, and once again setting USE_DB in order to get the
182 native interface is recommended. If USE_DB is *not* set, then you will have to
183 provide a suitable version of ndbm.h, because one does not come with the DB 2.x
184 distribution. A suitable version is:
186 /*************************************************
187 * ndbm.h header for DB 2.x *
188 *************************************************/
190 /* This header should replace any other version of ndbm.h when Berkeley DB
191 version 2.x is in use via the ndbm compatibility interface. Otherwise, any
192 extant version of ndbm.h may cause programs to misbehave. There doesn't seem
193 to be a version of ndbm.h supplied with DB 2.x, so I made this for myself.
195 Philip Hazel 12/Jun/97
198 #define DB_DBM_HSEARCH
203 When called via the compatibility interface, DB 2.x creates a single file with
204 a .db extension. When called via its native interface, no extension is added to
205 the file name handed to it.
207 DB 2.x does not do any automatic locking of its own; it does have a set of
208 functions for various forms of locking, but Exim does not use them.
210 Exim no longer supports these releases for current builds.
216 DB 3.0 was released in November 1999 and 3.1 in June 2000. The 3.x series is a
217 development of the 2.x series and the above comments apply. Exim can
218 automatically distinguish between the different versions, so it copes with the
219 changes to the API without needing any special configuration.
221 When Exim creates a DBM file using DB 3.x (e.g. when creating one of its hints
222 databases), it specified the "hash" format. However, when it opens a DB 3 file
223 for reading only, it specifies "unknown". This means that it can read DB 3
224 files in other formats that are created by other programs.
230 The 4.x series is a development of the 2.x and 3.x series, and the above
236 The 5.x versions seem to be current in several Linux disributions
237 as of writing (2024). Exim uses them under the assumption that the
238 API is the same as the version 4.1 library.
243 These versions exist, but documentation does not seem to be available.
244 Exim does not support them.
251 tdb 1.0.2 was released in September 2000. Its origin is the database functions
252 that are used by the Samba project.
256 Testing Exim's dbm handling
257 ---------------------------
259 Because there have been problems with dbm file locking in the past, I built
260 some testing code for Exim's dbm functions. This is very much a lash-up, but it
261 is documented here so that anybody who wants to check that their configuration
262 is locking properly can do so. Now that Exim does the locking on an entirely
263 separate file, locking problems are much less likely, but this code still
264 exists, just in case. Proceed as follows:
266 . Build Exim in the normal way. This ensures that all the makesfiles etc. get
269 . From within the build directory, obey "make test_dbfn". This makes a binary
270 file called test_dbfn. If you are experimenting with different configurations
271 you *must* do "make makefile" after changing anything, before obeying "make
272 test_dbfn" again, because the make target for test_dbfn isn't integrated
273 with the making of the makefile.
275 . Identify a scratch directory where you have write access. Create a sub-
276 directory called "db" in the scratch directory.
278 . Type the command "test_dbfn <scratch-directory>". This will output some
279 general information such as
281 Exim's db functions tester: interface type is db (v2)
282 DBM library: Berkeley DB: Sleepycat Software: DB 2.1.0: (6/13/97)
290 . At this point you can type commands to open a dbm file and read and write
291 data in it. First type the command "open <name>", e.g. "open junk". The
292 response should look like this
294 opened DB file <scratch-directory>/db/junk: flags=102
299 The tester will have created a dbm file within the db directory of the
300 scratch directory. It will also have created a file with the extension
301 ".lockfile" in the same directory. Unlike Exim itself, it will not create
302 the db directory for itself if it does not exist.
304 . To test the locking, don't type anything more for the moment. You now need to
305 set up another process running the same test_dbfn command, e.g. from a
306 different logon to the same host. This time, when you attempt to open the
307 file it should fail after a minute with a timeout error because it is
310 . If the second process doesn't produce any error message, but gets back to the
311 > prompt, then the locking is not working properly.
313 . You can check that the second process gets the lock when the first process
314 releases it by exiting from the first process with ^D, q, or quit; or by
315 typing the command "close".
317 . There are some other commands available that are not related to locking:
321 write abcde the quick brown fox
323 writes a record to the database,
328 read and delete a record, respectively, and
332 scans the entire database. Note that the database is purely for testing the
333 dbm functions. It is *not* one of Exim's regular databases, and you should
334 not try running this testing program on any of Exim's real database
338 Last update: June 2002
340 (+undates July 2024, jgh)
git://git.exim.org / exim.git / blob