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c-client Driver Characteristics
Mark Crispin
11 December 2006
Drivers are code modules that support different mailbox storage
technologies. A mailbox storage technology may be implemented by
1) files and directories on the local system
2) a database
3) a network protocol.
In the case of files and directories on the local system, a
driver supports a particular mailbox format. Mailbox formats are
discussed in more detail in the file formats.txt.
As of the date this document was written, there was no bundled
support for any databases in c-client. However, it should not be
particularly difficult to write a driver that communicates with a
database.
Network protocols supported by c-client drivers are the Internet
Mail Access Protocol (all versions: IMAP4rev1, IMAP4, IMAP2bis, and
IMAP2); the Post Office Protocol (version 3); and the Network News
Transport Protocol (NNTP). In addition, c-client also supports NNTP
and the Simple Mail Transport Protocol (SMTP) for mailbox transport.
By default, all drivers are enabled. There is little benefit to
be gained by disabling a driver, with one exception. The mbox driver
implements the behavior of automatically moving new mail from the
spool directory to the "mbox" file on the user's home directory, if
and *only* if the "mbox" exists and is in mailbox format. The mbox
driver is listed under EXTRADRIVERS; if you wish to disable it just
remove it from that list and rebuild.
I. Special name "INBOX"
The following rules to select INBOX and its format apply in
the order given if "black box mode" is not in effect:
1) mbox format is selected if file ~/mbox exists, and is in unix
format or is zero-length.
2) mx format is selected if file ~/INBOX/.mxindex exists.
3) mbx format is selected if file ~/INBOX exists and is in mbx format.
4) tenex format is selected if:
a) file ~/mail.txt exists, and is in tenex format or is zero-length.
b) file ~/INBOX exists and is in tenex format.
5) mtx format is selected if:
a) file ~/INBOX.MTX exists, and is in mtx format or is zero-length.
b) file ~/INBOX exists and is in mtx format.
6) mmdf format is selected if the spool directory file exists and is
in mmdf format.
7) unix format is selected if the spool directory file exists and is
in unix format.
8) the dummy driver is selected if the spool directory file does not
exist, or exists and is empty.
If "black box mode" is not in effect, messages are automatically
transferred ("snarfed") from the spool directory to an INBOX in mbox,
mx, mbx, tenex, and mtx formats.
The following rules to select INBOX and its format apply in the order
given if "black box mode" is in effect:
1) mx format is selected if file ~/INBOX/.mxindex exists.
2) mbx format is selected if file ~/INBOX exists and is in mbx format.
3) tenex format is selected if file ~/INBOX exists and is in tenex format.
4) mtx format is selected if file ~/INBOX exists and is in mtx format.
5) mmdf format is selected if file ~/INBOX exists and is in mmdf format.
6) unix format is selected if file ~/INBOX exists and is in unix format.
7) the dummy driver is selected if ~/INBOX does not exist, or exists
and is empty.
II. Special Name #mhinbox
#mhinbox always refers to the directory "inbox" in the MH path, which
is declared in the ~/.mh_profile file. Messages are automatically
transferred from the spool directory to #mhinbox mailbox.
III. Special Prefix "#mh/"
Any name prefixed with "#mh/" always refers to a directory in the MH
path, which is declared in the ~/.mh_profile file. For example, the name
"#mh/foo" refers to directory "foo" in the MH path.
IV. Special prefix "#news."
Any name prefixed with "#news" always refers to a newsgroup. For
example, the name "#news.comp.mail.misc" refers to newsgroup
"comp.mail.misc".
V. All Other Names
The driver is selected by generating a file name from the mailbox
name, and then examining the data of the object with the resulting
name. The formats are checked in order: mx, mbx, tenex, mtx, mmdf,
unix, and phile. The dummy driver is selected if the file is empty.
The file name is generated according to certain rules, based upon the
prefix of the mailbox name. On UNIX, the following rules apply:
Prefix Interpretation of Suffix
------ ------------------------
/ [black box] preceeds a user name; "/foo/bar" means
"black box user foo's mailbox bar"
[not black box] preceeds an absolute path name.
~ [not black box] preceeds a user name; "~foo/bar" means
"UNIX user foo's mailbox bar"
#ftp/ preceeds UNIX user ftp's mailbox name
#public/ preceeds UNIX user imappublic's mailbox name
#shared/ preceeds UNIX user imapshared's mailbox name
All other names are interpreted in the context of the UNIX user's home
directory (not black box), the black box user's black box directory
(black box), or UNIX user ftp's home directory (anonymous).
The strings "..", "//", and /~ are forbidden in names in:
black box mode
#ftp, #public, or #shared names
anonymous users
Anonymous users may only access:
INBOX (belonging to UNIX user ftp)
files in or below UNIX user ftp's home directory
#ftp, #news, and #public namespace
VI. Driver Comparison
The following information about the local file drivers is an
elaboration of a table compiled by Osma Ahvenlampi.
Driver CA CE UID Kwd Sub NFS Performance Layout
------ -- -- --- --- --- --- ----------- ------
unix no no yes yes no limited fair file
;;; traditional UNIX format
mbox no no yes yes no limited fair file
;;; traditional UNIX format, INBOX only, using ~/mbox with automatic
;;; moving from the mail spool directory.
mmdf no no yes yes no limited fair file
;;; default on SCO systems
mbx yes yes yes yes no no very good prefile
;;; best performing local file driver; preferred format at UW
tenex yes no no limited no no good prefile
;;; compatible with UNIX MM
mtx yes no no limited no no very good prefile
;;; PC Pine standard format; compatible with TOPS-20; identical to tenex
;;; but instead CRLF newlines instead of LF
mx yes buggy yes yes yes no poor ixdir
;;; fullest function; *not* recommended due to performance problems and bugs;
;;; to be redesigned/rewritten
mh yes no no no yes yes very poor dir
;;; compatible with mh; #mhinbox for INBOX, #mh/ prefix for all other names
news yes no yes no yes yes very poor ixdir
;;; local news spool access; #news. prefix for all names
phile no no no no no yes good file
;;; reads arbitrary file as a single readonly message
IMPORTANT: the "performance" ratings are relative to other drivers,
and not necessarily to other software which implements those formats.
They relate to the driver's performance in typical operations such as
an IMAP "FETCH ALL".
Key to headings:
CA: concurrent read/write access
CE: expunge permitted in concurrent read/write access
UID: sticky UIDs
Kwd: keyword flags
Sub: subfolders
NFS: usable over network filesystems (NFS, AFS, etc.)
Layout: file - single file
prefile - file with preallocated space for state
dir - directory, messages are files
ixdir - directory, messages are files, with helper index
In addition, drivers imap, nntp, and pop3 support IMAP4rev1, NNTP, and
POP3 protocols respectively.