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/* ======================================================================== * Copyright 1988-2006 University of Washington * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * * ======================================================================== / 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.