c-client Driver Characteristics			     Mark Crispin			     5 June 1999     Drivers are code modules that support different mailbox storagetechnologies.  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, adriver supports a particular mailbox format.  Mailbox formats arediscussed in more detail in the file formats.txt.     As of the date this document was written, there was no bundledsupport for any databases in c-client.  However, it should not beparticularly difficult to write a driver that communicates with adatabase.     Network protocols supported by c-client drivers are the InternetMail Access Protocol (all versions: IMAP4rev1, IMAP4, IMAP2bis, andIMAP2); the Post Office Protocol (version 3); and the Network NewsTransport Protocol (NNTP).  In addition, c-client also supports NNTPand the Simple Mail Transport Protocol (SMTP) for mailbox transport.     By default, all drivers are enabled.  There is little benefit tobe gained by disabling a driver, with one exception.  The mbox driverimplements the behavior of automatically moving new mail from thespool directory to the "mbox" file on the user's home directory, ifand *only* if the "mbox" exists and is in mailbox format.  The mboxdriver is listed under EXTRADRIVERS; if you wish to disable it justremove it from that list and rebuild.I. Special name "INBOX"The following rules to select INBOX and its format apply inthe 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 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 automaticallytransferred ("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 ordergiven 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, whichis declared in the ~/.mh_profile file.  Messages are automaticallytransferred from the spool directory to #mhinbox mailbox.III. Special Prefix "#mh/"Any name prefixed with "#mh/" always refers to a directory in the MHpath, 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.  Forexample, the name "#news.comp.mail.misc" refers to newsgroup"comp.mail.misc".V. All Other NamesThe driver is selected by generating a file name from the mailboxname, and then examining the data of the object with the resultingname.  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 theprefix 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 nameAll other names are interpreted in the context of the UNIX user's homedirectory (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 usersAnonymous 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 ComparisonThe following information about the local file drivers is anelaboration 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 formatmbox	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 systemsmbx	yes	yes	yes	yes	no	no	very good	prefile ;;; best performing local file driver; preferred format at UWtenex	yes	no	no	limited	no	no	good		prefile ;;; compatible with UNIX MMmtx	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 LFmx	yes	buggy	yes	yes	yes	no	poor		ixdir ;;; fullest function; *not* recommended due to performance problems and bugs; ;;; to be redesigned/rewrittenmh	yes	no	no	no	yes	yes	very poor	dir ;;; compatible with mh; #mhinbox for INBOX, #mh/ prefix for all other namesnews	yes	no	yes	no	yes	yes	very poor	ixdir ;;; local news spool access; #news. prefix for all namesphile	no	no	no	no	no	yes	good		file ;;; reads arbitrary file as a single readonly messageIMPORTANT: 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 asan 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 indexIn addition, drivers imap, nntp, and pop3 support IMAP4rev1, NNTP, andPOP3 protocols respectively.