mmdf(5) 							   User Manuals 							   mmdf(5)

NAME

       MMDF - Multi-channel Memorandum Distribution Facility mailbox format

DESCRIPTION

       This document describes the MMDF mailbox format used by some MTAs and MUAs (i.e.  scomail(1)) to store mail messages locally.

       An  MMDF  mailbox is a text file containing an arbitrary number of e-mail messages.  Each message consists of a postmark, followed by an e-
       mail message formatted according to RFC822 / RFC2822, followed by a postmark. The file format is line-oriented. Lines are separated by line
       feed characters (ASCII 10). A postmark line consists of the four characters "^A^A^A^A" (Control-A; ASCII 1).

       Example of a MMDF mailbox holding two mails:

	      ^A^A^A^A
	      From: example@example.com
	      To: example@example.org
	      Subject: test

	      >From what I learned about the MMDF-format:
	      ^A^A^A^A
	      ^A^A^A^A
	      From: example@example.com
	      To: example@example.org
	      Subject: test 2

	      bar
	      ^A^A^A^A

       In  contrast  to most other single file mailbox formats like MBOXO and MBOXRD (see mbox(5)) there is no need to quote/dequote "From "-lines
       in MMDF mailboxes as such lines have no special meaning in this format.

       If the modification-time (usually determined via stat(2)) of a nonempty mailbox file is greater than the access-time the file has new mail.
       Many MUAs place a Status: header in each message to indicate which messages have already been read.

LOCKING

       Since MMDF files are frequently accessed by multiple programs in parallel, MMDF files should generally not be accessed without locking.

       Three different locking mechanisms (and combinations thereof) are in general use:

       o      fcntl(2)	locking is mostly used on recent, POSIX-compliant systems. Use of this locking method is, in particular, advisable if MMDF
	      files are accessed through the Network File System (NFS), since it seems the only way to reliably invalidate NFS clients' caches.

       o      flock(2) locking is mostly used on BSD-based systems.

       o      Dotlocking is used on all kinds of systems. In order to lock an MMDF file named folder, an application  first  creates  a  temporary
	      file  with  a unique name in the directory in which the folder resides. The application then tries to use the link(2) system call to
	      create a hard link named folder.lock to the temporary file. The success of the link(2) system call should be  additionally  verified
	      using  stat(2)  calls.  If  the  link  has succeeded, the mail folder is considered dotlocked. The temporary file can then safely be
	      unlinked.

	      In order to release the lock, an application just unlinks the folder.lock file.

       If multiple methods are combined, implementors should make sure to use the non-blocking variants of the fcntl(2) and flock(2) system  calls
       in order to avoid deadlocks.

       If  multiple  methods  are  combined, an MMDF file must not be considered to have been successfully locked before all individual locks were
       obtained. When one of the individual locking methods fails, an application should release all locks it acquired successfully,  and  restart
       the entire locking procedure from the beginning, after a suitable delay.

       The  locking  mechanism	used  on  a  particular  system  is  a matter of local policy, and should be consistently used by all applications
       installed on the system which access MMDF files. Failure to do so may result in loss of e-mail data, and in corrupted MMDF files.

CONFORMING TO

       MMDF is not part of any currently supported standard.

HISTORY

       MMDF was developed at the University of Delaware by Dave Crocker.

SEE ALSO

       scomail(1), fcntl(2), flock(2), link(2), stat(2), mbox(5), RFC822, RFC2822

AUTHOR

       Urs Janssen <urs@tin.org>

Unix								February 18th, 2002							   mmdf(5)