mmdf

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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 spe
   cial 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:

         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.

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

         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  un
          linked.

          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)