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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 RFC5322, 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) and RFC4155) 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), RFC4155,
       RFC5322


AUTHOR

       Urs Janssen <urs@tin.org>

Unix                          November 5th, 2013                       mmdf(5)

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