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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 mes‐
       sage 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 AT example.com
              To: example AT example.org
              Subject: test

              >From what I learned about the MMDF-format:
              ^A^A^A^A
              ^A^A^A^A
              From: example AT example.com
              To: example AT 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  lock‐
              ing 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 direc‐
              tory 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 vari‐
       ants 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 success‐
       fully 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. Fail‐
       ure 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 AT tin.org>

Unix                                     February 18th, 2002                                 mmdf(5)