Ubuntu Manpage: MMDF - Multi-channel Memorandum Distribution Facility mailbox format

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:

• 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 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.

AUTHOR