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