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 RFC822 / RFC2822, followed by a post-
mark. The file format is line-oriented. Lines are separated by line
feed characters (ASCII 10). A postmark line consists of the four char-
acters "^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 mes-
sages have already been read.
LOCKING
Since MMDF files are frequently accessed by multiple programs in paral-
lel, 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 sys-
tems. 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 re-
sides. 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 ob-
tained. When one of the individual locking methods fails, an applica-
tion 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)
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