MMDF - Multi-channel Memorandum Distribution Facility mailbox format
This document describes the MMDF
mailbox format used by some MTAs and
MUAs (i.e. scomail
(1)) to store mail messages locally.
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
, 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:
>From what I learned about the MMDF-format:
Subject: test 2
In contrast to most other single file mailbox formats like MBOXO and MBOXRD (see
(5)) there is no need to quote/dequote "From "-lines in
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
files are frequently accessed by multiple programs in
files should generally not be accessed without locking.
Three different locking mechanisms (and combinations thereof) are in general
- 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
- flock(2) locking is mostly used on BSD-based
- 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
is not part of any currently supported standard.
was developed at the University of Delaware by Dave Crocker.
Urs Janssen <email@example.com>