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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. sco-
mail(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 sep-
arated 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
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:
o 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.
o flock(2) locking is mostly used on BSD-based systems.
o 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 di-
rectory 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 suc-
cess 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 tem-
porary 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 suc-
cessfully 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.
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)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
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