jcs.org
mutt stable branch with some hacks
1.\" Project : tin
2.\" Module : mmdf.5
3.\" Author : U. Janssen
4.\" Created : 2002-02-18
5.\" Updated :
6.\" Notes : needs a lot of work
7.\"
8.TH mmdf 5 "February 18th, 2002" "Unix" "User Manuals"
9.\"
10.SH NAME
11MMDF \- Multi\-channel Memorandum Distribution Facility mailbox format
12.\"
13.SH DESCRIPTION
14This document describes the
15.B MMDF
16mailbox format used by some MTAs and MUAs (i.e.
17.BR scomail (1))
18to store mail messages locally.
19.PP
20An
21.B MMDF
22mailbox is a text file containing an arbitrary number of e-mail messages.
23Each message consists of a postmark, followed by an e-mail message formatted
24according to \fBRFC822\fP / \fBRFC2822\fP, followed by a postmark. The file
25format is line-oriented. Lines are separated by line feed characters (ASCII
2610). A postmark line consists of the four characters "^A^A^A^A" (Control-A;
27ASCII 1).
28.TP
29Example of a \fBMMDF\fP mailbox holding two mails:
30.RS
31.nf
32.sp
33^A^A^A^A
34.br
35From: example@example.com
36.br
37To: example@example.org
38.br
39Subject: test
40.br
41.sp
42.br
43>From what I learned about the MMDF-format:
44.br
45.br
46^A^A^A^A
47.br
48^A^A^A^A
49.br
50From: example@example.com
51.br
52To: example@example.org
53.br
54Subject: test 2
55.br
56.sp
57.br
58bar
59.br
60^A^A^A^A
61.fi
62.RE
63.PP
64In contrast to most other single file mailbox formats like
65MBOXO and MBOXRD (see
66.BR mbox (5))
67there is no need to quote/dequote "From "\-lines in
68.B MMDF
69mailboxes as such lines have no special meaning in this format.
70.PP
71If the modification-time (usually determined via
72.BR stat (2))
73of a nonempty mailbox file is greater than the access-time
74the file has new mail. Many MUAs place a Status: header in
75each message to indicate which messages have already been
76read.
77.\"
78.SH LOCKING
79Since
80.B MMDF
81files are frequently accessed by multiple programs in parallel,
82.B MMDF
83files should generally not be accessed without locking.
84.PP
85Three different locking mechanisms (and combinations thereof) are in
86general use:
87.IP "\(bu"
88.BR fcntl (2)
89locking is mostly used on recent, POSIX-compliant systems. Use of
90this locking method is, in particular, advisable if
91.B MMDF
92files are accessed through the Network File System (NFS), since it
93seems the only way to reliably invalidate NFS clients' caches.
94.IP "\(bu"
95.BR flock (2)
96locking is mostly used on BSD-based systems.
97.IP "\(bu"
98Dotlocking is used on all kinds of systems. In order to lock an
99.B MMDF
100file named \fIfolder\fR, an application first creates a temporary file
101with a unique name in the directory in which the
102\fIfolder\fR resides. The application then tries to use the
103.BR link (2)
104system call to create a hard link named \fIfolder.lock\fR
105to the temporary file. The success of the
106.BR link (2)
107system call should be additionally verified using
108.BR stat (2)
109calls. If the link has succeeded, the mail folder is considered
110dotlocked. The temporary file can then safely be unlinked.
111.IP ""
112In order to release the lock, an application just unlinks the
113\fIfolder.lock\fR file.
114.PP
115If multiple methods are combined, implementors should make sure to
116use the non-blocking variants of the
117.BR fcntl (2)
118and
119.BR flock (2)
120system calls in order to avoid deadlocks.
121.PP
122If multiple methods are combined, an
123.B MMDF
124file must not be considered to have been successfully locked before
125all individual locks were obtained. When one of the individual
126locking methods fails, an application should release all locks it
127acquired successfully, and restart the entire locking procedure from
128the beginning, after a suitable delay.
129.PP
130The locking mechanism used on a particular system is a matter of
131local policy, and should be consistently used by all applications
132installed on the system which access
133.B MMDF
134files. Failure to do so may result in loss of e-mail data, and in
135corrupted
136.B MMDF
137files.
138.\"
139.\" .SH FILES
140.\" /usr/spool/mmdf/lock/home
141.\" $HOME/Mail/
142.\"
143.\" .SH SECURITY
144.\"
145.SH "CONFORMING TO"
146.B MMDF
147is not part of any currently supported standard.
148.\"
149.SH HISTORY
150.B MMDF
151was developed at the University of Delaware by Dave Crocker.
152.\"
153.SH "SEE ALSO"
154.BR scomail (1),
155.BR fcntl (2),
156.BR flock (2),
157.BR link (2),
158.BR stat (2),
159.BR mbox (5),
160.BR RFC822 ,
161.BR RFC2822
162
163.SH AUTHOR
164Urs Janssen <urs@tin.org>