--- /dev/null
+.\" Project : tin
+.\" Module : mmdf.5
+.\" Author : U. Janssen
+.\" Created : 2002-02-18
+.\" Updated :
+.\" Notes : needs a lot of work
+.\"
+.TH mmdf 5 "February 18th, 2002" "Unix" "User Manuals"
+.\"
+.SH NAME
+MMDF \- Multi\-channel Memorandum Distribution Facility mailbox format
+.\"
+.SH DESCRIPTION
+This document describes the
+.B MMDF
+mailbox format used by some MTAs and MUAs (i.e.
+.BR scomail (1))
+to store mail messages locally.
+.PP
+An
+.B 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 \fBRFC822\fP / \fBRFC2822\fP, 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).
+.TP
+Example of a \fBMMDF\fP mailbox holding two mails:
+.RS
+.nf
+.sp
+^A^A^A^A
+.br
+From: example@example.com
+.br
+To: example@example.org
+.br
+Subject: test
+.br
+.sp
+.br
+>From what I learned about the MMDF-format:
+.br
+.br
+^A^A^A^A
+.br
+^A^A^A^A
+.br
+From: example@example.com
+.br
+To: example@example.org
+.br
+Subject: test 2
+.br
+.sp
+.br
+bar
+.br
+^A^A^A^A
+.fi
+.RE
+.PP
+In contrast to most other single file mailbox formats like
+MBOXO and MBOXRD (see
+.BR mbox (5))
+there is no need to quote/dequote "From "\-lines in
+.B MMDF
+mailboxes as such lines have no special meaning in this format.
+.PP
+If the modification-time (usually determined via
+.BR 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.
+.\"
+.SH LOCKING
+Since
+.B MMDF
+files are frequently accessed by multiple programs in parallel,
+.B MMDF
+files should generally not be accessed without locking.
+.PP
+Three different locking mechanisms (and combinations thereof) are in
+general use:
+.IP "\(bu"
+.BR fcntl (2)
+locking is mostly used on recent, POSIX-compliant systems. Use of
+this locking method is, in particular, advisable if
+.B MMDF
+files are accessed through the Network File System (NFS), since it
+seems the only way to reliably invalidate NFS clients' caches.
+.IP "\(bu"
+.BR flock (2)
+locking is mostly used on BSD-based systems.
+.IP "\(bu"
+Dotlocking is used on all kinds of systems. In order to lock an
+.B MMDF
+file named \fIfolder\fR, an application first creates a temporary file
+with a unique name in the directory in which the
+\fIfolder\fR resides. The application then tries to use the
+.BR link (2)
+system call to create a hard link named \fIfolder.lock\fR
+to the temporary file. The success of the
+.BR link (2)
+system call should be additionally verified using
+.BR stat (2)
+calls. If the link has succeeded, the mail folder is considered
+dotlocked. The temporary file can then safely be unlinked.
+.IP ""
+In order to release the lock, an application just unlinks the
+\fIfolder.lock\fR file.
+.PP
+If multiple methods are combined, implementors should make sure to
+use the non-blocking variants of the
+.BR fcntl (2)
+and
+.BR flock (2)
+system calls in order to avoid deadlocks.
+.PP
+If multiple methods are combined, an
+.B 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.
+.PP
+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
+.B MMDF
+files. Failure to do so may result in loss of e-mail data, and in
+corrupted
+.B MMDF
+files.
+.\"
+.\" .SH FILES
+.\" /usr/spool/mmdf/lock/home
+.\" $HOME/Mail/
+.\"
+.\" .SH SECURITY
+.\"
+.SH "CONFORMING TO"
+.B MMDF
+is not part of any currently supported standard.
+.\"
+.SH HISTORY
+.B MMDF
+was developed at the University of Delaware by Dave Crocker.
+.\"
+.SH "SEE ALSO"
+.BR scomail (1),
+.BR fcntl (2),
+.BR flock (2),
+.BR link (2),
+.BR stat (2),
+.BR mbox (5),
+.BR RFC822 ,
+.BR RFC2822
+
+.SH AUTHOR
+Urs Janssen <urs@tin.org>
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