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mmdf(5)				 User Manuals			       mmdf(5)

       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.

       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:

	      Subject: test

	      >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 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.

       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'

       ·      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
	      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 addition‐
	      ally 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 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.

       MMDF is not part of any currently supported standard.

       MMDF was developed at the University of Delaware by Dave Crocker.

       scomail(1), fcntl(2),  flock(2),	 link(2),  stat(2),  mbox(5),  RFC822,

       Urs Janssen <>

Unix			      February 18th, 2002		       mmdf(5)

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