# man > mmdf(5)

[_mmdf_(5)](https://www.chedong.com/phpMan.php/man/mmdf/5/markdown)                                     User Manuals                                     [_mmdf_(5)](https://www.chedong.com/phpMan.php/man/mmdf/5/markdown)

## 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.  [**scomail**(1)](https://www.chedong.com/phpMan.php/man/scomail/1/markdown))
       to store mail messages locally.

       An  **MMDF **mailbox is a text file containing an arbitrary number of e-mail messages.  Each mes‐
       sage 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 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:

              ^A^A^A^A
              From: <example@example.com>
              To: <example@example.org>
              Subject: test

              >From what I learned about the MMDF-format:
              ^A^A^A^A
              ^A^A^A^A
              From: <example@example.com>
              To: <example@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)](https://www.chedong.com/phpMan.php/man/mbox/5/markdown))
       there is no need to quote/dequote "From "-lines in **MMDF **mailboxes as such lines have no  spe‐
       cial meaning in this format.

       If  the  modification-time  (usually  determined  via  [**stat**(2)](https://www.chedong.com/phpMan.php/man/stat/2/markdown)) 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:

       •      [**fcntl**(2)](https://www.chedong.com/phpMan.php/man/fcntl/2/markdown) locking is mostly used on recent, POSIX-compliant systems. Use of this  lock‐
              ing 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.

       •      [**flock**(2)](https://www.chedong.com/phpMan.php/man/flock/2/markdown) 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 direc‐
              tory in which the _folder_ resides. The application then tries to use the [**link**(2)](https://www.chedong.com/phpMan.php/man/link/2/markdown) system
              call to create a hard link named _folder.lock_ to the temporary file. The success of the
              [**link**(2)](https://www.chedong.com/phpMan.php/man/link/2/markdown)  system  call should be additionally verified using [**stat**(2)](https://www.chedong.com/phpMan.php/man/stat/2/markdown) 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 vari‐
       ants of the [**fcntl**(2)](https://www.chedong.com/phpMan.php/man/fcntl/2/markdown) and [**flock**(2)](https://www.chedong.com/phpMan.php/man/flock/2/markdown) system calls in order to avoid deadlocks.

       If  multiple  methods are combined, an **MMDF **file must not be considered to have been success‐
       fully 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. Fail‐
       ure 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)](https://www.chedong.com/phpMan.php/man/scomail/1/markdown), [**fcntl**(2)](https://www.chedong.com/phpMan.php/man/fcntl/2/markdown), [**flock**(2)](https://www.chedong.com/phpMan.php/man/flock/2/markdown), [**link**(2)](https://www.chedong.com/phpMan.php/man/link/2/markdown), [**stat**(2)](https://www.chedong.com/phpMan.php/man/stat/2/markdown), [**mbox**(5)](https://www.chedong.com/phpMan.php/man/mbox/5/markdown), **RFC822**, **RFC2822**


## AUTHOR
       Urs Janssen <<urs@tin.org>>

Unix                                     February 18th, 2002                                 [_mmdf_(5)](https://www.chedong.com/phpMan.php/man/mmdf/5/markdown)