| 1 | .\" Project : tin |
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| 2 | .\" Module : mmdf.5 |
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| 3 | .\" Author : U. Janssen |
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| 4 | .\" Created : 2002-02-18 |
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| 5 | .\" Updated : |
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| 6 | .\" Notes : needs a lot of work |
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| 7 | .\" |
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| 8 | .TH mmdf 5 "February 18th, 2002" "Unix" "User Manuals" |
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| 9 | .\" |
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| 10 | .SH NAME |
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| 11 | MMDF \- Multi\-channel Memorandum Distribution Facility mailbox format |
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| 12 | .\" |
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| 13 | .SH DESCRIPTION |
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| 14 | This document describes the |
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| 15 | .B MMDF |
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| 16 | mailbox format used by some MTAs and MUAs (i.e. |
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| 17 | .BR scomail (1)) |
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| 18 | to store mail messages locally. |
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| 19 | .PP |
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| 20 | An |
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| 21 | .B MMDF |
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| 22 | mailbox is a text file containing an arbitrary number of e-mail messages. |
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| 23 | Each message consists of a postmark, followed by an e-mail message formatted |
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| 24 | according to \fBRFC822\fP / \fBRFC2822\fP, followed by a postmark. The file |
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| 25 | format is line-oriented. Lines are separated by line feed characters (ASCII |
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| 26 | 10). A postmark line consists of the four characters "^A^A^A^A" (Control-A; |
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| 27 | ASCII 1). |
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| 28 | .TP |
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| 29 | Example of a \fBMMDF\fP mailbox holding two mails: |
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| 30 | .RS |
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| 31 | .nf |
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| 32 | .sp |
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| 33 | ^A^A^A^A |
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| 34 | .br |
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| 35 | From: example@example.com |
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| 36 | .br |
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| 37 | To: example@example.org |
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| 38 | .br |
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| 39 | Subject: test |
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| 40 | .br |
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| 41 | .sp |
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| 42 | .br |
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| 43 | >From what I learned about the MMDF-format: |
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| 44 | .br |
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| 45 | ... |
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| 46 | .br |
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| 47 | ^A^A^A^A |
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| 48 | .br |
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| 49 | ^A^A^A^A |
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| 50 | .br |
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| 51 | From: example@example.com |
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| 52 | .br |
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| 53 | To: example@example.org |
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| 54 | .br |
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| 55 | Subject: test 2 |
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| 56 | .br |
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| 57 | .sp |
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| 58 | .br |
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| 59 | bar |
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| 60 | .br |
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| 61 | ^A^A^A^A |
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| 62 | .fi |
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| 63 | .RE |
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| 64 | .PP |
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| 65 | In contrast to most other single file mailbox formats like |
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| 66 | MBOXO and MBOXRD (see |
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| 67 | .BR mbox (5)) |
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| 68 | there is no need to quote/dequote "From "-lines in |
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| 69 | .B MMDF |
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| 70 | mailboxes as such lines have no special meaning in this format. |
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| 71 | .PP |
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| 72 | If the modification-time (usually determined via |
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| 73 | .BR stat (2)) |
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| 74 | of a nonempty mailbox file is greater than the access-time |
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| 75 | the file has new mail. Many MUAs place a Status: header in |
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| 76 | each message to indicate which messages have already been |
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| 77 | read. |
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| 78 | .\" |
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| 79 | .SH LOCKING |
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| 80 | Since |
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| 81 | .B MMDF |
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| 82 | files are frequently accessed by multiple programs in parallel, |
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| 83 | .B MMDF |
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| 84 | files should generally not be accessed without locking. |
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| 85 | .PP |
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| 86 | Three different locking mechanisms (and combinations thereof) are in |
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| 87 | general use: |
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| 88 | .IP "\(bu" |
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| 89 | .BR fcntl (2) |
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| 90 | locking is mostly used on recent, POSIX-compliant systems. Use of |
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| 91 | this locking method is, in particular, advisable if |
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| 92 | .B MMDF |
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| 93 | files are accessed through the Network File System (NFS), since it |
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| 94 | seems the only way to reliably invalidate NFS clients' caches. |
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| 95 | .IP "\(bu" |
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| 96 | .BR flock (2) |
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| 97 | locking is mostly used on BSD-based systems. |
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| 98 | .IP "\(bu" |
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| 99 | Dotlocking is used on all kinds of systems. In order to lock an |
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| 100 | .B MMDF |
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| 101 | file named \fIfolder\fR, an application first creates a temporary file |
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| 102 | with a unique name in the directory in which the |
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| 103 | \fIfolder\fR resides. The application then tries to use the |
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| 104 | .BR link (2) |
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| 105 | system call to create a hard link named \fIfolder.lock\fR |
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| 106 | to the temporary file. The success of the |
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| 107 | .BR link (2) |
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| 108 | system call should be additionally verified using |
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| 109 | .BR stat (2) |
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| 110 | calls. If the link has succeeded, the mail folder is considered |
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| 111 | dotlocked. The temporary file can then safely be unlinked. |
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| 112 | .IP "" |
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| 113 | In order to release the lock, an application just unlinks the |
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| 114 | \fIfolder.lock\fR file. |
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| 115 | .PP |
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| 116 | If multiple methods are combined, implementors should make sure to |
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| 117 | use the non-blocking variants of the |
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| 118 | .BR fcntl (2) |
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| 119 | and |
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| 120 | .BR flock (2) |
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| 121 | system calls in order to avoid deadlocks. |
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| 122 | .PP |
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| 123 | If multiple methods are combined, an |
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| 124 | .B MMDF |
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| 125 | file must not be considered to have been successfully locked before |
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| 126 | all individual locks were obtained. When one of the individual |
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| 127 | locking methods fails, an application should release all locks it |
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| 128 | acquired successfully, and restart the entire locking procedure from |
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| 129 | the beginning, after a suitable delay. |
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| 130 | .PP |
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| 131 | The locking mechanism used on a particular system is a matter of |
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| 132 | local policy, and should be consistently used by all applications |
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| 133 | installed on the system which access |
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| 134 | .B MMDF |
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| 135 | files. Failure to do so may result in loss of e-mail data, and in |
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| 136 | corrupted |
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| 137 | .B MMDF |
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| 138 | files. |
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| 139 | .\" |
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| 140 | .\" .SH FILES |
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| 141 | .\" /usr/spool/mmdf/lock/home |
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| 142 | .\" $HOME/Mail/ |
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| 143 | .\" |
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| 144 | .\" .SH SECURITY |
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| 145 | .\" |
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| 146 | .SH "CONFORMING TO" |
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| 147 | .B MMDF |
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| 148 | is not part of any currently supported standard. |
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| 149 | .\" |
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| 150 | .SH HISTORY |
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| 151 | .B MMDF |
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| 152 | was developed at the University of Delaware by Dave Crocker. |
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| 153 | .\" |
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| 154 | .SH "SEE ALSO" |
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| 155 | .BR scomail (1), |
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| 156 | .BR fcntl (2), |
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| 157 | .BR flock (2), |
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| 158 | .BR link (2), |
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| 159 | .BR stat (2), |
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| 160 | .BR mbox (5), |
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| 161 | .BR RFC822 , |
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| 162 | .BR RFC2822 |
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| 163 | |
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| 164 | .SH AUTHOR |
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| 165 | Urs Janssen <urs@tin.org> |
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