Tribblix: manual page: mbox.5

mbox(5) User Manuals mbox(5)

NAME

mbox - Format for mail message storage.

DESCRIPTION

This document describes the format traditionally used by Unix hosts
to store mail messages locally. mbox files typically reside in the
system's mail spool, under various names in users' Mail directories,
and under the name mbox in users' home directories.

An mbox 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. The file format is
line-oriented. Lines are separated by line feed characters (ASCII
10).

A postmark line consists of the four characters "From", followed by a
space character, followed by the message's envelope sender address,
followed by whitespace, and followed by a time stamp. This line is
often called From_ line.

The sender address is expected to be addr-spec as defined in RFC2822
3.4.1. The date is expected to be date-time as output by asctime(3).
For compatibility reasons with legacy software, two-digit years
greater than or equal to 70 should be interpreted as the years 1970+,
while two-digit years less than 70 should be interpreted as the years
2000-2069. Software reading files in this format should also be
prepared to accept non-numeric timezone information such as "CET DST"
for Central European Time, daylight saving time.

Example:

>From example@example.com Fri Jun 23 02:56:55 2000

In order to avoid misinterpretation of lines in message bodies which
begin with the four characters "From", followed by a space character,
the mail delivery agent must quote any occurrence of "From " at the
start of a body line.

There are two different quoting schemes, the first (MBOXO) only
quotes plain "From " lines in the body by prepending a '>' to the
line; the second (MBOXRD) also quotes already quoted "From " lines by
prepending a '>' (i.e. ">From ", ">>From ", ...). The later has the
advantage that lines like

>From the command line you can use the '-p' option

aren't dequoted wrongly as a MBOXRD-MDA would turn the line into

>>From the command line you can use the '-p' option

before storing it. Besides MBOXO and MBOXRD there is also MBOXCL
which is MBOXO with a "Content-Length:"-field with the number of
bytes in the message body; some MUAs (like mutt(1)) do automatically
transform MBOXO mailboxes into MBOXCL ones when ever they write them
back as MBOXCL can be read by any MBOXO-MUA without any problems.

If the modification-time (usually determined via stat(2)) of a
nonempty mbox 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 mbox files are frequently accessed by multiple programs in
parallel, mbox files should generally not be accessed without
locking.

Three different locking mechanisms (and combinations thereof) are in
general use:

+o fcntl(2) locking is mostly used on recent, POSIX-compliant
systems. Use of this locking method is, in particular,
advisable if mbox files are accessed through the Network File
System (NFS), since it seems the only way to reliably
invalidate NFS clients' caches.

+o flock(2) locking is mostly used on BSD-based systems.

+o Dotlocking is used on all kinds of systems. In order to lock
an mbox 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 additionally 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 mbox 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.

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 mbox files. Failure to do so may
result in loss of e-mail data, and in corrupted mbox files.

FILES

/var/spool/mail/$LOGNAME
$LOGNAME's incoming mail folder.

$HOME/mbox
user's archived mail messages, in his $HOME directory.

$HOME/Mail/
A directory in user's $HOME directory which is commonly used
to hold mbox format folders.

AUTHOR

Thomas Roessler <roessler@does-not-exist.org>, Urs Janssen
<urs@tin.org>

HISTORY