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 RFC
822. The file format is line-oriented. Lines are sepa
rated 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 fol
lowed by a time stamp. The sender address is expected to
be an addrspec as defined in appendix D of RFC 822.

The date is expected to be formatted according to the fol
lowing syntax (represented in the augmented Backus-Naur
formalism used by RFC 822):

mbox-date = weekday month day time [ timezone ] year
weekday = "Mon" / "Tue" / "Wed" / "Thu" / "Fri"

/ "Sat" / "Sun"

month = "Jan" / "Feb" / "Mar" / "Apr" / "May"
/ "Jun" / "Jul" / "Aug" / "Sep"
/ "Oct" / "Nov" / "Dec"

day = 1*2DIGIT
time = 1*2DIGIT ":" 1*2DIGIT [ ":" 1*2DIGIT ]
timezone = ( "+" / "-" ) 4DIGIT
year = ( 4DIGIT / 2DIGIT )

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 pre
pared to accept non-numeric timezone information such as
"CET DST" for Central European Time, dailight saving time.

Example:

From roessler@does-not-exist.org Fri Jun 23 02:56:55 2000

In order to avoid mis-interpretation of lines in message
bodies which begin with the four characters "From", fol
lowed by a space character, the character ">" is commonly
prepended in front of such lines.

LOCKING

Since mbox files are frequently accessed by multiple pro
grams in parallel, mbox 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 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.

· flock(2) locking is mostly used on BSD-based sys
tems.

· Dotlocking is used on all kinds of systems. In
order to lock an mbox file named folder, an appli cation 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) sys
tem 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) sytem 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 individ
ual 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/user

user's incoming mail folder.

~user/mbox
user's archived mail messages, in his home direc
tory.

~user/Mail/
A directory in user's home directory which is com
monly used to hold mbox format folders.

SEE ALSO

elm(1), fcntl(2), flock(2), link(2), local(8), mail(1), maildir(5), mail.local(8), mutt(1), mutt_dotlock(1), pine(1), procmail(1), sendmail(8)

D. Crocker, Standard for the format of ARPA Internet text
messages, RFC 822

M. R. Horton, UUCP mail interchange format standard, RFC
976

AUTHOR

The present document was written by Thomas Roessler
<roessler@does-not-exist.org>.

HISTORY

The mbox format occured in Version 6 AT&T Unix.

A variant of this format was documented in RFC 976.

Unix August 2000 mbox(5)