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 RFC 822.  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. 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 following 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 prepared 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", followed by a space character,
       the character ">" is commonly prepended in front of such lines.

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) 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 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/user
              user's incoming mail folder.

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

       ~user/Mail/
              A directory in user's home directory which is commonly 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)
