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exim_lock(8) [opendarwin man page]

EXIM_LOCK(8)						      System Manager's Manual						      EXIM_LOCK(8)

NAME

       exim_lock - Mailbox maintenance

SYNOPSIS

       exim_lock [options]mailbox-file

DESCRIPTION

       The  exim_lock  utility	locks  a  mailbox  file  using	the same algorithm as Exim.  For a discussion of locking issues, see section 25.2.
       exim_lock can be used to prevent any modification of a mailbox by Exim or a user agent while investigating a problem.  The utility requires
       the  name  of  the file as its first argument.  If the locking is successful, the second argument is run as a command (using C's "system()"
       function); if there is no second argument, the value of the SHELL environment variable is used; if this is unset or empty, /bin/sh is  run.
       When the command finishes, the mailbox is unlocked and the utility ends.  The following options are available:

       -fcntl Use "fcntl()" locking on the open mailbox.

       -interval
	      This must be followed by a number, which is a number of seconds; it sets the interval to sleep between retries (default 3).

       -lockfile
	      Create a lock file before opening the mailbox.

       -mbx   Lock the mailbox using MBX rules.

       -q     Suppress verification output.

       -retries
	      This must be followed by a number; it sets the number of times to try to get the lock (default 10).

       -timeout
	      This must be followed by a number, which is a number of seconds; it sets a timeout to be used with a blocking "fcntl()" lock.  If it
	      is not set (the default), a non-blocking call is used.

       -v     Generate verbose output.

	      If none of -fcntl, -lockfile or -mbx are given, the default is to create a lock file and also use "fcntl()" locking on the  mailbox,
	      which  is  the same as Exim's default.  The use of -fcntl requires that the file be writable; the use of -lockfile requires that the
	      directory containing the file be writable.  Locking by lock file does not last for ever; Exim assumes that a lock file is expired if
	      it is more than 30 minutes old.

	      The  -mbx  option  is mutually exclusive with -fcntl.  It causes a shared lock to be taken out on the open mailbox, and an exclusive
	      lock on the file /tmp/.n.m where n and m are the device number and inode number of the mailbox file.  When the locking is  released,
	      if an exclusive lock can be obtained for the mailbox, the file in /tmp is deleted.

	      The  default  output  contains verification of the locking that takes place.  The -v option causes some additional information to be
	      given.  The -q option suppresses all output except error messages.

       A command such as

	 exim_lock /var/spool/mail/spqr

       runs an interactive shell while the file is locked, whereas

	 exim_lock -q /var/spool/mail/spqr <<End
	 <some commands>
	 End

       runs a specific non-interactive sequence of commands while the file is locked, suppressing all verification output.  A single  command  can
       be run by a command such as

	 exim_lock -q /var/spool/mail/spqr     "cp /var/spool/mail/spqr /some/where"

       Note that if a command is supplied, it must be entirely contained within the second argument - hence the quotes.

BUGS

       This manual page needs a major re-work. If somebody knows better groff than us and has more experience in writing manual pages, any patches
       would be greatly appreciated.

SEE ALSO

       exim(8), /usr/share/doc/exim4-base/

AUTHOR

       This manual page was stitched together from spec.txt by Andreas Metzler <ametzler at downhill.at.eu.org>, for the Debian  GNU/Linux  system
       (but may be used by others).

								  March 26, 2003						      EXIM_LOCK(8)

Check Out this Related Man Page

dotlock(1)							Mail Avenger 0.8.3							dotlock(1)

NAME

       dotlock - execute a command with a lock on a mailbox

SYNOPSIS

       dotlock [-LPW] mbox-file command [arg ...]

DESCRIPTION

       dotlock acquires a lock on the mailbox file mbox-file using both flock and a lock file, then executes command with any arguments specified.
       When command exits, dotlock releases the lock.

       dotlock attempts to clean up stale lockfiles.  If it succeeds in locking an mbox-file with flock, and roughly 30 seconds elapse without
       there being any changes to mbox-file or the lockfile, then dotlock will delete the lockfile and try again.

       While it holds a lock, lockfile will keep updating the modification time of the lockfile every 15 seconds, to prevent the lock from getting
       cleaned up in the event that command is slow.

   OPTION
       --noflock (-L)
	   Ordinarily, dotlock uses both flock and dotfile locking.  (It uses flock first, but releases that lock in the even that dotfile locking
	   fails, so as to avoid deadlocking with applications that proceed in the reverse order.)  The -L option disables flock locking, so that
	   dotlock only uses dotfile locking.

	   This is primarily useful as a wrapper around an application that already does flock locking, but to which you want to add dotfile
	   locking.  (Even if your mail delivery system doesn't use flock, flock actually improves the efficiency of dotlock, so there is no
	   reason to disable it.)

       --fcntl (-P)
	   This option enables fcntl (a.k.a. POSIX) file locking of mail spools, in addition to flock and dotfile locking.  The advantage of fcntl
	   locking is that it may do the right thing over NFS.	However, if either the NFS client or server does not properly support fcntl
	   locking, or if the file system is not mounted with the appropriate options, fcntl locking can fail in one of several ways.  It can
	   allow different processes to lock the same file concurrently--even on the same machine.  It can simply hang when trying to acquire a
	   lock, even if no other process holds a lock on the file.  Also, on some OSes it can interact badly with flock locking, because those
	   OSes actually implement flock in terms of fcntl.

       --nowait (-W)
	   With this option, dotlock simply exits non-zero and does not run command if it cannot immediately acquire the lock.

SEE ALSO

       avenger(1), deliver(1), avenger.local(8)

       The Mail Avenger home page: <http://www.mailavenger.org/>.

BUGS

       dotlock does not perform fcntl/lockf-style locking by default.  Thus, if your mail reader exclusively uses fcntl for locking, there will be
       race conditions unless you specify the --fcntl option.

       flock does not work over network file systems.  Thus, because of dotlock's mechanism for cleaning stale lock files, there is a possibility
       that a network outage could lead to a race condition where the lockfile is cleared before command finishes executing.  If lockfile detects
       that the lock has been stolen, it prints a message to standard error, but does not do anything else (like try to kill command).

AUTHOR

       David Mazieres

Mail Avenger 0.8.3						    2012-04-05								dotlock(1)
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