DOTLOCKFILE(1)							 Cistron Utilities						    DOTLOCKFILE(1)

NAME

       dotlockfile - Utility to manage lockfiles

SYNOPSIS

       /usr/bin/dotlockfile [-l [-r retries] |-u|-t|-c] [-p] [-m|lockfile]

DESCRIPTION

       dotlockfile  is	a  command  line  utility  to  reliably create, test and remove lockfiles.  It creates lockfiles reliably on local and NFS
       filesystems, because the crucial steps of testing for a preexisting lockfile and creating it are performed atomically by a single  call	to
       link(2).  Manpage lockfile_create(3) describes the used algorithm.

       dotlockfile  is installed with attribute SETGID mail and thus can also be used to lock and unlock mailboxes even if the mailspool directory
       is only writable by group mail.

       The name dotlockfile comes from the way mailboxes are locked for updates on a lot of UNIX systems.  A lockfile is  created  with  the  same
       filename as the mailbox but with the string ".lock" appended.

       The names dotlock and lockfile were already taken - hence the name dotlockfile :).

OPTIONS

       -l     Create  a  lockfile  if  no  preexisting	valid  lockfile  is found, else wait and retry according to option -r.	This option is the
	      default.

	      A lockfile is treated as valid,
	      o  if it holds the process-id of a running process,
	      o  or if it does not hold any process-id and has been touched less than 5 minutes ago (timestamp is younger than 5 minutes).

       -r retries
	      The number of times dotlockfile retries to acquire the lock if it failed the first time before giving up.  The initial  sleep  after
	      failing to acquire the lock is 5 seconds.  After each retry the sleep intervall is increased incrementally by 5 seconds up to a max-
	      imum sleep of 60 seconds between tries.  The default number of retries is 5.  To try only once, use "-r 0".   To	try  indefinitely,
	      use "-r -1".

       -u     Remove a lockfile.

       -t     Touch an existing lockfile (update the timestamp).  Useful for lockfiles on NFS filesystems.  For lockfiles on local filesystems the
	      -p option is preferable.

       -c     For debugging only:  Check for the existence of a valid lockfile.

	      Note:  Testing for a preexisting lockfile and writing of the lockfile must be done by the same "dotlockfile -l" or "dotlockfile  -m"
	      command, else the lockfile creation cannot be reliable.

       -p     Write  the  process-id of the calling process into the lockfile.	Also when testing for an existing lockfile, check the contents for
	      the process-id of a running process to verify if the lockfile is still valid.  Obviously useful only for lockfiles on local filesys-
	      tems.

       -m     Lock  or	unlock	the  current users mailbox.  The path to the mailbox is the default system mailspool directory (usually /var/mail)
	      with the username as gotten from getpwuid() appended.  If the environment variable $MAIL is set, that is	used  instead.	 Then  the
	      string ".lock" is appended to get the name of the actual lockfile.

       lockfile
	      The lockfile to be created or removed.  Must not be specified, if the -m option is in effect.

RETURN VALUE

       Zero  on  success,  and	non-zero on failure.  For the -c option, sucess means that a valid lockfile is already present.  When locking (the
       default, or with the -l option) dotlockfile returns the same values as the library function lockfile_create(3).	Unlocking  a  non-existant
       lockfile is not an error.

NOTES

       The lockfile is created exactly as named on the command line.  The extension ".lock" is not automatically appended.

       This utility is a lot like the lockfile(1) utility included with procmail, and the mutt_dotlock(1) utility included with mutt.  However the
       command-line arguments differ, and so does the return status.  It is believed, that dotlockfile is the most flexible implementation,  since
       it automatically detects when it needs to use priviliges to lock a mailbox, and does it safely.

       The above mentioned lockfile_create(3) manpage is present in the liblockfile-dev package.

BUGS

       None known.

SEE ALSO

       lockfile_create(3), maillock(3)

AUTHOR

       Miquel van Smoorenburg, miquels@cistron.nl

								 September 8, 2009						    DOTLOCKFILE(1)