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,
              •  if it holds the process-id of a running process,
              •  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 maximum 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 filesystems.

       -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)