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 0.8.5                                 2018-10-09                                         dotlock(1)