dotlock - execute a command with a lock on a mailbox
dotlock [-LPW] mbox-file command
dotlock acquires a lock on the mailbox file mbox-file
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
, 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
- --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
- --nowait (-W)
- With this option, dotlock simply exits non-zero and does
not run command if it cannot immediately acquire the lock.
The Mail Avenger home page: <http://www.mailavenger.org/>.
dotlock does not perform fcntl
-style locking by default.
Thus, if your mail reader exclusively uses fcntl
for locking, there
will be race conditions unless you specify the --fcntl
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
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