dotlock - Online in the Cloud

PROGRAM:

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.

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