.TH DOTLOCKFILE 1 "September 8, 2009" "" "Cistron Utilities"
.SH NAME
dotlockfile \- Utility to manage lockfiles
.SH SYNOPSIS
.B /usr/bin/dotlockfile
.RB [ \-l
.RB [ \-r
.IR retries ]
.RB | \-u | \-t | \-c ]
.RB [ \-p ]
.RB [ \-m | lockfile ]
.SH DESCRIPTION
.B dotlockfile
is a command line utility to reliably create, test and remove lockfiles.
It creates lockfiles
.I reliably
on local and NFS filesystems, because the crucial steps of testing for a
preexisting lockfile and creating it are performed
.I atomically
by a
.I single
call to
.IR link (2).
Manpage
.IR lockfile_create (3)
describes the used algorithm.
.PP
.B dotlockfile
is installed with attribute
.BI SETGID \ mail
and thus can also be used to lock and unlock mailboxes
.I even
if the mailspool directory is only writable by group mail.
.PP
The name
.B 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
"\fI.lock\fR" appended.
.PP
The names
.B dotlock
and
.B lockfile
were already taken \- hence the name \fBdotlockfile\fR \fI:)\fR.
.SH OPTIONS
.IP "\fB\-l\fR"
Create a lockfile if no preexisting valid lockfile is found, else wait and retry
according to option \fB\-r\fR.
This option is the default.

A lockfile is treated as valid,
.br
\[bu]\ \ if it holds the
.I process\-id
of a running process,
.br
\[bu]\ \ or if it does not hold any
.I process\-id
and has been touched less than 5\ minutes ago (timestamp is younger than
5\ minutes).
.IP "\fB\-r retries\fR"
The number of times
.B 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 "\fB\-r 0\fR".
To try indefinitely, use "\fB\-r -1\fR".
.IP "\fB\-u\fR"
Remove a lockfile.
.IP "\fB\-t\fR"
Touch an existing lockfile (update the timestamp).
Useful for lockfiles on NFS filesystems.
For lockfiles on local filesystems the
.B \-p
option is preferable.
.IP "\fB\-c\fR"
For debugging only:  Check for the existence of a \fIvalid\fR lockfile.

Note:  Testing for a preexisting lockfile and writing of the lockfile
.I must
be done by the
.I same
"\fBdotlockfile \-l\fR" or "\fBdotlockfile \-m\fR" command, else the lockfile
creation cannot be reliable.
.IP "\fB\-p\fR"
Write the
.I process\-id
of the calling process into the lockfile.
Also when testing for an existing lockfile, check the contents for the
.I process\-id
of a running process to verify if the lockfile is still valid.
Obviously useful only for lockfiles on local filesystems.
.IP "\fB\-m\fR"
Lock or unlock the current users mailbox.
The path to the mailbox is the default system mailspool directory (usually
.IR /var/mail )
with the username as gotten from
.IR getpwuid ()
appended.
If the environment variable
.I $MAIL
is set, that is used instead.
Then the string "\fI.lock\fR" is appended to get the name of the actual
lockfile.
.IP lockfile
The lockfile to be created or removed.
Must not be specified, if the \fB\-m\fR option is in effect.
.SH RETURN\ VALUE
Zero on success, and non\-zero on failure.
For the \fB\-c\fR option, sucess means that a valid lockfile is already present.
When locking (the default, or with the \fB\-l\fR option)
.B dotlockfile
returns the same values as the library function
.IR lockfile_create (3).
Unlocking a non\-existant lockfile is not an error.
.SH NOTES
The lockfile is created exactly as named on the command line.
The extension "\fI.lock\fR" is \fInot\fR automatically appended.
.PP
This utility is a lot like the
.IR lockfile (1)
utility included with
.IR procmail ,
and the
.IR mutt_dotlock (1)
utility included with
.IR mutt .
However the command\-line arguments differ, and so does the return status.
It is believed, that
.I dotlockfile
is the most flexible implementation, since it automatically detects when it
needs to use priviliges to lock a mailbox, and does it safely.
.PP
The above mentioned
.IR lockfile_create (3)
manpage is present in the
.I liblockfile\-dev
package.
.SH BUGS
None known.
.SH SEE\ ALSO
.IR lockfile_create (3),
.IR maillock (3)
.SH AUTHOR
Miquel van Smoorenburg, miquels@cistron.nl