dotlockfile Man page

DOTLOCKFILE(1) Cistron Utilities DOTLOCKFILE(1)

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. 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 dot‐
lockfile :).

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 sec‐
onds 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 filesys‐
tems the -p option is preferable.

-c For debugging only: Check for the existence of a valid lock‐
file.

Note: Testing for a preexisting lockfile and writing of the
lockfile must be done by the same “dotlockfile -l” or “dotlock‐
file -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 lock‐
file is still valid. Obviously useful only for lockfiles on
local filesystems.

-m Lock or unlock the current users mailbox. The path to the mail‐
box 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 utility included with proc‐
mail, 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)