Section: Cistron Utilities (1)
Updated: September 8, 2009
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?(2). 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.
- 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 seconds 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".:
- Remove a lockfile.:
- Touch an existing lockfile (update the timestamp). Useful for lockfiles on NFS filesystems. For lockfiles on local filesystems the -p option is preferable.:
- For debugging only: Check for the existence of a valid lockfile.
Note: Testing for a preexisting lockfile and writing of the lockfile must be done by the same "dotlockfile -l" or "dotlockfile -m" command, else the lockfile creation cannot be reliable.
- 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 lockfile is still valid. Obviously useful only for lockfiles on local filesystems.:
- Lock or unlock the current users mailbox. The path to the mailbox 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.:
- The lockfile to be created or removed. Must not be specified, if the -m option is in effect.:
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.
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?(1) utility included with procmail, 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.
Miquel van Smoorenburg, [email protected]