ulock(USP)

Name

ulock : Exclusive access control command

Synopsis

Usage   : ulock <lock-file>

          ulock -w <lock-file> <counter-file>

          ulock -r <lock-file> <counter-file> <command>

Option  : --timeout=<sec>

          --invalid=<sec>

Version : Mon Jan 17 11:50:14 JST 2022

Edition : 1

Description

Ulock command without option -w/-r creates an absolutely exclusive

zone.  In this case, ulock creates <lock-file> exclusively.

Ulock command optiwith on -r/-r creates a read-write lock.

When using -w (write lock), ulock creates <lock-file> exclusively

and waits until the link count on <counter-file> becomes 1.

When using -r (read lock), ulock increment the link count of

<counter-file> by one if <lock-file> does not exist, at which time

<command> is executed and the link count of <counter-file> is

decremented by one after <command> finished successfully.  The link

count is not decremented if <command> finished unsuccessfully.  In

this case, following message is issued.

Error(282)[ulock] : command '...' failed, file '...' is left.

The --timeout option specifies the maximum time to wait for

<command> to run.  If you specify -1 it will wait for ever. Default

is -1 (wait for ever).

The --invalid option deletes the <lock-files> which is created

before more tha specified seconds.  Default is 60 seconds.

Example 1 Create an Absolutely Exclusive Zone

$ cat lock.sh

#!/bin/bash

if ulock lock; then

  #

  # Read/Write Operation

  #

  rm -f lock

fi

Example 2 Read/Write Lock

$ cat writelock.sh

#!/bin/bash

if ulock -w lock counter; then

  # Write Operation (Example)

  up3 key=1 master tran > master.new

  mv master.new master

  rm -f lock

fi

$ cat readlock.sh

#!/bin/bash

ulock -r lock counter cat master

Note Read/Write Lock Algorithm

-- Write Lock

X-- Get a write lock (Atomic)

X-- Wait for counter to be zero

--- Write Operation

--- Release write lock

-- Read Lock

X-- Increment counter by 1 (Atomic)

X-- Confirm there is no write lock. If there is, decrement by 1 and return.

X-- Read only operation

X-- Decrement counter by 1 (Atomic)

Write locking in ulock uses the uniqueness of creating a hard link

file, and the counter operation uses the number of links (unique)

of the hard link file. In the above algorithms, ulock performs the

functions marked with X.

If lock-file and counter-file are created over NFS, then the read/write

lock is effective for every NFS client hosts.