Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Mail Systems
Eclipse Documentation

How To Guides
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Problem Solutions
Privacy Policy




Ruby Programming
Previous Page Home Next Page

Condition Variables

Using a mutex to protect critical data is sometimes not enough. Suppose you are in a critical section, but you need to wait for some particular resource. If your thread goes to sleep waiting for this resource, it is possible that no other thread will be able to release the resource because it cannot enter the critical section---the original process still has it locked. You need to be able to give up temporarily your exclusive use of the critical region and simultaneously tell people that you're waiting for a resource. When the resource becomes available, you need to be able to grab it and reobtain the lock on the critical region, all in one step.

This is where condition variables come in. A condition variable is simply a semaphore that is associated with a resource and is used within the protection of a particular mutex. When you need a resource that's unavailable, you wait on a condition variable. That action releases the lock on the corresponding mutex. When some other thread signals that the resource is available, the original thread comes off the wait and simultaneously regains the lock on the critical region.

require 'thread'
mutex =
cv =

a = {   mutex.synchronize {     puts "A: I have critical section, but will wait for cv"     cv.wait(mutex)     puts "A: I have critical section again! I rule!"   } }

puts "(Later, back at the ranch...)"

b = {   mutex.synchronize {     puts "B: Now I am critical, but am done with cv"     cv.signal     puts "B: I am still critical, finishing up"   } } a.join b.join
A: I have critical section, but will wait for cv(Later, back at the ranch...)

B: Now I am critical, but am done with cv B: I am still critical, finishing up A: I have critical section again! I rule!

For alternative implementations of synchronization mechanisms, see monitor.rb and sync.rb in the lib subdirectory of the distribution.
Ruby Programming
Previous Page Home Next Page

  Published under the terms of the Open Publication License Design by Interspire