Author: James CE Johnson

ACE Tutorial 016
Making ACE_Condition easier to use

The ACE framework has quite a few objects for syncronizing your threads and even processes. We've mentioned a few in passing already: ACE_Thread_Mutex and ACE_Barrier for instance.

Another interesting one is the ACE_Condition template. By using an ACE_Condition you can have your code wait for an arbitrary condition to occur. That condition is "embodied" in a variable of your choice. That variable can, in turn, be any data type you wish. This makes ACE_Condition much more flexible than a simple mutex, barrier or semaphore.

In this tutorial, I'll create a wrapper class around the ACE_Condition and the assorted housekeeping items necessary to make it work. I'll use a simple integer as the condition variable but keep in mind that you can use any data type you want.

Kirthika's abstract:

An ACE_Condition class is a synchronisation mechanism employed in situations where one or more threads cannot access the shared resource unless some 'condition' becomes true. The ACE_Condition is associated with a Mutex-lock which is released before blocking internally in the wait call. Once the blocked thread is signaled to wake up again it internally re-acquires the lock before checking the condition. Unless the condition is true and it has the lock, it cant go ahead. Once the shared resource is freed, a signal is sent to the waiting threads which can now contend for the lock and access the resource.

Pizza-delivery metaphor: (courtesy Dr.Schmidt)

Pizza delivery boy -- thread
keys to delivery van -- mutex-lock
pizza ready for delivery -- condition

Situation:

Five pizza delivery boys use the same van. While the van is unavailable, the boys go to sleep. When the van returns and the keys are returned, the current user of the van nudges the other sleeping boys to wake up and fight for the keys. Once the keys are obtained and the pizza is ready and out of the kitchen, the boy with the pizza and the keys can now deliver it.

This tutorial makes use of a wrapper class around the ACE_Condition and uses a integer value as the condition. The four kinds of condition implemented are: !=, >=, <=, == by using C++ operator overloading. Guards are used within the methods to make sure that the method is thread-safe. Once the thread completes its job, it broadcasts to the waiting on it.

An ACE_Task is used to stress test the various conditions. A test object is created for each condition and the main thread waits until the condition becomes true. For instance: <= condition: Five threads are spwaned which in turn increment the condition variable, which is initialised to 1 by 2. Remember that you are waiting on the <= condition. So once 3 threads have been thru it, the value becomes 6 and the condition becomes true!

This simple example shows us how to program and use the Condition variable for synchronisation.

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