condvar: +Using Condition Variables

condvarglobal

This +example shows the use of the global condition variable IPC mechanism. The +scope of a global condition variable is inter-process. It can be shared by +threads of any process in the system.

Download

Click +on the following link to download the example: condvarglobal .zip

Click browse to view the example code

Class summary

RCondVar RMutex RChunk CPeriodic

Description

This +example uses an adder and subtractor pattern to show the use of a global condition +variable. Two processes, the adder and the subtractor, modify a shared variable +by adding and subtracting random amounts. The condition variable ensures that +the value remains within given limits, KMaxValue and KMinValue, +by blocking one of the processes if the value crosses a warning threshold.

The +adder program creates a global shared memory chunk. It also creates a global +mutex to control access to the chunk and a global condition variable to signal +that the value in the chunk is 'ready for use'. It then initialises +the value in the chunk to zero and periodically tries to add a random value +between 1 and 10 . If, having added a value, it finds that the value of the +chunk is greater than KUpperThreshold, it waits for a signal +from the condition variable before adding another value.

The subtractor +program periodically tries to subtract a random value between 1 and 10 from +the global shared memory chunk. If, having subtracted a value, it finds that +the value of the chunk is less than KLowerThreshold, it waits +for a signal from the condition variable before subtracting another value.

Design +and implementation

+ +

Build

The +Symbian OS build process describes how to build this example. The ConditionVariable +example builds the following binary files in the standard location (\epoc32\release\winscw\ + <build_variant>) for Carbide.c++.

adder.exe: Demonstrates +the use of the global condition variable. It creates a global shared memory +chunk and periodically adds random values to the chunk.
subtractor.exe: Demonstrates +the use of the global condition variable. It periodically subtracts random +values from the chunk created by adder.exe.

How to run the Example

To run the example, perform +the following steps:

Run adder.exe.
Run eshell.
Run subtract.exe in +your new eshell.
Switch between eshells +by pressing CTRL+ALT+SHIFT+T.
To finish, stop each +application by pressing any key.

condvarlocal

This +example shows the use of the local condition variable IPC mechanism. The scope +of a local condition variable is intra-process. It can be shared by threads +of the process that creates the condition variable.

Download

Click +on the following link to download the example: condvarlocal .zip

Click browse to view the example.

Class summary

RMutex RThread CPeriodic RCondVar

Description

This +example uses the producer and the consumer model to show the use of the local +condition variable.

The example creates two local threads: a producer +and a consumer. The two threads share a buffer, which is an object of the CQueue class. +The CQueue object creates a local condition variable using the RCondVar::CreateLocal() function. +It also defines the methods to insert and remove a token from the queue. The CQueue::Insert() function +inserts a token into the queue and signals the condition variable. The CQueue::Remove() function +tries to remove a token from the queue. If the queue is empty, it must wait +for a signal from the condition variable.

An object of the CProducer class +creates and calls the producer thread. The producer thread is called once +every two seconds using an object of the CPeriodic class. +This thread inserts a token into the queue when it is called. It calls the CQueue::Insert() function +on the shared CQueue object.

An object of the CConsumer class +creates and calls the consumer thread. The consumer thread is called once +a second using an object of the CPeriodic class. This thread +removes a token from the queue when it is called. It calls the CQueue::Remove() function +on the shared CQueue object.

For more information, +refer to Condition Variables.

Symbian is not +responsible for the content of external websites.

Design +and implementation

+ +

Build

condvarlocal.exe: +Demonstrates the use of the local condition variable.

How to run +the Example

To run the example, perform the following steps:

Run condvarlocal.exe. +The program calls the producer and the consumer threads periodically as shown +in the description section. It also displays a menu.
Press ‘d’ to +display the contents of the queue.
Press ‘p’ to +insert a token into the queue.
Press any key to stop +the program.