<concept id="GUID-5C1E776D-5AEF-5326-BEA6-F2108F42CB71" xml:lang="en"><title>condvar: Using Condition Variables</title><shortdesc>Examples that explains how to use condition variables. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<section id="GUID-8209AA7E-0DFB-512D-9BEB-9063BA617BC5"><title>condvarglobal</title> <p>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. </p> <p><b>Download</b> </p> <p>Click on the following link to download the

example: <xref href="guid-6013a680-57f9-415b-8851-c4fa63356636/zips/guid-ddcb07c8-2646-4414-b33f-086f5758cbfe.zip" scope="external">condvarglobal .zip</xref></p><p>Click <xref href="guid-6013a680-57f9-415b-8851-c4fa63356636/guid-ddcb07c8-2646-4414-b33f-086f5758cbfe.html" scope="peer"> browse </xref> to view the example code</p><p><b>Class

summary</b></p><p> <xref href="GUID-D16EF740-78E6-3D08-AE2F-AFA5E812FF2B.dita"><apiname>RCondVar </apiname></xref> <xref href="GUID-C0FEA3A0-7DD3-3B87-A919-CB973BC05766.dita"><apiname>RMutex </apiname></xref> <xref href="GUID-326A2F4D-0E99-31C0-A35D-E8BF45913F07.dita"><apiname>RChunk </apiname></xref> <xref href="GUID-3CECC9FC-58C1-3117-AAF2-FDF88341F56F.dita"><apiname>CPeriodic</apiname></xref> </p> <p><b>Description</b> </p> <p>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, <codeph>KMaxValue</codeph> and <codeph>KMinValue</codeph>, by blocking one of the processes if the value

crosses a warning threshold. </p> <p>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 '<codeph>ready for use</codeph>'. 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 <codeph>KUpperThreshold</codeph>, it waits for a signal from the condition variable before adding

another value. </p> <p>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 <codeph>KLowerThreshold</codeph>, it waits for

a signal from the condition variable before subtracting another value. </p> <p><b>Design and implementation</b> </p> <fig id="GUID-1FAFB189-D2E4-5A05-8D8B-B3F7D28467FC">

<image href="GUID-857A523E-E660-5AFA-98B1-3A5440A8372F_d0e357778_href.jpg" placement="inline"/>

</fig> <p><b>Build</b> </p> <p>The ConditionVariable example builds

the following binary files in the standard location (<filepath>\epoc32\release\winscw\

<li id="GUID-A699F4AF-3A5F-5C24-83CB-1762348AA1D2"><p> <b>adder.exe:</b> Demonstrates the use of the global condition variable. It creates

a global shared memory chunk and periodically adds random values to

the chunk. </p> </li>

<li id="GUID-30A5BE77-8F99-5206-AD42-51E843D0D3C2"><p> <b>subtractor.exe:</b> Demonstrates the use of the global condition variable. It periodically

subtracts random values from the chunk created by <filepath>adder.exe</filepath>. </p> </li>

</ul> <p><b>How to run the Example</b> </p> <p>To run the example,

perform the following steps: </p> <ol id="GUID-312A1E16-C92B-5C33-9D80-83080412E657">

<li id="GUID-457E6D8F-A383-5CF6-BA6E-D98C8FAC20F4"><p>Run <filepath>subtract.exe</filepath> in your new eshell. </p> </li>

<li id="GUID-122618AE-028B-5BF9-BB4B-4F95099EE839"><p>Switch between

eshells by pressing <b>CTRL+ALT+SHIFT+T</b>. </p> </li>

<li id="GUID-24CCA548-D350-53A7-9919-270EA0C71EC1"><p>To finish, stop

each application by pressing any key. </p> </li>

<section id="GUID-8CF8C4E2-4D74-54C8-9601-97CC44B9C943"><title>condvarlocal</title> <p>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. </p> <p><b>Download</b> </p> <p>Click on the following link

to download the example: <xref href="guid-6013a680-57f9-415b-8851-c4fa63356636/zips/guid-527239ad-3eff-4d62-a7fe-b7e43658dd2f.zip" scope="external">condvarlocal .zip </xref></p><p>Click <xref href="guid-6013a680-57f9-415b-8851-c4fa63356636/guid-527239ad-3eff-4d62-a7fe-b7e43658dd2f.html" scope="peer"> browse </xref> to view the example.</p> <p><b>Class

summary</b></p><p> <xref href="GUID-C0FEA3A0-7DD3-3B87-A919-CB973BC05766.dita"><apiname>RMutex </apiname></xref><xref href="GUID-B0E661BC-4058-3256-B9C3-5A4FD52F6DE5.dita"><apiname>RThread </apiname></xref>  <xref href="GUID-3CECC9FC-58C1-3117-AAF2-FDF88341F56F.dita"><apiname>CPeriodic </apiname></xref> <xref href="GUID-D16EF740-78E6-3D08-AE2F-AFA5E812FF2B.dita"><apiname>RCondVar</apiname></xref></p> <p><b>Description</b> </p> <p>This example uses the producer and the

consumer model to show the use of the local condition variable. </p> <p>The example creates two local threads: a producer and a consumer.

The two threads share a buffer, which is an object of the <codeph>CQueue</codeph> class. The CQueue object creates a local condition

variable using the <codeph>RCondVar::CreateLocal()</codeph> function.

It also defines the methods to insert and remove a token from the

queue. The <codeph>CQueue::Insert()</codeph> function inserts a token

into the queue and signals the condition variable. The <codeph>CQueue::Remove()</codeph> function tries to remove a token from the queue. If the queue is

empty, it must wait for a signal from the condition variable. </p> <p>An object of the <codeph>CProducer</codeph> class creates and

calls the producer thread. The producer thread is called once every

two seconds using an object of the <xref href="GUID-3CECC9FC-58C1-3117-AAF2-FDF88341F56F.dita"><apiname>CPeriodic</apiname></xref> class.

This thread inserts a token into the queue when it is called. It calls

the <codeph>CQueue::Insert()</codeph> function on the shared <codeph>CQueue</codeph> object. </p> <p>An object of the <codeph>CConsumer</codeph> class creates and calls the consumer thread. The consumer thread

is called once a second using an object of the <xref href="GUID-3CECC9FC-58C1-3117-AAF2-FDF88341F56F.dita"><apiname>CPeriodic</apiname></xref> class. This thread removes a token from the queue when it is called.

It calls the <codeph>CQueue::Remove()</codeph> function on the shared <codeph>CQueue</codeph> object. </p> <p>For more information, refer to <xref href="http://support.entegrity.com/private/doclib/docs/osfhtm/develop/appdev/Appde179.htm" scope="external">Condition Variables</xref>. </p> <p> <note> Symbian

is not responsible for the content of external websites.</note>  </p> <p><b>Design and implementation</b> </p> <fig id="GUID-CA344322-0982-59C3-B93A-7A1F175728F3">

<image href="GUID-CAB30473-7829-5F2E-9F45-A2344DEDFC35_d0e357977_href.jpg" placement="inline"/>

</fig> <p><b>Build</b> </p> <p>The Symbian build process describes

how to build this example. The ConditionVariable example builds the

following binary files in the standard location (<filepath>\epoc32\release\winscw\

             &lt;build_variant&gt;</filepath>) for Carbide.c++. </p> <p> <filepath>condvarlocal.exe</filepath>: Demonstrates the use of the

local condition variable. </p> <p><b>How to run the Example</b> </p> <p>To run the example, perform the following steps: </p> <ol id="GUID-D07D5782-6A32-5552-B42B-7D2672F1C94F">

<li id="GUID-CD0F5EAF-780F-5BC7-A216-2FE9CA3B284D"><p>Run <filepath>condvarlocal.exe</filepath>. The program calls the producer and the

consumer threads periodically as shown in the description section.

It also displays a menu. </p> </li>

<li id="GUID-106E7D1D-4ECB-5FA2-A42C-CB1549F4811F"><p>Press <b>‘d’</b> to display the contents of the queue. </p> </li>

<li id="GUID-AA9ABD22-E280-5F21-A1A1-F47FFAAC58F2"><p>Press <b>‘p’</b> to insert a token into the queue. </p> </li>

<li id="GUID-96C5D556-0700-5632-80A1-E728C97A62C1"><p>Press any key

to stop the program. </p> </li>