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<concept id="GUID-056B9A65-EC59-4B2E-9F10-E487343B5F5F" xml:lang="en"><title>SemaphoreExample:
thread synchronisation using semaphores</title><shortdesc>Explains how to use Semaphores to achieve thread synchronization.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>It is recommended to use semaphores instead of thread priorities for the
following reasons:<ul>
<li><p> In a uniprocessor environment, setting the thread priorities does
not guarantee prioritized or serialized execution.</p></li>
<li><p>In a multiprocessor or SMP environment, thread priorities are irrelevant
as multiple threads can run simultaneously.</p></li>
</ul></p>
<section id="GUID-48668C9B-6B42-4FE8-899B-403D5EB5FB53"><title>Download</title> <p>Click on the following link to download
the example: <xref href="guid-6013a680-57f9-415b-8851-c4fa63356636/zips/guid-34775913-c5c2-41ba-a591-827039bbe7af.zip" scope="external">SemaphoreExample.zip</xref></p><p>Click: <xref href="guid-6013a680-57f9-415b-8851-c4fa63356636/guid-34775913-c5c2-41ba-a591-827039bbe7af.html" scope="peer">browse</xref> to view the example code. </p></section>
<section id="GUID-2E531F3D-EEC8-40FD-9F32-2071FDECAE61"><title>Description: </title><p> The example creates a database containing
a table of three columns with integer entries.</p><p> The example creates
two threads:</p><ul>
<li><p><codeph>WriterThread</codeph>: This thread calls a periodic function
at an interval of one second, and in each iteration it inserts one row of
integers in the table. </p></li>
<li><p><codeph>ReaderThread</codeph>: This thread also calls a periodic function
at an interval of one second, and in each iteration it reads one row of integers
following the last row read.</p></li>
</ul><p> The example uses a macro <codeph>USE_SEMAPHORE</codeph> to show the
difference between the code using semaphores and the code which doesn't use
semaphores. When not using semaphores, it is not guaranteed that the <codeph>ReaderThread</codeph> will
be able to read the database because in an SMP environment, data may not yet
have been written by the <codeph>WriterThread</codeph>.</p><p>Using a semaphore
serializes the thread execution. The read operation waits on the semaphore
until it gets a signal from the write operation. This ensures that the <codeph>ReaderThread</codeph> only
reads a row from the database after the <codeph>WriterThread</codeph> has
written it. </p></section>
<section id="GUID-07CAD3CA-86DB-4245-8565-C618E59718A5"><title>Class summary</title><ul>
<li><p><xref href="GUID-B0E661BC-4058-3256-B9C3-5A4FD52F6DE5.dita"><apiname>RThread</apiname></xref> - A handle to a thread.</p></li>
<li><p><xref href="GUID-F4A68645-2265-307F-B951-8295827E000B.dita"><apiname>RDbStoreDatabase</apiname></xref> - DBMS Store database implementation</p></li>
<li><p><xref href="GUID-AED27A76-3645-3A04-B80D-10473D9C5A27.dita"><apiname>RSemaphore</apiname></xref> - Used for thread synchronization. </p></li>
</ul></section>
<section id="GUID-863B0EDF-F3F8-4345-9FEE-824350A2DBCE"><title>Building and configuring</title> <p>To build the example: </p> <ul>
<li id="GUID-037442C7-2436-5152-9A3E-9DC0B4283A9C"><p>The example builds an
executable called <filepath>semaphoreexample.exe</filepath> in the standard
location. </p> </li>
<li id="GUID-B7C849B9-0885-5253-A78B-71854F86D772"><p>You can build the example
from your IDE or the command line. </p> <p>If you use an IDE, import the <filepath>bld.inf</filepath> file
of the example into your IDE, and use the build command of the IDE. </p> <p>If
you use the command line, open a command prompt, and set the current directory
to the source code directory of the example. You can then build the example
with the SBSv1 build tools with the following commands: </p> <p><userinput>bldmake
bldfiles</userinput> </p> <p><userinput>abld build</userinput> </p> <p><xref href="GUID-793A5EF9-CC16-5EEB-9011-6431EA76EB15.dita">How to use bldmake</xref> and <xref href="GUID-B6B54E07-3B34-5D5C-8815-93383FA8FB4B.dita">How to use abld</xref> describe
how to use the SBSv1 build tools. </p> </li>
</ul> </section>
<section id="GUID-278DD8CF-28AF-4826-B938-20EC19F3A5B0"><title>Running the example</title> <p>The example code contains a
macro<codeph>USE_SEMAPHORE</codeph> in the <filepath>mmp</filepath> file.
By default it is commented out and on execution the example runs without using
semaphores; the macro should be uncommented to use semaphores. The console
output will generally look the same whether using semaphores or not if there
is a single processor, but differences will occur in an SMP environment. Run
the example following the instructions provided in the console. </p> </section>
</conbody></concept>