<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>

<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>

<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-GENID-1-12-1-18-1-1-9-1-4-1-25-1-31-1-3-5-3-1"><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-GENID-1-12-1-18-1-1-9-1-4-1-25-1-31-1-3-5-3-2"><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> </li>

<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>