SemaphoreExample: +thread synchronisation using semaphores

It is recommended to use semaphores instead of thread priorities for the +following reasons:

In a uniprocessor environment, setting the thread priorities does +not guarantee prioritized or serialized execution.
In a multiprocessor or SMP environment, thread priorities are irrelevant +as multiple threads can run simultaneously.

Download

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

Click: browse to view the example code.

Description:

The example creates a database containing +a table of three columns with integer entries.

The example creates +two threads:

WriterThread: 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.
ReaderThread: 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.

The example uses a macro USE_SEMAPHORE 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 ReaderThread will +be able to read the database because in an SMP environment, data may not yet +have been written by the WriterThread.

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 ReaderThread only +reads a row from the database after the WriterThread has +written it.

Class summary

RThread - A handle to a thread.
RDbStoreDatabase - DBMS Store database implementation
RSemaphore - Used for thread synchronization.

Building and configuring

To build the example:

The example builds an +executable called semaphoreexample.exe in the standard +location.
You can build the example +from your IDE or the command line.

If you use an IDE, import the bld.inf file +of the example into your IDE, and use the build command of the IDE.

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:

bldmake +bldfiles

abld build

How to use bldmake and How to use abld describe +how to use the SBSv1 build tools.

Running the example

The example code contains a +macroUSE_SEMAPHORE in the mmp 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.