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.

+ + + + + +SemaphoreExample: thread synchronisation using semaphoresExplains how to use Semaphores to achieve thread synchronization. +

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 .

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.

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