Explains how to use Semaphores to achieve thread synchronization.
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.
Click on the following link to download the example: SemaphoreExample.zip
Click: browse to view the example code.
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.
RThread - A handle to a thread.
RDbStoreDatabase - DBMS Store database implementation
RSemaphore - Used for thread synchronization.
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 .
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.