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perform its processing and return quickly. To run a program that loops until some event (e.g.,
recording start/stop or system shutdown), use StartTask to kick-off a program that monitors for
the desired event, sleeping when there’s nothing to do.
To run a program at system shutdown, create a subroutine named Stop_Program. All subroutines
having this name are run at system shutdown. This subroutine should perform cleanup quickly
and return.
Note: when defining Start_Program and Stop_Program, don’t declare them as Public, since there
could be other subroutines having these names in other .bas files.
The following example shows how to start a program at system start, and then signal that
program to stop at system shutdown:
TimeToStop = 0
Public Function ProcessingLoop(Parm)
Sum = 0.0
' Return 0 to schedule the function for the next minute
ProcessingLoop = 0
' Average A/D channel 1 once per second for the next 10 seconds
For i = 1 To 10
Sum = Sum + Ad(1, 1)
' Exit and stop processing if the TimeToStop event is raised
' otherwise delay a second between samples
If WaitEvent(1, TimeToStop) = 1 Then
ProcessingLoop = -1
Exit Function
End If
Next i
End Function
Sub START_PROGRAM
StatusMsg "Start Program"
ResetEvent TimeToStop
REM Run the main processing loop every minute on the minute
StartTask "ProcessingLoop", 0, TimeSerial(0, 0, 0), TimeSerial(0, 1, 0)
End Sub
Sub STOP_PROGRAM
StatusMsg "Stop Program"
SetEvent TimeToStop
StopTask "ProcessingLoop"
End Sub
Multi-threading
Xpert Basic programs often run in independent program threads (e.g., programs started with
StartTask, scheduled subroutines, and setup blocks). There are several concerns related to multithreaded operation to address:
Resource Contention
The first concern is that subroutines executing in different threads may require access to the
same variable, allowing the possibility of data corruption. When a subroutine uses only local
variables, there is no concern of data corruption. However, when a subroutine uses global
resources (e.g., a global variable or com port), it is important to synchronize thread access to the
resource so that only a single thread will use it at any given time.
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