How to communicate save between different task classes AsSem

Ask Questions
1

Because of the fact that the AutomationRuntime has a task system it means, every program can be interrupted during the runtime, also in the middle of the execution, this means we can have a producer and consumer problem.
I can Imagine that for this case the library AsSem is available.
Unfortunately in the with AutomationStudio delivered example, it is only shown one task not both, so I am not pretty sure how to handle the function block to save for example a global variable.

The SemCreate ident must be shared globally between the two tasks which communication between each other right?

So one task create the semaphore with SemCreate_0 and SemCreate is locally defined in that task memory. After that this task writes the Ident to a global variable.
Is that right and a good idea?

Is there any case that a semaphore gets deleted of a program transfer?
Or does the delete 100% only happen with the function block SemDelete?

2

Hi,

Yes, if you want to use the same semaphore in different tasks (for example to control the access into critical code sections for handling shared ressources - I assume that’s the usecase you’re talking about), you have to share the ident from SemCreate() to all of the tasks they want to use SemAcquire() and SemRelease().

As I know, SemCreate() uses native Automation Runtime semaphores (I mean semaphores with a operating system-wide scope), so I can’t imagine that they’re deleted by a program transfer. I assume they’re really only delete by reboot or by SemDelete()… but to be honest, I haven’t had a closer look into it until today.

Best regards!

3

Hi again,

because I was curious, I did a short test → in a nutshell: it works like expected.

What I tried:

  • task1 that creates the semaphore und stores the ident in a global var
  • tasks2 and task3 using the semaphores ident
  • after installation, checked if the semaphore was created successfully and checked acquiring and releasing
  • then, deleted task1 from the PLC (by removing from deployment list and transferring)
  • task2 and task3 still were able to access the semaphore, even if the local SemCreate() instance from task1 wasn’t longer existing
  • after rebooting the PLC (task1 stayed removed), access to the semaphore wasn’t longer possible for task2 and task3

Best regards.