Experimental Physics and
Industrial Control System

Marty Kraimer <[email protected]> · Mon, 26 Jul 1999 10:50:19 -0500

Steven M Beard wrote:

> Now all of a sudden you have three copies of myFunction all sharing the
> same resource, and if the genSub records happen to process at the same
> time there can be a clash. I had a database like this and saw some very
> strange behaviour (a different record generating an "illegal choice"
> message when I tried to write a valid value to one of its inputs) until I
> explicitly protected pgensub->vala with a mutex semaphore. I have the
> following questions:
> 
> 1) How can myFunction tell that the output field it is writing to is
>    connected to a shared resource? This depends on how the database has
>    been connected up.
> 
> 2) Can EPICS handle such resource sharing transparently? It would be a lot
>    easier to deal with if shared links like this are recognised and
>    protected with a mutex semaphore automatically.
> 
>    I am using EPICS 3.12.2. Is the behaviour different in EPICS 3.13?
> 
> 3) Am I missing something? Is there a technique I should be following
>    to protect the shared resource automatically (e.g. using the PHAS field
>    or something similar)?
> 

I am not sure what happened to cause the behavior you describe but it should not
be the result of multiple genSub records executing simultaneously. Lock sets are
designed to prevent such problems. Please read the chapter "Database Locking,
Scanning, and Processing" in the Application Developer's Guide.

Marty Kraimer

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System