Experimental Physics and
Industrial Control System

"Christopher A. Larrieu" <[email protected]> · Mon, 29 Nov 1999 11:57:15 -0500

Eric,

  I've partially implemented a C++ wrapper around CA which
runs the CA client code in a separate thread in order to hide
the asynchronous "untidy bits" (passing control off to to the
CA library to handle callbacks, etc.) from the user, though 
have not yet had the opportunity to test it.  The one major 
immediately discernable issue is how to communicate between
the threads without ultimately falling back to a polling loop
inside the CA thread.

  My solution to this was the following:

  (1) set up a callback for getting CA's fd's
  (2) create a special fd.  Call it fd_foo
  (3) within the CA thread, select on all of CA's
      file descriptors, as well as fd_foo
  (4) when select returns, check to see if the active
      file descriptor is fd_foo.  
        + If so, then do whatever
          CA stuff you want (e.g. add a monitor, unconnect
          a chid, etc.).
        + If not, then pass off control to CA so that
          it can do whatever work needs doing on the active
          file descriptor.
  (5) select again within the CA thread.

  The whole point of this is to ensure that CA is in a stable
state before calling any of the client code.  When a thread wants
to perform some CA task, it writes to fd_foo its thread ID, which
causes select() to return in the CA thread, which then yields
to the thread which initiated the whole sequence of events.

  Unfortunately, I'm not the one to say authoritatively whether
or not this is the best approach, but hought I would at least
share the idea with you.

  Best Regards,

Chris
-- 
Christopher A. Larrieu
Jefferson Laboratory
(757) 269-5097

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System