Experimental Physics and
Industrial Control System

"J. Lewis Muir" <[email protected]> · Thu, 06 Nov 2014 16:51:57 -0600

Dear Tech-Talkers,

Is it permissible, in a C program using the EPICS CA client library
using a preemptive CA context, to call CA functions that perform I/O
from a user-supplied callback function?

Here's an example.  A CA client creates a CA context like this:

  status = ca_context_create(ca_enable_preemptive_callback);

It creates a channel and then registers a state change subscription on
it like this:

  status = ca_create_subscription(DBR_LONG, 1, ctx->ch_image_id,
      DBE_VALUE, on_image_id_state_change, ctx, NULL);

And in the on_image_id_state_change callback function, it reads a PV
value like this:

  status = ca_get(DBR_LONG, ctx->ch_nx, &nx);
  if (status != ECA_NORMAL) goto err;
  status = ca_pend_io(ctx->timeout);
  if (status != ECA_NORMAL) goto err;

Is this permissible?

One advantage of doing things this way is that the client doesn't need
a separate thread for handling state change events.  It essentially
just uses "for free" a thread managed by the CA client library.  One
disadvantage is that, since the CA client library guarantees that it
will always wait for a user-supplied callback function to complete
before calling another one, the client won't learn of any state changes
that occur while performing work in the callback function until the
callback function completes.

The client could obviously have it's own "worker" thread that would
handle CA state change events.  The user-supplied callback function
would just notify the worker thread whenever it received a state change
event.  But as I said, this would mean managing a worker thread, which
isn't a big deal, but does increase the complexity.

Thank you!

Lewis

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System