Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Mon, 11 Nov 2013 10:52:23 -0600

Hi Matt,

On 11/10/2013 04:46 AM, Gibbs, Matt wrote:

I have a simple test script where I create a channel, then create a
new subscription, and pass in an event callback function which just
prints the current value of the PV to the terminal.  Most of the
time, this works great, but maybe 20% of the time, I get two updates,
then nothing else for the duration of the program.  This is always
using the same code, with the same PV.  If I run ‘camonitor’ on the
PV repeatedly, I never see the same problem.

I don’t get any exception messages, and the rest of the script
doesn’t crash or halt, so it seems like the event callback just stops
getting called.

Any ideas about what might be happening, or things I can do to
further debug this?

As Dirk said its easier for us to help if you can show us the code, but

I can suggest a couple of possibilities.

Are you configuring the CA library to use pre-emptive callbacks or not?

See http://www.aps.anl.gov/epics/base/R3-14/12-docs/CAref.html#Thread

for an explanation. If you want libca run your callbacks from its

internal threads at any time (pre-emptive enabled) you must handle any

necessary locking to protect your data structures against race

conditions. If on the other hand you disable pre-emptive callbacks you

must call ca_pend_event() regularly because libca can only call your

event callbacks while your main thread is inside that routine.

Note that the callbacks will still happen in the context of those

internal libca threads, but you don't have to worry about using mutexes

to protect your data in most cases.

There is a difference between ca_pend_io() and ca_pend_event(); the

former will only result in monitor callbacks being run while the library

is waiting for channel connections and ca_get() and ca_put() operations

to complete. If the only I/O outstanding is waiting for monitor events

to fire, ca_pend_io() will return immediately without processing them,

thus you usually need to use ca_pend_event().

If that doesn't help we'll probably need to see your code to work out

what's going on.

- Andrew
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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System