Re: caClient with PCAS

[pthomas@caltech.edu]

Hi Xiaoqiang,

Thank you for your response. I think it turns out that the problem was
that I actually had preemptive callback enabled and I should not have. I
think with preemptive callback enabled, the callback functions were being
run while the fileDescriptorManager.process was processing. Now they only
run when I call ca_poll or ca_pend_event, which I can separate from
fileDescriptorManager.process.

-Patrick

>
> Hi,
>
> It is difficult to say anything without looking at your source.
> But one thing come to my mind is, you are using
> fileDescriptorManager.process to poll for updates.
> It would be good to test again using preemptive callback in caCleint code.
> Check here for more infos.
> http://www.aps.anl.gov/epics/base/R3-14/12-docs/CAref.html#Thread
>
> On the other hand, if you are not bound to use C++,
> one can write such an application in Python, like I just did, in 3
> minutes, 33 lines of code.
>
>
>
>
> It uses module PCASpy[1] for PCAS server programming.
> I use module CaChannel[2] for caClient, but other modules[3] should work
> the same.
>
> Best
> Xiaoqiang
>
> [1] https://code.google.com/p/pcaspy/
> [2]
> http://controls.web.psi.ch/cgi-bin/twiki/view/Main/NewPythonEpicsInterface
> [3] http://www.aps.anl.gov/epics/extensions/index.php
>
> On Feb 22, 2013, at 8:00 PM, pthomas@caltech.edu wrote:
>
>> Hi,
>>
>> I am trying to add a portable channel access server to a channel access
>> client, similar to the EPICS gateway I believe. The values for the
>> process
>> variables that are created by the PCAS are written to from the
>> connection
>> and event handler callbacks for the caClient. The problem I think I am
>> running into, is that when I call fileDescriptorManager.process, these
>> callbacks get called similar to as if a ca_pend_event was called. So
>> after
>> the call to fileDescriptorManager.process and before it completes, new
>> values are being written to the PCAS process variables, and postEvent is
>> being called for these changed values. This leads to funny behavior
>> where
>> the same value is posted for a PCAS channel multiple times in a row (as
>> seen with camonitor). For example, it should post 1, 2, 3 but posts 3,
>> 3,
>> 3 instead, all with the same time stamps. Is there a way around this?
>>
>> I also ran a test, with just the PCAS and leaving out the caClient. I
>> wrote to a PCAS process variable continuously with increasing values in
>> one thread and called fileDescriptorManager.process continuously in
>> another thread. This gave me similar results, that is the same value was
>> posted multiple times in a row for stretches of time before it
>> increased.
>> I was not surprised that I got funny behavior doing this, but the fact
>> that it was similar to the caClient helped me diagnose what may be
>> happening.
>>
>> Also, what is the usual way to update the values for the PCAS process
>> variables from the application itself (i.e. not with a caput)? I added a
>> public function to the PCAS Server that calls functions in the
>> appropriate
>> PV instance to write a value and call the postEvent function. Is this
>> typical? If so, I am guessing that I should also check in this function
>> to
>> see if the new value will actually be different before writing it, so
>> that
>> postEvent is not called when the value has not really changed? Or is
>> there
>> another way to do that?
>>
>> Thank you for your time,
>> Patrick
>>
>>
>
>