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Subject: Re: What is void* puser in ca_create_channel
From: Florian Feldbauer <florian@ep1.ruhr-uni-bochum.de>
To: "Hill, Jeffrey O" <johill@lanl.gov>
Cc: EPICS tech talk <tech-talk@aps.anl.gov>
Date: Fri, 30 Mar 2012 14:29:05 +0200
Thanks for your answers.

Just one other question: Can I use the CA methods ca_create_channel and ca_create_subscription within a c++ class?
The project we are working on should use OO techniques.
We need not only an interface to EPICS, but also to a simulation.
So we wanted to have an abstract base class, which is used by the rest of our project, and derived classes for the implementation of the individual "control systems". One derived class would contain the CA for EPICS and another the interface to our simulation.

I can compile my test program for this object-structure, but I don't get a conncetion to my EPICS ioc.
The two examples from makeBaseApp.pl are working correctly.

Any idea based on this few information, or should I post my code?

Best regards,
Florian

On 03/29/2012 05:19 PM, Hill, Jeffrey O wrote:
Hi,

The puser pointer you pass to ca_create_channel is saved by the library with the channel and is later supplied as a puser argument to your channel connectione state update callback function. There is also a ca function "void * ca_puser ( CHID )" that will return the puser pointer that was supplied when the channel was created.

Likewise, the puser pointer you pass to ca_create_subscription is saved by the library with the subscription and is later supplied as a puser argument to your subscription update callback function.

How does this parameter have to look like?
Can I use a C-Struct with any member I want to have (even members which
are not for the CA, but for another part of my program)? Are there any
members which are mandatory? (like value or chid)
A pointer to any data structure can be used. The CA library does not in any way access, or place any restrictions on the content of, this user application private data structure. The user application is of course responsible for allocation, initialization, maintenance, and de-allocation (proper cleanup) of this data structure.

Use of void pointers passed to callbacks is an old technique which is roughly equivalent to a virtual function in modern software interfaces, BTW.

HTH, and feel free to send any other questions that you might have, and I will do my best to answer them.

Jeff

-----Original Message-----
From: tech-talk-bounces@aps.anl.gov [mailto:tech-talk-bounces@aps.anl.gov]
On Behalf Of Florian Feldbauer
Sent: Thursday, March 29, 2012 4:01 AM
To: EPICS tech talk
Subject: What is void* puser in ca_create_channel

Dear all,

I currently write a CA Client that should monitor all measured
temperatures in my experiment
to regulate the chillers.
This is my first try to program a CA Client, and I have a question
concerning this:

The ca_create_channel and the ca_create_subscription methods both have
a parameter void* puser. How does this parameter have to look like?
Can I use a C-Struct with any member I want to have (even members which
are not
for the CA, but for another part of my program)? Are there any members
which are mandatory? (like value or chid)

In the camonitor.c example generated with makeBaseApp.pl this puser is
just a pointer to a C-Struct containing
a c-string value, the chid and evid of the channel.

Best regards,
Florian

--
-------------------------------------
| Florian Feldbauer                 |
| Ph.D. student                     |
|                                   |
| Institut für Experimentalphysik I |
| Ruhr-Universität Bochum           |
| Universitätsstr. 150              |
| D-44780 Bochum                    |
|                                   |
| Office: NB 2/173                  |
| Phone:  (+49)234 / 32-23538       |
| Fax:    (+49)234 / 32-14170       |
| Web:    http://www.ep1.rub.de     |
-------------------------------------


--
-------------------------------------
| Florian Feldbauer                 |
| Ph.D. student                     |
|                                   |
| Institut für Experimentalphysik I |
| Ruhr-Universität Bochum           |
| Universitätsstr. 150              |
| D-44780 Bochum                    |
|                                   |
| Office: NB 2/173                  |
| Phone:  (+49)234 / 32-23538       |
| Fax:    (+49)234 / 32-14170       |
| Web:    http://www.ep1.rub.de     |
-------------------------------------


Replies:
RE: What is void* puser in ca_create_channel Hill, Jeffrey O
References:
What is void* puser in ca_create_channel Florian Feldbauer
RE: What is void* puser in ca_create_channel Hill, Jeffrey O

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