RE: What is void* puser in ca_create_channel

["Hill, Jeffrey O" <johill@lanl.gov>]

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
> 
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