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Subject: RE: Calling ca_context_create(1) too late
From: "Hill, Jeff" <johill@lanl.gov>
To: "michael.abbott@diamond.ac.uk" <michael.abbott@diamond.ac.uk>, "tech-talk@aps.anl.gov" <tech-talk@aps.anl.gov>
Date: Mon, 22 Apr 2013 15:30:15 +0000
Michael,

There can be many CA contexts in a system; In fact, as Mark has discovered, for backwards compatibility reasons the first call to the CA library implicitly creates a CA context for the current thread. Several threads can also agree together to share a CA context. If the ca_create_context fails it returns failure status.

epicsShareFunc int epicsShareAPI ca_context_create (enum ca_preemptive_callback_select select);

In an IOC, by default only the threads associated with the DB CA links and the sequencer maintain CA contexts; for good reasons those two contexts are separate and independent BTW. There is no CA context associated with the startup script, the EPICS shell, or the vxWorks shell although if you called a CA function from those threads a CA context would be implicitly created but this typically isn't a good idea.

Jeff

> -----Original Message-----
> From: tech-talk-bounces@aps.anl.gov [mailto:tech-talk-
> bounces@aps.anl.gov] On Behalf Of michael.abbott@diamond.ac.uk
> Sent: Friday, April 19, 2013 7:44 AM
> To: tech-talk@aps.anl.gov
> Subject: Calling ca_context_create(1) too late
> 
> I've just discovered that calling ca_context_create(1) after iocInit() appears to
> succeed (it returns ECA_NORMAL) ... but the resulting context is not very
> healthy!  In my particular case I find that ca_array_get_callback delivers zero
> length data, but I expect other deeper problems are to be found.
> 
> I understand that I need to call ca_context_create() a good deal earlier, but I
> don't really understand why it appears to succeed -- is it possible for an error
> code to be returned instead, please?  (In some future version of EPICS, I'm
> testing this on 3.14.11.)
> 
> Alternatively, is there some reliable way I can test whether it has succeeded?
> I don't have full control over when my library is initialised, so detecting an
> error immediately would be more friendly to the user.
> 
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References:
Calling ca_context_create(1) too late michael.abbott

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