Re: ca_sg_put problem with DISP=1

[Andrew Johnson <anj@aps.anl.gov>]

Dirk Zimoch wrote:
> I got a strange effect with ca_sg_put if writing to a record where DISP=1.
>
> The ca_sg_put succeeds (returns ECA_NORMAL) but then,
> an error message is printed asynchronously:
>
> CA.Client.Exception...............................................
>     Warning: "Channel write request failed"
>     Context: "op=1, channel=DZ-AO3, type=DBR_invalid, count=0, 
> ctx="write notify request rejected""
>     Source File: ../syncGroupWriteNotify.cpp line 98
>     Current Time: Wed Feb 28 2007 16:40:34.421658000
> ..................................................................
>
> Finally, ca_sg_block times out, long after it was clear to CA that 
> writing failed.
> Especially the last point is strange to me. Is this a bug or is it 
> designed this way?

If you look at the documentation for ca_sg_block(), which is at
http://www.aps.anl.gov/epics/base/R3-14/9-docs/CAref.html#ca_sg_block
You'll see that it doesn't currently have any return code that means 
"operation failed". ECA_NORMAL means success so it can't return that, 
and if the time period you specified hasn't passed yet then ECA_TIMEOUT 
isn't appropriate.  The other two codes documented are associated with 
bad arguments, so it must have been designed to work that way.

> If it works as intended, what is the correct way to handle failing 
> synchronous writes to records with DISP=1 without timing out?

What is setting the record's DISP field, and why do you expect any 
different behaviour than that which you're seeing?  The DISP field is 
documented in the Record Reference Manual as meaning "disable/ignore all 
puts to other fields of this record" and CA flags that as a failure. 
The CA timeout operations don't mean "retry the operations for this time 
period"; if you want retries you'll have to code them yourself I'm afraid.

I think you may have discovered one reason why the ca_sg_ functions 
aren't quite as useful as they initially seem, which is probably why 
they're not used very much.

- Andrew
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