Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Mon, 19 Jan 2009 11:42:03 -0600

Hi Alex,

Your subject line is slightly misleading, I don't think you're actually asking 
about support for variable-sized arrays but about the maximum size of an 
array that can be transferred over Channel Access.

On Monday 19 January 2009 04:01:46 [email protected] wrote:
>
> Currently, if we dealing with array, its max length is determined at
> "compile-time" as specified by environment variable
> EPICS_CA_MAX_ARRAY_BYTES, this means we can't change the max array length
> when a CA client is built. If this client connects an IOC supports larger
> size of arrays, we need to rebuild the client. So do we have any plan to
> support non-fixed size array?

You are incorrect about this being a compile-time only setting.  A default 
value for the maximum length is compiled into the libCom library, but this 
can be overridden at runtime by setting the environment variable before you 
initialize the CA library — I'm not sure if Win32 supports putenv() but if 
not it will have an equivalent that you can use instead.  Once CA has been 
initialized though, changing the value will not affect the maximum array size 
that can be transferred.

The maximum size that can be used for a particular client/server combination 
is calculated as the smaller of the two settings on the client and server, so 
an IOC that may not have much memory can limit how much space it has to 
allocate for buffering data for clients.  Here at the APS we make the limit 
fairly large on our client workstations, and allow the IOCs to control how 
big it really gets.

HTH,

- Andrew
-- 
The best FOSS code is written to be read by other humans -- Harold Welte

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System