Experimental Physics and
Industrial Control System

"Kasemir, Kay via Core-talk" <[email protected]> · Wed, 10 Apr 2019 12:30:16 +0000

Hi:

Google pointed me to http://epics-pvdata.sourceforge.net/pvAccess_Protocol_Specification.html, "Fourth (in progress) Public Working Draft, 16-October-2015", "<to be> followed by publication of the specification."

It has a lot of information, but the very first specific information about size encoding is already wrong:
"""
If the number of elements is less than 255, the size MUST be encoded as a single byte containing an unsigned 8-bit integer indicating the number of elements
If the number of elements is less than 2^31-1, then the size MUST be encoded as an unsigned 8-bit integer with value 255, followed by a positive signed 32-bit integer indicating the number of elements
If the number of elements is greater than or equal to 2^31-1, then the size MUST be encoded as an unsigned 8-bit integer with value 255, followed by a positive signed 32-bit integer with value 2^31-1, followed by a positive signed 64-bit integer indicating the number of elements. This implies a maximum size of 2^63-1.
"""

The current implementations (https://github.com/epics-base/pvDataCPP/blob/master/src/misc/serializeHelper.cpp#L34, https://github.com/epics-base/epicsCoreJava/blob/master/pvDataJava/src/org/epics/pvdata/misc/SerializeHelper.java#L35) use 254 instead of 255 as the "larger size follows" marker, 255 is used to encode 'null', and values beyond 0x7fffffff=2^31-1 are not implemented.

Is there a current PVA protocol specification?

Thanks,
Kay

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System