Re: Size encoding

[Ralph Lange <ralph.lange@gmx.de>]

Hi Bruno,

I agree that the C++ implementation seemingly uses -1 in the first byte to encode "empty" and -2 to encode "string of size >= 2^8-2".

Houston, we have a problem.

(The value you are missing in the third case of the spec is explicitly mentioned there as 2^31-1.)

Cheers,
~Ralph

On Tue, May 15, 2018 at 3:13 PM Bruno Martins <brunoseivam@gmail.com> wrote:

Hi,

I was taking a look at how messages are de/serialized by pvData and pvAccess and I think there is an inconsistency between the spec and the implementation regarding how sizes are encoded.

The spec says, basically [1]:

If size < 2^8-1, encode byte directly

If size < 2^31-1, encode -1 as byte and then size as signed 32-bit integer

If size < 2^63-1, encode -1 as byte, then a positive 32-bit int (which value? -1?), then size as a signed 64-bit integer.

The implementation (C++) does [2]:

If size == -1, encode -1 as byte (what is this used for?)

If size < 2^8 - 2, encode size directly as byte

Else encode -2 as byte, size as signed 32-bit integer

[1] http://epics-pvdata.sourceforge.net/pvAccess_Protocol_Specification.html#dataEncoding:sizes

[2] https://github.com/epics-base/pvDataCPP/blob/207efca15cf471bbc2990cbd336000e04dcfcd22/src/misc/serializeHelper.cpp#L34

Bruno