Re: [Cbor] Decoding of 64-bit CBOR ints on 32-bit systems

Carsten Bormann <cabo@tzi.org> Sat, 27 April 2019 22:06 UTC

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From: Carsten Bormann <cabo@tzi.org>
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Date: Sun, 28 Apr 2019 00:06:30 +0200
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To: Felipe Gasper <felipe@felipegasper.com>
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Subject: Re: [Cbor] Decoding of 64-bit CBOR ints on 32-bit systems
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Hi Felipe,

> On Apr 27, 2019, at 23:14, Felipe Gasper <felipe@felipegasper.com>; wrote:
> 
> 	Is there either a custom or a standard that guides interpretation of 64-bit CBOR integers on 32-bit systems that don’t have 64-bit emulation?

How to interpret this question depends a bit on what you are trying to do.  Let me try to list the possibilities:

(1) I have an application protocol that makes use of integers that don’t fit into the 32-bit integers I have on my systems.

(2) I have an application protocol that doesn’t make use of such integers.  A CBOR encoder could still use a 64-bit form for such an integer (non-preferred encoding).

Orthogonal to this is the question whether you are employing a generic CBOR decoder or an application-specific one.

I think for (2) with a generic CBOR decoder, my recommendation would be to handle this in the generic CBOR decoder (e.g., by checking that the upper 32 bits are zero); similarly for an application specific CBOR decoder.  Alternatively, the application protocol could outright forbid 64-bit forms, just like it could forbid floating point numbers.

For (1), any application-specific CBOR decoder would then have to handle this case in a way that makes sense for the application.  A generic CBOR decoder could define its own 64-bit integer type (not necessarily with all the usual operations implemented), e.g. as a struct of two 32-bit unsigned integers.  Such a generic decoder would, of course, also work for (2); I’d probably still recommend that the decoder deliver integers where the upper half is zero as simple 32-bit integers.  An application of type (2) would handle any input that has those 64-bit structures as “unexpected” input.

Similar considerations apply for other data types not supported by the platform, e.g. 64-bit floating point values (or floating point values at all); it is maybe more obvious how to handle these.

After trying to handle your question in a general way, I think you can see what we would like to know about the specific circumstances to really answer this question.

Grüße, Carsten