Re: [Qemu-devel] [PATCH] qemu: json: Fix parsing of integers >= 0x8000000000000000

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, May 23, 2011 at 08:50:55AM -0500, Anthony Liguori wrote:
> >>The actual value of the alert will surprise you :-)
> >>
> >>Integers in Javascript are actually represented as doubles
> >>internally which means that integer constants are only accurate up
> >>to 52 bits.
> >>
> >>So really, we should cap integers at 32-bit :-/
> >>
> >>Have I mentioned recently that I really dislike JSON...
> >
> >NB, I am distinguishing between JSON the generic specification and
> >JSON as implemented in web browsers. JSON the specification has *no*
> >limitation on integers.
> 
> The spec has no notion of integers at all.  Here's the relevant
> text. Note that the BNF only has a single entry point for numbers.
> It does not distinguish between integers and floating point numbers.
> Also, the only discussion of valid numbers is about whether the
> number can be represented as a rational number.  I think the only
> way to read the spec here is that *all* numbers are meant to be
> represented as floating point numbers.

I don't agree. It means that JSON as a format can represent arbitrary
numbers, whether rational or not. Interpretation as 32/64/128 bit
floating point, or as 16/32/64 bit integers is entirely a matter for
the parser.

The only issue is whether the implementation parser can hold the
numbers without loosing precision. eg, so if JavaScript gets a
number which doesn't fit in 52-bits, it'll loose some precision
due to floating point storage. Nothing in the JSON spec requires
other languages to throw away precision when parsing or formatting
JSON.

Daniel
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