Re: ffi.cast: weird behaviour
- From: Daurnimator <quae@xxxxxxxxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Wed, 20 Nov 2013 13:29:11 +0100
Would it be safe just checking the type after this operation?
e.g.
function read_float(s)
local x = ffi.cast("const float*",s)[0]
if type(x) ~= "number" then x = 0/0 end
return x
end
print(read_float("\xff\xff\xff\xff"))
I ask as I've written various parsers that I've only just realised do this
unsafely.
The alternative is to check that the string does not contain an invalid NaN
upfront: what would this look like?
D
P.S; any chance we'll see you (Mike Pall) at the lua workshop this weekend
in Toulouse, France?
On 27 June 2012 00:04, Mike Pall <mike-1206@xxxxxxxxxx> wrote:
> Wolfgang Pupp wrote:
> > I've recently been surprised by weird (IMHO) behaviour of ffi.cast;
> > Consider this code:
> >
> > local ffi = require'ffi'
> > local zeroneg = '\x00\x00\x00\x80'
> > local nan = '\xff\xff\xff\xff'
> >
> > print(ffi.cast('float *', zeroneg)[0])
> > print(ffi.cast('float *', nan)[0])
> >
> > I would have expected this to print -0 and nan. But it doesn't. I get
> > -0 and *nil* instead.
>
> LuaJIT reserves certain NaN bit patterns for itself. Your cast and
> the following conversion to a double creates one of these reserved
> patterns (which happens to map to nil).
>
> > Am I doing it wrong in the first place?
>
> Only use the canonical NaN bit patterns: ffc00000 for floats and
> fff80000 00000000 for doubles. Or the positive equivalents on
> non-x86 platforms (but the sign bit doesn't matter for NaNs).
>
> --Mike
>
>
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