[luatex] fio library byte order

[luigi scarso]

On Tue, Jun 23, 2020 at 4:29 AM Henri Menke <henrimenke at gmail.com> wrote:

>
>
> No, type punning *is* undefined behaviour.
>
>     6.2.6.1 General
>
>     7 When a value is stored in a member of an object of union type, the
>     bytes of the object representation that do not correspond to that
> member
>     but do correspond to other members take unspecified values.
>
> http://port70.net/~nsz/c/c11/n1570.html#6.2.6.1p7


hm, unspecified value, not undefined behaviour.

Probably this one is ok:
\directlua{
ffi.cdef [[
 unsigned int c ;
 union endianess {
   uint32_t i;
   uint8_t c[sizeof(uint32_t)];
 }
]]
local x = ffi.new('union endianess x',{i=0xAABBCCDD})
if ffi.sizeof('uint8_t')==1 and ffi.sizeof('uint32_t')==4 then
 if(x.c[0]==0xDD and x.c[1]==0xCC and x.c[2]==0xBB and x.c[3]==0xAA) then
   print "little endian"
 elseif (x.c[0]==0xAA and x.c[1]==0xBB and x.c[2]==0xCC and x.c[3]==0xDD)
then
  print "big endian"
 else
  print "mixed endian"
 end
else
 print "unable to check endianess"
end
}
\end

see eg Modern-C on unions
https://modernc.gforge.inria.fr/#orgd2172c0


-- 
luigi
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