[luatex] fio library byte order

[Henri Menke]

On 20/06/20, 13:26, luigi scarso wrote:
> On Fri, Jun 19, 2020 at 11:17 PM Reinhard Kotucha <reinhard.kotucha at web.de>
> wrote:
> 
> > Hi,
> > it's nice that with the fio library LuaTeX can now process binary
> > files.  What I'm missing is the ability to specify the byte order
> > (little vs. big endian).
> >
> >
> I don't knwo if it can help,
> but if you can use ffi I think you can know the endianness of the current
> platform with with this code:
> \directlua{
> ffi.cdef [[
>  union endianness {
>    int32_t i;
>    char c[4];
>  }
> ]]
> local x = ffi.new('union endianness x',{i=1})
> if(x.c[0]==1) then
>   print "little endian"
> else
>  print "big endian"
> end
> }
> \end

Type punning is undefined behaviour.  When you read from a different
union member than you wrote to, there is no guarantee that you are
actually reading the same memory.  It is much safer to just do a memcpy
to an array of char of the correct length.

local x = ffi.new('int[1]',1)
local size = ffi.sizeof(x, 0)
local y = ffi.new('char[' .. size .. ']')
ffi.copy(y, x, ffi.sizeof(x))
if (y[0] == 0x01) then
    print "little endian"
elseif (y[size - 1] == 0x01) then
    print "big endian"
else
    error("could not detect endianness")
end

Cheers, Henri

> 
> -- 
> luigi