Re: FFI and endian conversion
- From: Mike Pall <mike-1207@xxxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Wed, 25 Jul 2012 12:45:27 +0200
Luke Gorrie wrote:
> I'm using LuaJIT's FFI to access things like ethernet frames as if they
> were C structures, and I'm wondering what the best way to take care of
> endian conversion is. Like: can I easily tell LuaJIT that I want automagic
> endian conversion on certain structure fields?
It's a low-level API, so the magic is limited. But it's easy
enough to add this functionality yourself:
local ffi = require("ffi")
local bit = require("bit")
local function n32(x) return x end
local n16 = n32
if ffi.abi("le") then
local shr = bit.rshift
n32 = bit.bswap
function n16(x) return shr(n32(x), 16) end
end
Then use n32(foo.field) or n16(foo.field) to access fields in
network byte order (aka ntohl or ntohs).
> } __attribute__ ((__packed__));
If you know you're on a host that allows unaligned access (e.g.
x86 and some ARMs), you can directly access the fields. Otherwise
you'll have to jump through some hoops: map it to a byte buffer
and piece the bytes together by hand.
--Mike
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