Re: LuaJIT FFI cdata introspection

[Alexander Nasonov <alnsn@...>]

Leo Razoumov wrote:
> Michal,
> were you able to make FFI introspection work the way you wanted? If
> yes, could you, please, provide the latest version of the patch you
> posted to the Lua mailing list back in February 2011.
> 
> Thanks,
> --Leo--

I'd like to bring up the topic and paste my hack for those who don't
want to modify luajit binary.

Alex

local ffi = require 'ffi'

local defs = [[
struct A {
	int16_t a;
	int16_t b;
	char c[7];
} __attribute__((packed));

struct B {
	char a[6];
	int16_t b;
	int16_t c;
	int8_t d;
} __attribute__((packed));
]]

ffi.cdef(defs)

local member_candidates = {}
for w in string.gmatch(defs, '%w+') do
	member_candidates[w] = true
end

local memorized_members = {}

local function members(obj, ct)
	ct = ct or ffi.typeof(obj)

	local f = memorized_members[ct]
	if not f then
		local om = {} -- {{offset, "member"}, ...}
		for m,_ in pairs(member_candidates) do
			local o = ffi.offsetof(ct, m)
			if o then table.insert(om, {o, m}) end
		end

		table.sort(om, function(a, b) return a[1] < b[1] end)

		local mms = {}
		for i = 1, #om do
			mms[i] = om[i][2]
		end

		f = function(obj)
			local i = 0
			return function()
				i = i + 1
				if i <= #mms then
					local m = mms[i]
					return m, obj[m]
				end
			end
		end

		memorized_members[ct] = f
	end

	return f(obj)
end

local pa = ffi.cast('struct A *', 'abracadabra')
local pb = ffi.cast('struct B *', 'abracadabra')

print('struct A:')
for k,v in members(pa[0]) do
	print(k,v)
end

print('struct B:')
for k,v in members(pb[0]) do
	print(k,v)
end

$ luajit test.lua
struct A:
a	25185
b	24946
c	cdata<char (&)[7]>: 0x40269320
struct B:
a	cdata<char (&)[6]>: 0x402731f8
b	24932
c	29282
d	97