luajit segfault on garbage collection

Question

I'm having an issue with a userdata defined with ffi.metatype. When the object if garbage collected, I get a segfault. Here is the code.

ffi = require("ffi")
srate = 48000

ffi.cdef[[
typedef struct sin_state {
    float _now;   // time of last eval
    float _last;  // result of last eval
    float _freq;  // frequency
    float _gain;  // gain
    float _phase; // phase
    float _w;     // angular speed in radians/sample
    float _p;     // update parameter
    float _y[3];  // last 3 steps
} sin_t;
]]

sin = {}

sin.mt = {
    __call = function(s,now)
        if s._now < now then
            s._now = now
            s._phase = (s._phase + s._w) % 2 * math.pi
            s._y[1] = s._p * s._y[2] - s._y[3]
            s._y[3] = s._y[2]
            s._y[2] = s._y[1]
            s._last = s._y[1] * s._gain
        end
        return s._last 
    end,

    __index = function(s,k)
        return s["_"..k]
    end,

    __newindex = function(s,k,v)
        -- update parameter
        s["_"..k] = v
        -- update internal state
        sin.update(s)
    end
}

function sin.update(s)
    s._w = s._freq * 2 * math.pi / srate
    s._p = 2 * math.cos(s._w)
    s._y[3] = math.sin(-2 * s._w + s._phase)
    s._y[2] = math.sin(-1 * s._w + s._phase)
    s._y[1] = s._p * s._y[2] - s._y[3]
    s._last = s._y[1] * s._gain
end

sin.state = ffi.metatype("sin_t",sin.mt)

function sin.new(t)
    local t = t or {}
    local params = {
        _freq = t.freq or 440,
        _gain = t.gain or 1,
        _phase = t.phase or 0,
    }
    local s = sin.state(params)
    sin.update(s)
    return s
end

s = sin.new()
print(s(1))
s = nil
print("garbage collecting")
collectgarbage()
print("done")

Running :

% luajit test.lua
6.6929342068534e-09
garbage collecting
[1]    20398 segmentation fault (core dumped)  luajit test.lua

Am I missing something ? Should I define a __gc metamethod ? If so, What should I put in there ?

Answer 1

Ok, as usual I found the mistake a few minutes after posting.

I messed up with the indexes of the _y array. Somehow I had the impression the luaJIT's FFI library was doing the index conversion (starting from 1 instead of 0) but it was not.

I just substracted 1 to all the _y[X] and it works fine.