How to get SIGINT to give a backtrace using ljsyscall

sigint backtrace

local S = require "syscall"
local t = S.t
local c = S.c
local ffi = require "ffi"

local ip
if ffi.arch == "x86" then ip = c.REG.EIP
elseif ffi.arch == "x64" then ip = c.REG.RIP
else error "unsupported architecture" end

local g = ffi.cast("void (*)(void)", function() error("sigint") end)

local f = t.sa_sigaction(function(s, info, ucontext)
  ucontext.uc_mcontext.gregs[ip] = ffi.cast("intptr_t", ffi.cast("void *", g)) -- set instruction pointer to g
end)
assert(S.sigaction("int", {sigaction = f}))

-- example code to get interesting stack trace
function bb(x)
  assert(S.kill(S.getpid(), "int"))
  return x + 1
end

function aa(x)
  local c = 2 * bb(x + 1)
  print("not here")
  return c
end

aa(2)