metatypes for bounds checking

gistfile1.txt

ffi = require "ffi"

local VHOST_VRING_SIZE = 32*1024

ffi.cdef [[
enum { VHOST_VRING_SIZE = 32*1024 };

struct vring_avail {
  uint16_t flags;
  uint16_t idx;
  uint16_t ring[VHOST_VRING_SIZE];
};
]]

vring_avail = ffi.metatype("struct vring_avail", {__index = {
  ring_get = function(v, i)
               if i < 0 or i > VHOST_VRING_SIZE then error "out of bounds" end
               return v.ring[i]
             end,
  ring_set = function(v, i, k)
               if i < 0 or i > VHOST_VRING_SIZE then error "out of bounds" end
               v.ring[i] = k
             end,
}})

v = vring_avail()

v:ring_set(2, 4)
assert(v:ring_get(2) == 4)

v:ring_get(65000)