lua_touserdata
return lua_touserdata(L, idx);
void *p = lua_touserdata(L, ud);
LUA_API void *(lua_touserdata) (lua_State *L, int idx);
ri = lua_touserdata(state, -1);
nvlist_t *nv = (nvlist_t *)lua_touserdata(state, 2);
const char *key = (const char *)lua_touserdata(state, 1);
zcp_run_info_t *ri = lua_touserdata(state, -1);
nvlist_t **props = lua_touserdata(state, 1);
nvlist_t **props = lua_touserdata(state, lua_upvalueindex(1));
nvpair_t *pair = lua_touserdata(state, lua_upvalueindex(2));
zcp_list_info_t *info = lua_touserdata(state, lua_upvalueindex(1));
zcp_synctask_info_t *info = lua_touserdata(state, lua_upvalueindex(1));