Function index2stack

lib/lua/src/lapi.c:95–108

** Convert a valid actual index (not a pseudo-index) to its address. */

** Convert a valid actual index (not a pseudo-index) to its address.
*/
l_sinline StkId index2stack (lua_State *L, int idx) {
  CallInfo *ci = L->ci;
  if (idx > 0) {
    StkId o = ci->func.p + idx;
    api_check(L, o < L->top.p, "invalid index");
    return o;
  }
  else {    /* non-positive index */
    api_check(L, idx != 0 && -idx <= L->top.p - (ci->func.p + 1),
                 "invalid index");
    api_check(L, !ispseudo(idx), "invalid index");
    return L->top.p + idx;
  }
}


LUA_API int lua_checkstack (lua_State *L, int n) {