Function index2value

lua/src/lapi.c:60–87

** Convert an acceptable index to a pointer to its respective value. ** Non-valid indices return the special nil value 'G(L)->nilvalue'. */

** Non-valid indices return the special nil value 'G(L)->nilvalue'.
*/
static TValue *index2value (lua_State *L, int idx) {
  CallInfo *ci = L->ci;
  if (idx > 0) {
    StkId o = ci->func + idx;
    api_check(L, idx <= L->ci->top - (ci->func + 1), "unacceptable index");
    if (o >= L->top) return &G(L)->nilvalue;
    else return s2v(o);
  }
  else if (!ispseudo(idx)) {  /* negative index */
    api_check(L, idx != 0 && -idx <= L->top - (ci->func + 1), "invalid index");
    return s2v(L->top + idx);
  }
  else if (idx == LUA_REGISTRYINDEX)
    return &G(L)->l_registry;
  else {  /* upvalues */
    idx = LUA_REGISTRYINDEX - idx;
    api_check(L, idx <= MAXUPVAL + 1, "upvalue index too large");
    if (ttisCclosure(s2v(ci->func))) {  /* C closure? */
      CClosure *func = clCvalue(s2v(ci->func));
      return (idx <= func->nupvalues) ? &func->upvalue[idx-1]
                                      : &G(L)->nilvalue;
    }
    else {  /* light C function or Lua function (through a hook)?) */
      api_check(L, ttislcf(s2v(ci->func)), "caller not a C function");
      return &G(L)->nilvalue;  /* no upvalues */
    }
  }
}