Function lua_settop

3rd/lua-5.4.3/src/lapi.c:174–199

LUA_API void lua_settop (lua_State *L, int idx) {
  CallInfo *ci;
  StkId func, newtop;
  ptrdiff_t diff;  /* difference for new top */
  lua_lock(L);
  ci = L->ci;
  func = ci->func;
  if (idx >= 0) {
    api_check(L, idx <= ci->top - (func + 1), "new top too large");
    diff = ((func + 1) + idx) - L->top;
    for (; diff > 0; diff--)
      setnilvalue(s2v(L->top++));  /* clear new slots */
  }
  else {
    api_check(L, -(idx+1) <= (L->top - (func + 1)), "invalid new top");
    diff = idx + 1;  /* will "subtract" index (as it is negative) */
  }
  api_check(L, L->tbclist < L->top, "previous pop of an unclosed slot");
  newtop = L->top + diff;
  if (diff < 0 && L->tbclist >= newtop) {
    lua_assert(hastocloseCfunc(ci->nresults));
    luaF_close(L, newtop, CLOSEKTOP, 0);
  }
  L->top = newtop;  /* correct top only after closing any upvalue */
  lua_unlock(L);
}


LUA_API void lua_closeslot (lua_State *L, int idx) {