关于sendarp函数问题

Question

为什么下面代码所运行得到的ErrCode都是87啊!求各位高手帮忙.Function TForm1.sendarp(ipaddr:ulong;
                        temp:dword;ulmacaddr:pointer;
                        ulmacaddrleng:pointer) : DWord;
                        StdCall; External 'Iphlpapi.dll' Name 'SendARP';procedure TForm1.Button1Click(Sender: TObject);
var
  myip:ulong;
  mymac:array[0..5] of byte;
  mymaclength:ulong;
  ErrCode:integer;
begin
    myip:=inet_addr(PChar(edit1.text));
    ShowMessage(IntToHex(myip,8));
    mymaclength:=6;
    ErrCode:=sendarp(myip,0,@mymac,@mymaclength);
     ShowMessage( 'dwResult:   '+IntToStr(ErrCode));
    label1.caption:='errorcode:'+inttostr(ErrCode);
    label2.caption:=format('%2.2x:%2.2x:%2.2x:%2.2x:%2.2x:%2.2x',[mymac[0],mymac[1],mymac[2],mymac[3],mymac[4],mymac[5]]);
end;