Multicast on Windows
lazarus / 38-Networking and Web Programming

----------------------------------------------------------------------
iwilson · 2013-01-09 18:28

Hey, I can't seem to get this to work... yet I have no issues on
Linux. The below code works perfectly fine on my Linux machine but not
on any Windows.
Firewall is disabled, 10.0.0.3 is a NIC, 224.126.126.126 is a valid
multicast address receiving messages.

Code: [Select]program project1;

{$mode objfpc}{$H+}

uses

  Classes, Sockets,
  {$IFDEF MSWINDOWS}
  Windows,
  Winsock,
  {$ELSE}
  BaseUnix,
  UnixType,
  {$ENDIF}
   Sysutils;

type
TIP_mreq = packed record
    imr_multiaddr: in_addr;     // IP multicast address of group
    imr_interface: in_addr;     // local IP address of interfacetype
  end;

var
  ASock_Addr : SockAddr_In;
  FSocket    : LongInt;
  ipmreq     : TIP_mreq;
  MasterFD,
  TempFD     : TFDSet;
  TestInt    : Integer;
  Timeout    : TTimeVal;
begin
  FSocket := fpSocket(AF_INET, LongInt(SOCK_DGRAM), 0);
  if (FSocket > 0) then
  begin
    fpSetSockOpt(FSocket, SOL_SOCKET, SO_REUSEADDR, PChar('True'),
Length('True'));
    FillByte(ASock_Addr, SizeOf(SockAddr_In), 0);
    ASock_Addr.sin_family      := AF_INET;
    ASock_Addr.sin_port        := HtoNS(15510);
    ASock_Addr.sin_addr.S_addr := INADDR_ANY;

    if (fpbind(FSocket, @ASock_Addr, SizeOf(SockAddr_In)) = 0) then
    begin
      FillByte(ipmreq, SizeOf(Tip_mreq),0);
      ipmreq.imr_multiaddr.s_addr :=
StrToNetAddr('224.126.126.126').s_addr;
      ipmreq.imr_interface.s_addr := StrToNetAddr('10.0.0.3').s_addr;
      fpSetsockOpt(FSocket, IPPROTO_IP, IP_ADD_MEMBERSHIP,  @ipmreq,
SizeOf(ipmreq));
      {$IFDEF MSWINDOWS}
          FD_SET(FSocket, MasterFD);
      {$ELSE}
          fpFD_SET(FSocket, MasterFD);
      {$ENDIF}

      while (true) do
      begin
        TempFD := MasterFD;
        TimeOut.tv_sec  := 0;
        TimeOut.tv_usec := 20;

        {$IFDEF MSWINDOWS}
           TestInt := Select(FSocket, @TempFD, NIL, NIL, @TimeOut);
        {$ELSE}
           TestInt := fpSelect(FSocket, @TempFD, NIL, NIL, @TimeOut);
        {$ENDIF}

        if (TestInt > 0) then
        begin
          TestInt := 0;
        end;
        Sleep(1);
      end;
    end;
  end;
end.

Any tips would be greatly appreciated. Thanks.

EDIT: I should have been more clear, everything works other than
Select never being > on 0 (not receiving messages). It stays within
the loop.

----------------------------------------------------------------------
iwilson · 2013-01-09 18:45

I would like to add that I just got this to work on a Windows XP VM
running on my host machine that cannot do the same (Windows 7).

Looks like this may be a Windows 7 (vista maybe) issue only. Has
anyone successfully pulled this off on 7?

----------------------------------------------------------------------
Zoran · 2013-01-09 22:19

I can't see break in your while loop.

----------------------------------------------------------------------
iwilson · 2013-04-03 15:01

bump, still having the same issue.

----------------------------------------------------------------------
iwilson · 2013-04-04 15:55

The answer is to use the Winsock calls for Bind and SetSockOpt on
Windows 7 and up. This isn't necessary on XP.

----------------------------------------------------------------------
BigChimp · 2013-04-04 19:52

iwilson,

Thanks for posting the solution! (And sorry I couldn't help)