Blitz3D+ Command Reference

UDPMsgIP ( udp_stream )

Parameters

udp_stream - a UDP stream handle returned by CreateUDPStream

Description

Returns the IP address the most recently received UDP packet came from.

After RecvUDPMsg delivers a packet, UDPMsgIP tells you which machine sent it - the same packed integer RecvUDPMsg itself returned, kept around so you can ask again while handling the message. Pair it with UDPMsgPort and you have the sender's full return address: pass both straight to SendUDPMsg to answer, which is exactly how a game host welcomes each new player in the example.

Because one UDP stream can hear from any number of machines, checking the sender of every packet is normal practice - it is how a server tells its players apart (and how it ignores strangers). DottedIP formats the value for a lobby screen or log.

The value only means something after a successful RecvUDPMsg on that stream; before the first packet it just echoes the stream's own bound address (0). It is refreshed by each newly received packet.

See also: UDPMsgPort, RecvUDPMsg, SendUDPMsg, DottedIP.

Example

; UDPMsgIP Example
; ----------------

; This program is both ends of a UDP conversation at once: two
; streams on this machine swap packets over the loopback address.

; The loopback address 127.0.0.1 as a packed integer
loopback=(127 Shl 24) Or 1

host=CreateUDPStream(5405)
If host=0 Then
    Print "Could not bind port 5405 (it may be in use)"
Else
    player=CreateUDPStream()
    If player=0 Then
        Print "Player: could not create a UDP stream"
    Else
        WriteLine player,"Anyone there?"
        SendUDPMsg player,loopback,5405
        Print "Player: sent a packet from port "+UDPStreamPort(player)

        UDPTimeouts 500
        If RecvUDPMsg(host)=0 Then
            Print "Host: no packet arrived"
        Else
            Print "Host: message = '"+ReadLine$(host)+"'"

            ; UDPMsgIP returns the sender's IP address, so a game
            ; server knows who each packet came from
            sender_ip=UDPMsgIP(host)
            sender_port=UDPMsgPort(host)
            Print "Host: UDPMsgIP says it came from "+DottedIP$(sender_ip)

            ; ...and gives us everything needed to reply
            WriteLine host,"Welcome aboard!"
            SendUDPMsg host,sender_ip,sender_port
            If RecvUDPMsg(player)<>0 Then
                Print "Player: reply = '"+ReadLine$(player)+"'"
            EndIf
        EndIf

        CloseUDPStream player
    EndIf
    CloseUDPStream host
EndIf

Print ""
Print "Press any key to close the example"
WaitKey

End

Index