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Asked: 2026-06-11T03:25:43+00:00

I am trying to make a simple UDP packet decoder. packet_decoder(Packet)-> <<Opts:8,MobIdLength:8,MobId:64,MobIdType:8,MgeType:8,SeqNum:16,Rest/binary>> = Packet,

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I am trying to make a simple UDP packet decoder. 

packet_decoder(Packet)->
<<Opts:8,MobIdLength:8,MobId:64,MobIdType:8,MgeType:8,SeqNum:16,Rest/binary>> = Packet,
io:format("Options:~p~n",Opts),
io:format("MobIdLength:~p~n",MobIdLength),
io:format("MobId:~p~n",MobId),
io:format("MobIdType:~p~n",MobIdType),
io:format("MgeType:~p~n",MgeType),
io:format("SeqNum:~p~n",SeqNum).

Packet is passed by a receive loop:

rcv_loop(Socket) ->
inet:setopts(Socket, [{active, once}, binary]),
io:format("rcvr started:~n"),
receive
    {udp, Socket, Host, Port, Bin} ->
        packet_decoder(Bin),
        rcv_loop(Socket)
end.

I keep getting(following error edited 9/7/12 9:30 EST): 

** exception error: no match of right hand side value 
                    <<131,8,53,134,150,4,149,0,80,15,1,2,1,2,0,16,80,71,115,
                      52,80,71,115,53,24,63,227,197,211,...>>
     in function  udp_server:packet_decoder/1
        called as udp_server:packet_decoder(<<131,8,53,134,150,4,149,0,80,15,
                                              1,2,1,2,0,16,80,71,115,52,80,71,
                                              115,53,24,63,227,197,...>>)
     in call from udp_server:rcv_loop/1 
     in call from udp_server:init/0

If I create the same variable in the Erlang shell as a binary, i.e. 

Packet = <<131,8,53,134,150,4,149,0,80,15,1,2,1,2,0,16,80,71,115,52,80,71,115,53,24,63,227,197,211,228,89,72,0,0,0,0,0,0,0,16,0,5,5,32,1,4,255,159,15,18,28,0,34,62,2,0,0,0,0,0,0,0,47,67>>.

<<Opts:8,MobIdLength:8,MobId:64,MobIdType:8,MgeType:8,SeqNum:16,Rest/binary>> = Packet.

It works just fine. Is there some subtlety in passing this to a function that I am missing? I have tried what I think is everything(except the right way). I tried setting the type and size. I also just tried 

<<Rest/binary>> = Packet.

To no avail. Any help much appreciated.

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1 Answer

  1. Editorial Team

    I figured it out(kinda). I had been working on this in erlide in eclipse which had worked fine for all of the other parts of the. I tried compiling it from the erl shell and it worked fine. There must be some minor difference in the way eclipse is representing the source or the way it invokes the erlang compiler and shell. I will take it up with erlide.org. Thanks for the help!

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