[erlang-questions] segments of variable length

Gamoto <>
Fri Apr 3 18:13:06 CEST 2009


-module(aa).
-define(X,<<131,107,0,18,36,65,86,82,77,67,44,49,50,51,52,44,49,53,49,55,51,54>>).
-export([start/0]).

start()->
   case binary_to_term(?X) of 
	    {<<H,U,T>>}->
	     io:format("Header = ~s~n",[H]),
	     io:format("Unit ID = ~s~n",[U]),
	     io:format("Time = ~s~n",[T])
	end.

This gives me an error.
I was waiting for
Header = $AVRMC
Unit ID = 1234
Time =  151736

The most important for me is to obtain H,U and T
>
>
>> I receive from several machines messages and I would like to extract data.
>> My first approach was:
>> 
>> handler(Data)->
>>     case Data of 
>>         <>
>>         etc...
>> 
>> My problem is that some segment have a fixed length (header, Status) and some 
>> others a variable length (Time <= 6 bytes, UnitID <= 8 bytes).
>> My approach is bad. I thought to do:      Segments = string:tokens(Data,",") 
>> This separates the segments but I don't have their name 
>> (header,unitid,time,status) !
>> Could you advice me a better approach. My goal is to check each segment and make 
>> actions according to their values.
>
>You need to define a message format such that the required information is present to determine how to parse the rest of the message. This isn't really an erlang-specific problem but erlang is very good at supporting this.
>
>As an example you could use erlang:term_to_binary() and binary_to_term() for the message encoding on the wire. binary_to_term() might fail if the message is incomplete. This is where the {packet, N} inet options for connect/listen come in, eg. {packet,2} will only return complete messages; the length header is added automatically on send and stripped on receive so you only need to do, eg.
>
>gen_tcp:send( Sock, term_to_binary( MessageTerm ) )
>
>and if you receive {tcp, Sock, Data}, then it is safe to call
>
>MessageTerm = binary_to_term(Data)
>
>Check out the inet module documentation. There are lots of useful options.
>
>Hope that is useful
>
>--
>  Rich
>
>
>      The new Internet Explorer 8 optimised for Yahoo!7: Faster, Safer, Easier.




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