# [erlang-questions] Binary Parsing

Torben Hoffmann <>
Sat Mar 21 22:14:31 CET 2009

```A classical problem!

My preferred solution to this is to do it in steps:

parse(<<Type:8,FullLength:8,TempRest/binary>>) ->
Length = FullLength - 2,
<< Value:Length/binary,Rest/binary >> = TempRest,
{Value,Rest}.

This approach has served us well, but I would have liked to be able to do
this more directly but the language does not allow it.

Cheers,
Torben

On Sat, Mar 21, 2009 at 8:52 PM, Pete Stapley <> wrote:

> I am attempting to parse some Radius attributes with Erlang and I have
> run into a bit of a problem. I pass some binary data to parseAttributes
> and I want to start peeling off the attributes one by one. I first peel
> off Type which is one byte, then Length which is also one byte. Next I
> want to peel off the Attribute. The Length variable is the length of the
> attribute plus the Type and Length byte, so I have to subtract 2 bytes
> from the Length variable before peeling off the attribute. I have tried
> the method below without any luck. Is there a way to do this?
>
> Thanks,
>
> -define(BYTE, 8/unsigned-big-integer).
>
> parseAttributes(Bin) ->
>     parseAttributes(Bin, []).
>
> parseAttributes(<<>>, Atts) ->
>     {ok, Atts};
> %%problem is here, Length - 2 does not seem to work
> parseAttributes( <<Type:?BYTE, Length:?BYTE, Att:Length - 2/binary,
> Rest/binary>>, Atts) ->
>     io:format("Type: ~p Length: ~p Att:~p Length: ~p~n", [Type, Length,
> Att, size(Rest)]),
>     parseAttributes(Rest, Atts).
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