[erlang-questions] Binary Parsing

wolfmanjm <>
Sun Mar 22 04:35:48 CET 2009


Actually Length is bound as it is bound in the preceding Expression,
so what you are doing is valid I do it all the time, I think what you
need to do is put parenthesis around...

(Att:Length - 2)/binary

as mentioned in the docs...

"Used in a bit string construction, Value is an expression which
should evaluate to an integer, float or bit string. If the expression
is something else than a single literal or variable, it should be
enclosed in parenthesis."

On Mar 21, 12:52 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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