What is << >> ?

[Ulf Wiger]

<< and >> are used for the "Bit Syntax", which allows for
constructing binaries and matching  binary patterns.

http://www.erlang.org/doc/r8b/doc/extensions/bit_syntax.html

The following example from the documentation illustrates how
the bit syntax can be used in e.g. protocol programming -- in
this case doing pattern matching directly on an incoming IP
datagram.

-define(IP_VERSION, 4).
-define(IP_MIN_HDR_LEN, 5).

   DgramSize = size(Dgram),
   case Dgram of
      <<?IP_VERSION:4, HLen:4, SrvcType:8, TotLen:16,
        ID:16, Flgs:3, FragOff:13,
        TTL:8, Proto:8, HdrChkSum:16,
        SrcIP:32,
        DestIP:32, RestDgram/binary>> when HLen >= 5,
                                           4*HLen =< DgramSize ->
        OptsLen = 4*(HLen - ?IP_MIN_HDR_LEN),
        <<Opts:OptsLen/binary,Data/binary>> = RestDgram,
        ...
   end.

/Uffe

On Mon, 12 Nov 2001, Tanja Godau wrote:

>Hello Erlang Experts,
>
>Could someone please explain to me the purpose of <<>> and how
>of use it?
>
>As seen for example in:
>
>=> io:format("   global code:END (of attribute) from
> ~p~n",[<<?WBXML_END>>]),
>    	 ^^          ^^
>or
>
>=> case catch receive_data(Socket,Timeout,Method,<<>>) of
>						  ^^^^
>>Thankyou
>
>Tanja

-- 
Ulf Wiger, Senior Specialist,
   / / /   Architecture & Design of Carrier-Class Software
  / / /    Strategic Product & System Management
 / / /     Ericsson Telecom AB, ATM Multiservice Networks