[erlang-questions] using variable in bit syntax

Gaspar Chilingarov <>
Sat Jul 14 09:01:23 CEST 2007


Haobo Yu wrote:
> Hi,
> 
> I'm rather new to erlang, so please pardon my ignorance.
> 
> I'm trying to parse a packet header using the bit syntax.  There is a  
> byte in the header that indicates the endianness of the subsequent  
> integers in the packet header.  For example:
> 
> endianness : 8/unsigned-integer
> %% The following integers follow the above endianness
> value1     : 32/integer
> value2	   : 32/integer
> ...
> 
> Because the endianness is specified in the bit syntax, such as  
> unsigned-little-integer, it would be nice to encode the endianness in  
> a variable when I parse that byte, then use this variable to parse  
> the following integers in the packet header.  I would like to have  
> something like this:
> 
> if Endianness == 1 ->
>    Spec = "32/little-integer";
>     true ->
>    Spec = "32/big-integer";
> end,
> <<Value1:Spec, Value2:Spec>> = Header,
> 
> However, putting a variable in the place of bit type specifier does  
> not seem to work.  I can't put a BIF, such as list_to_atom, there  
> either.
> 
> Any suggestions whether and how this might be done?
> 

The most straightforward method is something like this :)


  if Endianness == 1 ->
      <<Value1:32/little-integer, Value2:32/little-integer>> = Header;
      true ->
      <<Value1:32/big-integer, Value2:32/big-integer>> = Header;
  end,

:)

In other hand you can construct string and then
transform it to erlang code, see
http://www.erlang.org/pipermail/erlang-questions/2005-April/015274.html

This will allow you construct fun before parsing data, and them apply 
that fun on each packet.


-- 
Gaspar Chilingarov

System Administrator,
Network security consulting

t +37493 419763 (mob)
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