Streaming Input

[orbitz]

I am working with a protocol where the size of the following block is 
told to me so I can just convert the next N bytes to, say, a string.  
The problem is though, I'm trying to write this so it handles a stream 
properly, so in the binary I have could be all N bytes that I need, or 
something less than N. So at first I tried:

extract_string(Tail, 0, Res) ->
  {ok, {string, Res}, Tail};
extract_string(<<H, Tail/binary>>, Length, Res) ->
  extract_string(Tail, Length - 1, lists:append(Res, [H]));
extract_string(<<>>, Length, Res) ->
  case dispatch_message() of
    {decode, _, Data} ->
      extract_string(Data, Length, Res)
  end.

When the binary is empty but I still need more data it waits for more.  
I don't know if this is the proper idiom (it seems gross to me but I am 
unsure of how to do it otherwise).  This is incredibly slow though.  
With a long string that I need to extract it takes a lot of CPU and far 
too long.  So I decided to do:

extract_string(Data, Length, _) ->
  <<String:Length/binary, Tail/binary>> = Data,
  {ok, {string, binary_to_list(String)}, Tail}.

In terms of CPU and time this is much much better, but if I don't have 
all N bytes it won't work.  Any suggestions?