[erlang-questions] Encoding empty strings using ei

[Peter Membrey]

Hi Serge,

Thanks for taking the time to go through that, much appreciated!

Cheers,

Pete

----- Original Message -----
From: "Serge Aleynikov" <serge@REDACTED>
To: "Peter Membrey" <peter@REDACTED>
Cc: "Erlang Questions" <erlang-questions@REDACTED>
Sent: Thursday, 1 August, 2013 1:00:19 PM
Subject: Re: [erlang-questions] Encoding empty strings using ei

term_to_binary/1 is your friend:

1> term_to_binary({mylist, []}).
<<131,104,2,100,0,6,109,121,108,105,115,116,106>>

As you see at the end of that binary, the empty list gets encoded with a
special 106 ('j') tag, which is what ei_x_encode_empty_list() call does
under the hood.

$ grep ERL_NIL_EXT lib/erl_interface/include/ei.h
#define ERL_NIL_EXT           'j'

So the correct way to encode the {mylist, []} tuple in ei is to call:

      ei_x_new_with_version(pbuf);
      ei_x_encode_tuple_header(pbuf, 2);
      ei_x_encode_atom(pbuf, "mylist");
      ei_x_encode_empty_list(pbuf);

BR,

Serge

On 7/31/2013 11:57 AM, Peter Membrey wrote:
> Hi guys,
> 
> I've got a C program that encodes a list of tuples to send to an Erlang
> server, something like this:
> 
> {mylist, [{1,2,3} , {2,3,4} , {3,4,5}]}
> 
> To do this I encode a list header with:
> 
> ei_x_encode_list_header(ei_x_buff* x, int arity);
> 
> and all is well.
> 
> The question I have though, is what should I do if I encode this tuple?
> 
> {mylist, [] }
> 
> Should I encode the tuple, followed by an empty list
> (ei_x_encode_empty_list) or do I need to put in a list header with an
> arity of 0 and then add the empty list?
> 
> In short, what is the correct way to encode an empty list using the ei
> interface.
> 
> Thanks in advance!
> 
> Cheers,
> 
> Pete
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