Example sending empty binary over a TCP socket

gistfile1.txt

Let's actually see what's going on. Change client to send via this "tunnel":

socat -v TCP4-LISTEN:9898,fork TCP4:localhost:5678     
> 2012/09/06 14:05:24.413264  length=5 from=0 to=4
hello


This confirms that sending <<>> is, eventually, a nop in the sense of no data sent over the wire. But it takes a number of function calls to not send anything.

output.txt

9> spawn(fun() -> tcp1:server() end).
<0.54.0>
10> tcp1:client(<<>>).
ok
S: accepted

This is bogus. What you're seeing is that the socket gets closed by the client that sends nothing and then the empty list which is passed into do_recv is converted to binary and then printed.

S: received: '<<>>'
11> 

tcp1.erl

-module(tcp1).

-compile([export_all]).


server() ->
    {ok, LSock} = gen_tcp:listen(5678, [binary, {packet, 0}, 
                                        {active, false}]),
    {ok, Sock} = gen_tcp:accept(LSock),
    io:format("S: accepted~n"),
    {ok, Bin} = do_recv(Sock, []),
    io:format("S: received: '~p'~n", [Bin]),
    ok = gen_tcp:close(Sock),
    Bin.

do_recv(Sock, Bs) ->
    case gen_tcp:recv(Sock, 0) of
        {ok, B} ->
            do_recv(Sock, [Bs, B]);
        {error, closed} ->
            {ok, list_to_binary(Bs)}
    end.

client(Bin) ->
    SomeHostInNet = "localhost", % to make it runnable on one machine
    {ok, Sock} = gen_tcp:connect(SomeHostInNet, 9898, 
                                 [binary, {packet, 0}]),
    ok = gen_tcp:send(Sock, Bin),
    ok = gen_tcp:close(Sock).

Good suggestions. The bit with socat is a cheap and fast substitute for wireshark. It provides a text dump of anything sent to the socket and you can see that nothing arrives in the empty case.