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).

Try changing line 21 from:

{ok, list_to_binary(Bs)}

to:

{ok, list_to_binary([Bs, <<"closed">>])}

and you'll see that all you get is the closed message.

Better yet, change line 19 from:

do_recv(Sock, [Bs, B]);

to:

do_recv(Sock, [Bs, <<B/binary, "XX">>]);

and you won't see any "XX" in your result if you send an empty binary from the client.

You can also watch your localhost interface and port 5678 with wireshark, and if you try to send an empty binary you'll never see a PSH sent but you will if you send actual data.

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.