The following code does work for the {packet,0} option in the gen_tcp:connect() function call but not for 1 , 2 and 4 (although I only tested it for 4 and I'm assuming 1 and 2 doesn't work either). The question I have is why not and is it important to use one over the other? Basically the Erlang documentation does not address a detailed explanation of the subject regarding the packet options and Programming Erlang by Joe Armstrong doesn't give any great detail either; he just explains that the packets aren't reassembled in order although I always thought tcp packets are received as they are sent, unlike UDP. One interesting note I have is that the client-server on this page has {packet,4} as the option and it works fine and its very similar to this code below. Heres the server shell output for {packet,4} option used in the code below.
Erlang R16A (erts-5.10) [smp:8:8] [async-threads:10]
Eshell V5.10 (abort with ^G)
1> cd("c:/erlang").
c:/erlang
ok
2> c(cp3).
{ok,cp3}
3> cp3:server().
Started Server:
<0.41.0>
Accept Server:
Pid <0.43.0>
Connection accepted
Accept Server:
Loop Server:
Error on socket #Port<0.2256> reason: einval
This is the client side shell output.
Erlang R16A (erts-5.10) [smp:8:8] [async-threads:10]
Eshell V5.10 (abort with ^G)
1> cd("c:/erlang").
c:/erlang
ok
2> cp3:client().
exit
3>
and this is the code used,
-module(cp3).
-export([client/0, server/0,start/0,accept/1,enter_loop/1,loop/1]).
client() ->
{ok, Socket} = gen_tcp:connect("localhost", 4001,[binary, {packet, 4}]),
ok = gen_tcp:send(Socket, "packet"),
receive
{tcp,Socket,String} ->
io:format("Client received = ~p~n",[String]),
io:format("Client result = ~p~n",[String]),
gen_tcp:close(Socket)
after 1000 ->
exit
end.
server() ->
Pid = spawn(fun()-> start() end),
Pid.
start() ->
io:format("Started Server:~n"),
{ok, Socket} = gen_tcp:listen(4001, [binary, {packet, 4},{reuseaddr, true},{active, false}]),
accept(Socket).
accept(ListenSocket) ->
io:format("Accept Server:~n"),
case gen_tcp:accept(ListenSocket) of
{ok, Socket} ->
Pid = spawn(fun() ->
io:format("Connection accepted ~n", []),
enter_loop(Socket)
end),
io:format("Pid ~p~n",[Pid]),
gen_tcp:controlling_process(Socket, Pid),
Pid ! ack,
accept(ListenSocket);
Error ->
exit(Error)
end.
enter_loop(Socket) ->
%% make sure to acknowledge owner rights transmission finished
receive ack -> ok end,
loop(Socket).
loop(Socket) ->
io:format("Loop Server:~n"),
case gen_tcp:recv(Socket, 6) of
{ok, Data} ->
case Data of
<<"packet">> ->
io:format("Server replying = ~p~n",[Data]),
gen_tcp:send(Socket, Data),
loop(Socket)
end;
{error, Reason} ->
io:format("Error on socket ~p reason: ~p~n", [Socket, Reason])
end.
Your code is nearly ok, just modify in the loop(Socket) the line
case gen_tcp:recv(Socket, 6) of
by
case gen_tcp:recv(Socket, 0) of
see documentation:
recv(Socket, Length) -> {ok, Packet} | {error, Reason} recv(Socket,Length, Timeout) -> {ok, Packet} | {error, Reason}
Types: Socket = socket() Length = integer() >= 0 Timeout = timeout() Packet = string() | binary() | HttpPacket Reason = closed | inet:posix() HttpPacket = term() See the description of HttpPacket in erlang:decode_packet/3.
This function receives a packet from a socket in passive mode. A closed socket is indicated by a return value {error, closed}.
The Length argument is only meaningful when the socket is in raw mode and denotes the number of bytes to read. If Length = 0, all available bytes are returned. If Length > 0, exactly Length bytes are returned, or an error; possibly discarding less than Length bytes of data when the socket gets closed from the other side.
The optional Timeout parameter specifies a timeout in milliseconds. The default value is infinity.
With this modification it works, but as the client is closing the socket after each call, you get an error message, I am not sure it is useful to close the socket from the client. You can change this behavior by modifying the client this way:
client() ->
{ok, Socket} = gen_tcp:connect("localhost", 4001,[binary, {packet, 4}]),
ok = gen_tcp:send(Socket, "packet"),
receive
{tcp,Socket,String} ->
io:format("Client received = ~p~n",[String]),
io:format("Client result = ~p~n",[String])
%gen_tcp:close(Socket)
after 1000 ->
exit
end.