Socket Usage

Introduction

The socket interface (module) is basically a "thin" layer on top of the OS socket interface. It is assumed that, unless you have special needs, gen_[tcp|udp|sctp] should be sufficient (when they become available).

Note that just because we have a documented and described option, it does not mean that the OS supports it. So its recommended that the user reads the platform specific documentation for the option used.

Asynchronous calls

Some functions allow for an asynchronous call (accept/2, connect/3, recv/3,4, recvfrom/3,4, recvmsg/2,3,5, send/3,4, sendmsg/3,4 and sendto/4,5). This is achieved by setting the Timeout argument to nowait. For instance, if calling the recv/3 function with Timeout set to nowait (i.e. recv(Sock, 0, nowait)) when there is actually nothing to read, it will return with:

• On Unix - {select,SelectInfo}

  SelectInfo contains the SelectHandle.

• On Windows - {completion,CompletionInfo}

  CompletionInfo contains the CompletionHandle.

When data eventually arrives a 'select' or 'completion' message will be sent to the caller:

• On Unix - {'$socket', socket(), select, SelectHandle}

  The caller can then make another call to the recv function and now expect data.

  Note that all other users are locked out until the 'current user' has called the function (recv in this case). So either immediately call the function or cancel.

• On Windows - {'$socket', socket(), completion, {CompletionHandle, CompletionStatus}}

  The CompletionStatus contains the result of the operation (read).

The user must also be prepared to receive an abort message:

• {'$socket', socket(), abort, Info}

If the operation is aborted for whatever reason (e.g. if the socket is closed "by someone else"). The Info part contains the abort reason (in this case that the socket has been closed Info = {SelectHandle, closed}).

The general form of the 'socket' message is:

• {'$socket', Sock :: socket(), Tag :: atom(), Info :: term()}

Where the format of Info is a function of Tag:

Table: socket message info value type

The select_handle() is the same as was returned in the SelectInfo.

The completion_handle() is the same as was returned in the CompletionInfo.

Socket Registry

The socket registry is how we keep track of sockets. There are two functions that can be used for interaction: socket:number_of/0 and socket:which_sockets/1.

In systems which create and delete many sockets dynamically, it (the socket registry) could become a bottleneck. For such systems, there are a couple of ways to control the use of the socket registry.

Firstly, its possible to effect the global default value when building OTP from source with the two configure options:

--enable-esock-socket-registry (default) | --disable-esock-socket-registry

Second, its possible to effect the global default value by setting the environment variable ESOCK_USE_SOCKET_REGISTRY (boolean) before starting the erlang.

Third, its possible to alter the global default value in runtime by calling the function use_registry/1.

And finally, its possible to override the global default when creating a socket (with open/2 and open/4) by providing the attribute use_registry (boolean) in the their Opts argument (which effects that specific socket).

Example

This example is intended to show how to create a simple (echo) server (and client).

-module(example).

-export([client/2, client/3]).
-export([server/0, server/1, server/2]).


%% ======================================================================

%% === Client ===

client(#{family := Family} = ServerSockAddr, Msg)
  when is_list(Msg) orelse is_binary(Msg) ->
    {ok, Sock} = socket:open(Family, stream, default),
    ok         = maybe_bind(Sock, Family),
    ok         = socket:connect(Sock, ServerSockAddr),
    client_exchange(Sock, Msg);

client(ServerPort, Msg)
  when is_integer(ServerPort) andalso (ServerPort > 0) ->
    Family   = inet, % Default
    Addr     = get_local_addr(Family), % Pick an address
    SockAddr = #{family => Family,
		 addr   => Addr,
		 port   => ServerPort},
    client(SockAddr, Msg).

client(ServerPort, ServerAddr, Msg)
  when is_integer(ServerPort) andalso (ServerPort > 0) andalso
       is_tuple(ServerAddr) ->
    Family   = which_family(ServerAddr),
    SockAddr = #{family => Family,
		 addr   => ServerAddr,
		 port   => ServerPort},
    client(SockAddr, Msg).

%% Send the message to the (echo) server and wait for the echo to come back.
client_exchange(Sock, Msg) when is_list(Msg) ->
    client_exchange(Sock, list_to_binary(Msg));
client_exchange(Sock, Msg) when is_binary(Msg) ->
    ok = socket:send(Sock, Msg, infinity),
    {ok, Msg} = socket:recv(Sock, byte_size(Msg), infinity),
    ok.


%% ======================================================================

%% === Server ===

server() ->
    %% Make system choose port (and address)
    server(0).

%% This function return the port and address that it actually uses,
%% in case server/0 or server/1 (with a port number) was used to start it.

server(#{family := Family, addr := Addr, port := _} = SockAddr) ->
    {ok, Sock} = socket:open(Family, stream, tcp),
    ok         = socket:bind(Sock, SockAddr),
    ok         = socket:listen(Sock),
    {ok, #{port := Port}} = socket:sockname(Sock),
    Acceptor = start_acceptor(Sock),
    {ok, {Port, Addr, Acceptor}};

server(Port) when is_integer(Port) ->
    Family   = inet, % Default
    Addr     = get_local_addr(Family), % Pick an address
    SockAddr = #{family => Family,
		 addr   => Addr,
		 port   => Port},
    server(SockAddr).

server(Port, Addr)
  when is_integer(Port) andalso (Port >= 0) andalso
       is_tuple(Addr) ->
    Family   = which_family(Addr),
    SockAddr = #{family => Family,
		 addr   => Addr,
		 port   => Port},
    server(SockAddr).


%% --- Echo Server - Acceptor ---

start_acceptor(LSock) ->
    Self = self(),
    {Pid, MRef} = spawn_monitor(fun() -> acceptor_init(Self, LSock) end),
    receive
	{'DOWN', MRef, process, Pid, Info} ->
	    erlang:error({failed_starting_acceptor, Info});
	{Pid, started} ->
	    %% Transfer ownership
	    socket:setopt(LSock, otp, owner, Pid),
	    Pid ! {self(), continue},
	    erlang:demonitor(MRef),
	    Pid
    end.
    
acceptor_init(Parent, LSock) ->
    Parent ! {self(), started},
    receive
	{Parent, continue} ->
	    ok
    end,
    acceptor_loop(LSock).

acceptor_loop(LSock) ->
    case socket:accept(LSock, infinity) of
	{ok, ASock} ->
	    start_handler(ASock),
	    acceptor_loop(LSock);
	{error, Reason} ->
	    erlang:error({accept_failed, Reason})
    end.


%% --- Echo Server - Handler ---

start_handler(Sock) ->
    Self = self(),
    {Pid, MRef} = spawn_monitor(fun() -> handler_init(Self, Sock) end),
    receive
	{'DOWN', MRef, process, Pid, Info} ->
	    erlang:error({failed_starting_handler, Info});
	{Pid, started} ->
	    %% Transfer ownership
	    socket:setopt(Sock, otp, owner, Pid),
	    Pid ! {self(), continue},
	    erlang:demonitor(MRef),
	    Pid
    end.

handler_init(Parent, Sock) ->
    Parent ! {self(), started},
    receive
	{Parent, continue} ->
	    ok
    end,
    handler_loop(Sock, undefined).

%% No "ongoing" reads
%% The use of 'nowait' here is clearly *overkill* for this use case,
%% but is intended as an example of how to use it.
handler_loop(Sock, undefined) ->
    case socket:recv(Sock, 0, nowait) of
	{ok, Data} ->
	    echo(Sock, Data),
	    handler_loop(Sock, undefined);

	{select, SelectInfo} ->
	    handler_loop(Sock, SelectInfo);

	{completion, CompletionInfo} ->
	    handler_loop(Sock, CompletionInfo);

	{error, Reason} ->
	    erlang:error({recv_failed, Reason})
    end;

%% This is the standard (asyncronous) behaviour.
handler_loop(Sock, {select_info, recv, SelectHandle}) ->
    receive
	{'$socket', Sock, select, SelectHandle} ->
	    case socket:recv(Sock, 0, nowait) of
		{ok, Data} ->
		    echo(Sock, Data),
		    handler_loop(Sock, undefined);

		{select, NewSelectInfo} ->
		    handler_loop(Sock, NewSelectInfo);

		{error, Reason} ->
		    erlang:error({recv_failed, Reason})
	    end
    end;

%% This is the (asyncronous) behaviour on platforms that support 'completion',
%% currently only Windows.
handler_loop(Sock, {completion_info, recv, CompletionHandle}) ->
    receive
	{'$socket', Sock, completion, {CompletionHandle, CompletionStatus}} ->
	    case CompletionStatus of
		{ok, Data} ->
		    echo(Sock, Data),
		    handler_loop(Sock, undefined);
		{error, Reason} ->
		    erlang:error({recv_failed, Reason})
	    end
    end.

echo(Sock, Data) when is_binary(Data) ->
    ok = socket:send(Sock, Data, infinity),
    io:format("** ECHO **"
	      "~n~s~n", [binary_to_list(Data)]).


%% ======================================================================

%% === Utility functions ===

maybe_bind(Sock, Family) ->
    maybe_bind(Sock, Family, os:type()).

maybe_bind(Sock, Family, {win32, _}) ->
    Addr     = get_local_addr(Family),
    SockAddr = #{family => Family,
                 addr   => Addr,
                 port   => 0},
    socket:bind(Sock, SockAddr);
maybe_bind(_Sock, _Family, _OS) ->
    ok.

%% The idea with this is extract a "usable" local address
%% that can be used even from *another* host. And doing
%% so using the net module.

get_local_addr(Family) ->
    Filter =
	fun(#{addr  := #{family := Fam},
	      flags := Flags}) ->
		(Fam =:= Family) andalso (not lists:member(loopback, Flags));
	   (_) ->
		false
	end,
    {ok, [SockAddr|_]} = net:getifaddrs(Filter),
    #{addr := #{addr := Addr}} = SockAddr,
    Addr.

which_family(Addr) when is_tuple(Addr) andalso (tuple_size(Addr) =:= 4) ->
    inet;
which_family(Addr) when is_tuple(Addr) andalso (tuple_size(Addr) =:= 8) ->
    inet6.

Socket Options

Options for level otp:

Table: option levels

Options for level socket:

Table: socket options

Options for level ip:

Table: ip options

Options for level ipv6:

Table: ipv6 options

Options for level tcp:

Table: tcp options

Options for level udp:

Table: udp options

Options for level sctp:

Table: sctp options

Socket Configure Flags

There are a couple of configure flags, that can be used when (configure and) building Erlang/OTP, which effect the functionality of the 'socket' nif.

Builtin Socket Support

Support for the builtin 'socket' (as a nif) can be explicitly enabled and disabled:

--enable-esock (default) | --disable-esock

RCVTIMEO/SNDTIMEO socket options

Support for these (socket) options has to be explicitly enabled. For details, see the specific option descriptions in the socket option table):

--enable-esock-rcvsndtimeo | --disable-esock-rcvsndtimeo (default)

Extended Error Info

The use of Extended Error Info (currently only used on Windows) can be explicitly enabled and disabled:

--enable-esock-extended-error-info (default) | --disable-esock-extended-error-info

Verbose Mutex Names

The 'socket' nif uses several mutex(s). Specifically, two for each socket; One for read and one for write. These mutex(s) are named as: esock.r[FD] & and esock.w[FD] (where FD is the file descriptor). Example: esock.r[10]. This is not normally a problem, but in some very specific debug scenarious, it can become a bottleneck. Therefor these names can be simplified to just e.g. "esock.r". (that is, all read mutex(s) have the same "name").

The use of these verbose mutex names (in the 'socket' nif) can be explicitly enabled and disabled:

--enable-esock-verbose-mtx-names (default) | --disable-esock-verbose-mtx-names

Counter Size

The 'socket' nif uses counters for various things (diagnistics and statistics). The size (in number of bits) of these counters can be explictly configured:

--with-esock-counter-size=SZ

Where SZ is one of 16 | 24 | 32 | 48 | 64. Defaults to 64.

Socket Registry

The socket registry keeps track of 'socket' sockets. This can be explicitly enabled and disabled:

--enable-esock-socket-registry (default) | --disable-esock-socket-registry

See socket registry for more info.