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# 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`](`m:socket#accept-nowait`),
[`connect/3`](`m:socket#connect-nowait`), [`recv/3,4`](`m:socket#recv-nowait`),
[`recvfrom/3,4`](`m:socket#recvfrom-nowait`),
[`recvmsg/2,3,5`](`m:socket#recvmsg-nowait`),
[`send/3,4`](`m:socket#send-nowait`), [`sendmsg/3,4`](`m:socket#sendmsg-nowait`)
and [`sendto/4,5`](`m:socket#sendto-nowait`)). This is achieved by setting the
`Timeout` argument to `nowait`. For instance, if calling the
[`recv/3`](`m:socket#recv-nowait`) 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`](`t:socket:select_info/0`)`}`

  `SelectInfo` contains the [`SelectHandle`](`t:socket:select_handle/0`).

- **On Windows** -
  `{completion, `[`CompletionInfo`](`t:socket:completion_info/0`)`}`

  `CompletionInfo` contains the
  [`CompletionHandle`](`t:socket:completion_handle/0`).

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`](`socket:cancel/2`).

- **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`:

| Tag        | Info value type                           |
| ---------- | ----------------------------------------- |
| select     | select_handle()                           |
| completion | \{completion_handle(), CompletionStatus\} |
| abort      | \{select_handle(), Reason :: term()\}     |

_Table: socket message info value type_

The `select_handle()` is the same as was returned in the
[`SelectInfo`](`t:socket:select_info/0`).

The `completion_handle()` is the same as was returned in the
[`CompletionInfo`](`t:socket:completion_info/0`).

## 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:

```text
--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`](`socket:use_registry/1`).

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


## Examples

### Completion asynchronous sendv

This is a simple example function that illustrates how to use
`socket:sendv/3` with asynchronous (nowait) on completion systems (Windows).\
Observe that this is not an illustration how to write a asynchronous
sendv function. Its just an example of what kind of messages and results
that can be expected. The example below basically (re-) implements:
[`socket:sendv(Sock, IOV, infinity)`](`socket:sendv/3`).

```erlang
completion_sendv(Sock, IOV) ->
    case socket:sendv(Sock, IOV, nowait) of
        ok -> % Complete success - We are done
            ok;
        {completion, {CompletionInfo, RestIOV0}} ->
            %% Some of IOV was sent, but the rest, RestIOV0, was scheduled
            case completion_sendv_await_result(Sock,
                                               CompletionInfo, RestIOV0) of
                ok -> % We done
                    ok;
                {ok, RestIOV} ->
                    completion_sendv(Sock, RestIOV);
                {error, Reason} ->
                    {error, {Reason, RestIOV0}}
            end;
        {completion, CompletionInfo} ->
            %% Nothing was sent, IOV was scheduled
            case completion_sendv_await_result(Sock,
                                               CompletionInfo, IOV) of
                ok -> % We done
                    ok;
                {ok, RestIOV} ->
                    completion_sendv(Sock, RestIOV);
                {error, _} = ERROR ->
                    ERROR
            end;
        {error, {_Reason, _RestIOV}} = ERROR ->
            %% Some part of the I/O vector was sent before an error occured
            ERROR;
        {error, _} = ERROR ->
            %% Note that 
            ERROR
    end.

completion_sendv_await_result(Sock,
                              {completion_info, _, Handle},
                              IOV) ->
  receive
      {'$socket', Sock, abort, {Handle, Reason}} ->
          ?P("unexpected abort: "
             "~n   Reason: ~p", [Reason]),
          {error, {abort, Reason}};

      {'$socket', Sock, completion, {Handle, {ok, Written}}} ->
          %% Partial send; calculate rest I/O vector
          case socket:rest_iov(Written, IOV) of
              [] -> % We are done
                  ok;
              RestIOV ->
                  {ok, RestIOV}
          end;

      {'$socket', Sock, completion, {Handle, CompletionStatus}} ->
          CompletionStatus

  end.
```

### Completion asynchronous recv

This is a simple example function that illustrates how to use
`socket:recv/3` with asynchronous (nowait) on completion systems (Windows).
Observe that this is not an illustration how to write a asynchronous
read function. Its just an example of what kind of messages and results
that can be expected. The example below basically (re-) implements:
[`socket:recv(Sock, Sz)`](`socket:recv/2`).

```erlang
completion_recv(Sock, Sz) when (Sz > 0) ->
    completion_recv(Sock, Sz, []).

completion_recv(_Sock, 0, [Bin] = _Acc) ->
    {ok, Bin};
completion_recv(_Sock, 0, Acc) ->
    {ok, erlang:iolist_to_binary(lists:reverse(Acc))};
completion_recv(Sock, Sz, Acc) ->
    case socket:recv(Sock, Sz, nowait) of
        {ok, Bin} when (byte_size(Bin) =:= Sz) ->
            completion_recv(Sock, 0, [Bin|Acc]);
        {ok, Bin} ->
            completion_recv(Sock, Sz-byte_size(Bin), [Bin|Acc]);

	{completion, CompletionInfo} ->
            case completion_recv_await_result(Sock, CompletionInfo) of
                {ok, Bin} ->
                    completion_recv(Sock, Sz-byte_size(Bin), [Bin|Acc]);
                {error, {_Reason, _Data}} = ERROR ->
                    ERROR;
                {error, _Reason} = ERROR ->
                    ERROR
	    end;

	{error, {_Reason, _Data}} = ERROR ->
	    ERROR;
	{error, _Reason} = ERROR ->
	    ERROR

    end.

completion_recv_await_result(Sock,
                             {completion_info, _, Handle}) ->
    receive
	{'$socket', Sock, abort, {Handle, Reason}} ->
	    {error, {abort, Reason}};

	{'$socket', Sock, completion, {Handle, {ok, _Bin} = OK}} ->
            %% We "should" be done
	    OK;
	{'$socket', Sock, completion, {Handle, {more, Bin}}} ->
            %% There is more to read
	    {ok, Bin};

	{'$socket', Sock, completion, {Handle, CompletionStatus}} ->
	    CompletionStatus

    end.
```

### Echo server (and client)

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

```erlang
-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 }

## Socket Options

[](){: #socket_options_otp }
Options for level `otp`:

| Option Name         | Value Type                                                    | Set | Get | Other Requirements and comments                                                                                                                |
| ------------------- | ------------------------------------------------------------- | --- | --- | ---------------------------------------------------------------------------------------------------------------------------------------------- |
| assoc_id            | integer()                                                     | no  | yes | type = seqpacket, protocol = sctp, is an association                                                                                           |
| debug               | boolean()                                                     | yes | yes | none                                                                                                                                           |
| iow                 | boolean()                                                     | yes | yes | none                                                                                                                                           |
| controlling_process | pid()                                                         | yes | yes | none                                                                                                                                           |
| rcvbuf              | default \| pos_integer() \| \{pos_integer(), pos_ineteger()\} | yes | yes | The tuple format is _not_ allowed on Windows. 'default' only valid for set. The tuple form is only valid for type 'stream' and protocol 'tcp'. |
| rcvctrlbuf          | default \| pos_integer()                                      | yes | yes | default only valid for set                                                                                                                     |
| sndctrlbuf          | default \| pos_integer()                                      | yes | yes | default only valid for set                                                                                                                     |
| fd                  | integer()                                                     | no  | yes | none                                                                                                                                           |
| use_registry        | boolean()                                                     | no  | yes | the value is set when the socket is created, by a call to [`open/2`](`socket:open/2`) or [`open/4`](`socket:open/4`).                          |

_Table: option levels_

[](){: #socket_options_socket }
Options for level `socket`:

| Option Name      | Value Type        | Set | Get | Other Requirements and comments                                                                                                                                                                                                                                                                                                                                                                                                                      |
| ---------------- | ----------------- | --- | --- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| acceptconn       | boolean()         | no  | yes | none                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| bindtodevice     | string()          | yes | yes | Before Linux 3.8, this socket option could be set, but not get. Only works for some socket types (e.g. `inet`). If empty value is set, the binding is removed.                                                                                                                                                                                                                                                                                       |
| broadcast        | boolean()         | yes | yes | type = dgram                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| bsp_state        | map()             | no  | yes | Windows only                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| debug            | integer()         | yes | yes | may require admin capability                                                                                                                                                                                                                                                                                                                                                                                                                         |
| domain           | domain()          | no  | yes | _Not_ on FreeBSD (for instance)                                                                                                                                                                                                                                                                                                                                                                                                                      |
| dontroute        | boolean()         | yes | yes | none                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| exclusiveaddruse | boolean()         | yes | yes | Windows only                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| keepalive        | boolean()         | yes | yes | none                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| linger           | abort \| linger() | yes | yes | none                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| maxdg            | integer()         | no  | yes | Windows only                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| max_msg_size     | integer()         | no  | yes | Windows only                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| oobinline        | boolean()         | yes | yes | none                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| peek_off         | integer()         | yes | yes | domain = local (unix). Currently disabled due to a possible infinite loop when calling recv(\[peek]) the second time.                                                                                                                                                                                                                                                                                                                                |
| priority         | integer()         | yes | yes | none                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| protocol         | protocol()        | no  | yes | _Not_ on (some) Darwin (for instance)                                                                                                                                                                                                                                                                                                                                                                                                                |
| rcvbuf           | non_neg_integer() | yes | yes | none                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| rcvlowat         | non_neg_integer() | yes | yes | none                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| rcvtimeo         | timeval()         | yes | yes | This option is not normally supported. OTP has to be explicitly built with the `--enable-esock-rcvsndtime` configure option for this to be available. Since our implementation is _nonblocking_, its unknown if and how this option works, or even if it may cause malfunctions. Therefore, we do not recommend setting this option. Instead, use the `Timeout` argument to, for instance, the [`recv/3`](`socket:recv/3`) function. |
| reuseaddr        | boolean()         | yes | yes | none                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| reuseport        | boolean()         | yes | yes | domain = inet \| inet6                                                                                                                                                                                                                                                                                                                                                                                                                               |
| sndbuf           | non_neg_integer() | yes | yes | none                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| sndlowat         | non_neg_integer() | yes | yes | not changeable on Linux                                                                                                                                                                                                                                                                                                                                                                                                                              |
| sndtimeo         | timeval()         | yes | yes | This option is not normally supported. OTP has to be explicitly built with the `--enable-esock-rcvsndtime` configure option for this to be available. Since our implementation is _nonblocking_, its unknown if and how this option works, or even if it may cause malfunctions. Therefore, we do not recommend setting this option. Instead, use the `Timeout` argument to, for instance, the [`send/3`](`socket:send/3`) function. |
| timestamp        | boolean()         | yes | yes | none                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| type             | type()            | no  | yes | none                                                                                                                                                                                                                                                                                                                                                                                                                                                 |

_Table: socket options_

[](){: #socket_options_ip }
Options for level `ip`:

| Option Name            | Value Type            | Set | Get | Other Requirements and comments                            |
| ---------------------- | --------------------- | --- | --- | ---------------------------------------------------------- |
| add_membership         | ip_mreq()             | yes | no  | none                                                       |
| add_source_membership  | ip_mreq_source()      | yes | no  | none                                                       |
| block_source           | ip_mreq_source()      | yes | no  | none                                                       |
| drop_membership        | ip_mreq()             | yes | no  | none                                                       |
| drop_source_membership | ip_mreq_source()      | yes | no  | none                                                       |
| freebind               | boolean()             | yes | yes | none                                                       |
| hdrincl                | boolean()             | yes | yes | type = raw                                                 |
| minttl                 | integer()             | yes | yes | type = raw                                                 |
| msfilter               | null \| ip_msfilter() | yes | no  | none                                                       |
| mtu                    | integer()             | no  | yes | type = raw                                                 |
| mtu_discover           | ip_pmtudisc()         | yes | yes | none                                                       |
| multicast_all          | boolean()             | yes | yes | none                                                       |
| multicast_if           | any \| ip4_address()  | yes | yes | none                                                       |
| multicast_loop         | boolean()             | yes | yes | none                                                       |
| multicast_ttl          | uint8()               | yes | yes | none                                                       |
| nodefrag               | boolean()             | yes | yes | type = raw                                                 |
| pktinfo                | boolean()             | yes | yes | type = dgram                                               |
| recvdstaddr            | boolean()             | yes | yes | type = dgram                                               |
| recverr                | boolean()             | yes | yes | none                                                       |
| recvif                 | boolean()             | yes | yes | type = dgram \| raw                                        |
| recvopts               | boolean()             | yes | yes | type =/= stream                                            |
| recvorigdstaddr        | boolean()             | yes | yes | none                                                       |
| recvttl                | boolean()             | yes | yes | type =/= stream                                            |
| retopts                | boolean()             | yes | yes | type =/= stream                                            |
| router_alert           | integer()             | yes | yes | type = raw                                                 |
| sendsrcaddr            | boolean()             | yes | yes | none                                                       |
| tos                    | ip_tos()              | yes | yes | some high-priority levels may require superuser capability |
| transparent            | boolean()             | yes | yes | requires admin capability                                  |
| ttl                    | integer()             | yes | yes | none                                                       |
| unblock_source         | ip_mreq_source()      | yes | no  | none                                                       |

_Table: ip options_

[](){: #socket_options_ipv6 }
Options for level `ipv6`:

| Option Name            | Value Type         | Set | Get | Other Requirements and comments                                                                                                                                                                         |
| ---------------------- | ------------------ | --- | --- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| addrform               | inet               | yes | no  | allowed only for IPv6 sockets that are connected and bound to a v4-mapped-on-v6 address                                                                                                                 |
| add_membership         | ipv6_mreq()        | yes | no  | none                                                                                                                                                                                                    |
| authhdr                | boolean()          | yes | yes | type = dgram \| raw, obsolete?                                                                                                                                                                          |
| drop_membership        | ipv6_mreq()        | yes | no  | none                                                                                                                                                                                                    |
| dstopts                | boolean()          | yes | yes | type = dgram \| raw, requires superuser privileges to update                                                                                                                                            |
| flowinfo               | boolean()          | yes | yes | type = dgram \| raw, requires superuser privileges to update                                                                                                                                            |
| hoplimit               | boolean()          | yes | yes | type = dgram \| raw. On some platforms (e.g. FreeBSD) is used to set in order to get `hoplimit` as a control message heeader. On others (e.g. Linux), `recvhoplimit` is set in order to get `hoplimit`. |
| hopopts                | boolean()          | yes | yes | type = dgram \| raw, requires superuser privileges to update                                                                                                                                            |
| mtu                    | boolean()          | yes | yes | Get: Only after the socket has been connected                                                                                                                                                           |
| mtu_discover           | ipv6_pmtudisc()    | yes | yes | none                                                                                                                                                                                                    |
| multicast_hops         | default \| uint8() | yes | yes | none                                                                                                                                                                                                    |
| multicast_if           | integer()          | yes | yes | type = dgram \| raw                                                                                                                                                                                     |
| multicast_loop         | boolean()          | yes | yes | none                                                                                                                                                                                                    |
| recverr                | boolean()          | yes | yes | none                                                                                                                                                                                                    |
| recvhoplimit           | boolean()          | yes | yes | type = dgram \| raw. On some platforms (e.g. Linux), `recvhoplimit` is set in order to get `hoplimit`                                                                                                   |
| recvpktinfo \| pktinfo | boolean()          | yes | yes | type = dgram \| raw. On some platforms (e.g. FreeBSD) is used to set in order to get `hoplimit` as a control message heeader. On others (e.g. Linux), `recvhoplimit` is set in order to get `hoplimit`. |
| recvtclass             | boolean()          | yes | yes | type = dgram \| raw. On some platforms is used to set (=true) in order to get the `tclass` control message heeader. On others, `tclass` is set in order to get `tclass` control message heeader.        |
| router_alert           | integer()          | yes | yes | type = raw                                                                                                                                                                                              |
| rthdr                  | boolean()          | yes | yes | type = dgram \| raw, requires superuser privileges to update                                                                                                                                            |
| tclass                 | integer()          | yes | yes | Set the traffic class associated with outgoing packets. RFC3542.                                                                                                                                        |
| unicast_hops           | default \| uint8() | yes | yes | none                                                                                                                                                                                                    |
| v6only                 | boolean()          | yes | no  | none                                                                                                                                                                                                    |

_Table: ipv6 options_

[](){: #socket_options_tcp }
Options for level `tcp`:

| Option Name | Value Type | Set | Get | Other Requirements and comments                                                                          |
| ----------- | ---------- | --- | --- | -------------------------------------------------------------------------------------------------------- |
| congestion  | string()   | yes | yes | none                                                                                                     |
| cork        | boolean()  | yes | yes | 'nopush' one some platforms (FreeBSD)                                                                    |
| keepcnt     | integer()  | yes | yes | On Windows (at least), it is illegal to set to a value greater than 255.                                 |
| keepidle    | integer()  | yes | yes | none                                                                                                     |
| keepintvl   | integer()  | yes | yes | none                                                                                                     |
| maxseg      | integer()  | yes | yes | Set not allowed on all platforms.                                                                        |
| nodelay     | boolean()  | yes | yes | none                                                                                                     |
| nopush      | boolean()  | yes | yes | 'cork' on some platforms (Linux). On Darwin this has a different meaning than on, for instance, FreeBSD. |

_Table: tcp options_

[](){: #socket_options_udp }
Options for level `udp`:

| Option Name | Value Type | Set | Get | Other Requirements and comments |
| ----------- | ---------- | --- | --- | ------------------------------- |
| cork        | boolean()  | yes | yes | none                            |

_Table: udp options_

[](){: #socket_options_sctp }
Options for level `sctp`:

| Option Name       | Value Type             | Set | Get | Other Requirements and comments |
| ----------------- | ---------------------- | --- | --- | ------------------------------- |
| associnfo         | sctp_assocparams()     | yes | yes | none                            |
| autoclose         | non_neg_integer()      | yes | yes | none                            |
| disable_fragments | boolean()              | yes | yes | none                            |
| events            | sctp_event_subscribe() | yes | no  | none                            |
| initmsg           | sctp_initmsg()         | yes | yes | none                            |
| maxseg            | non_neg_integer()      | yes | yes | none                            |
| nodelay           | boolean()              | yes | yes | none                            |
| rtoinfo           | sctp_rtoinfo()         | yes | yes | none                            |

_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:

```text
--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](socket_usage.md#socket_options_socket) table):

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

### Extended Error Info

The use of [`Extended Error Info`](`t:socket:eei/0`) (currently only used on
Windows) can be explicitly enabled and disabled:

```text
--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:

```text
--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:

```text
--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:

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

See [socket registry](socket_usage.md#socket-registry) for more info.
