socket(3)

NAME
socket - Socket interface.

DESCRIPTION

This module provides an API for network socket. Functions are provided to create, delete and manipulate the sockets as well as sending and receiving data on them.

The intent is that it shall be as "close as possible" to the OS level socket interface. The only significant addition is that some of the functions, e.g. recv/3, have a time-out argument.

    Note:
Some functions allow for an asynchronous call. This is achieved by setting the Timeout argument to nowait or to a (select or completion) handle .

For instance, if calling the recv/3 function with Timeout set to nowait (recv(Sock, 0, nowait)) when there is actually nothing to read, it will return with either one of:

    :   {completion, CompletionInfo}

    :   {select, SelectInfo}

CompletionInfo contains the CompletionHandle and SelectInfo contains the SelectHandle.

We have two different implementations. One on Unix (select, based directly on the synchronous standard socket interface) and one on Windows (completion, based on the asynchronous I/O Completion Ports).

These two implementations have a slightly different behaviour and message interface.

The difference will only manifest for the user, if calls are made with the timeout argument set to 'nowait' (see above).

When an completion message is received (with the result of the operation), that means that the operation (connect, send, recv, ...) has been completed (successfully or otherwise). When a select message is received, that only means that the operation can now be completed, via a call to, for instance, connect/1.

The completion message has the format:

    :   {'$socket', socket(), completion, {CompletionHandle, CompletionStatus}}

The select message has the format:

    :   {'$socket', socket(), select, SelectHandle}

Note that, on select "system", all other users are locked out until the 'current user' has called the function (recv for instance) and its return value shows that the operation has completed. Such an operation can also be cancelled with cancel/2.

Instead of Timeout = nowait it is equivalent to create a SelectHandle or CompletionHandle with make_ref() and give as Timeout. This will then be the Handle in the 'completion' or 'select' message, which enables a compiler optimization for receiving a message containing a newly created reference() (ignore the part of the message queue that had arrived before the the reference() was created).

Another message the user must be prepared for (when making asynchronous calls) is the abort message:

    :   {'$socket', socket(), abort, Info}

This message indicates that the (asynchronous) operation has been aborted. If, for instance, the socket has been closed (by another process), Info will be {SelectHandle, closed}.

    Note:
The Windows support has currently pre-release status.

Support for IPv6 has been implemented but not fully tested.

SCTP has only been partly implemented (and not tested).

DATA TYPES
invalid() = {invalid, What :: term()}
domain() = inet | inet6 | local | unspec

    A lowercase atom() representing a protocol domain on the platform named AF_* (or PF_*).

    The calls supports(), is_supported(ipv6) and is_supported(local) tells if the IPv6 protocol for the inet6 protocol domain / address family, and if the local protocol domain / address family is supported by the platform's header files.
type() = stream | dgram | raw | rdm | seqpacket

    A lowercase atom() representing a protocol type on the platform named SOCK_*.
protocol() = atom()

    An atom() means any protocol as enumerated by the C library call getprotoent() on the platform, or at least the supported ones of ip | ipv6 | tcp | udp | sctp.

    See open/2,3,4

    The call supports(protocols) returns which protocols are supported, and is_supported(protocols, Protocol) tells if Protocol is among the enumerated.
socket() = {'$socket', socket_handle()}

    As returned by open/1,2,3,4 and accept/1,2.
socket_handle()

    An opaque socket handle unique for the socket.
select_tag()

    A tag that describes the (select) operation, contained in the returned select_info().
select_handle() = reference()

    A reference() that uniquely identifies the (select) operation, contained in the returned select_info().
select_info() =
    {select_info,
     SelectTag :: select_tag(),
     SelectHandle :: select_handle()}

    Returned by an operation that requires the caller to wait for a select message containing the SelectHandle.
completion_tag()

    A tag that describes the ongoing (completion) operation, contained in the returned completion_info().
completion_handle() = reference()

    A reference() that uniquely identifies the (completion) operation, contained in the returned completion_info().
completion_info() =
    {completion_info,
     CompletionTag :: completion_tag(),
     CompletionHandle :: completion_handle()}

    Returned by an operation that requires the caller to wait for a completion message containing the CompletionHandle and the result of the operation; the CompletionStatus.
info() =
    #{counters := #{atom() := integer() >= 0},
      iov_max := integer() >= 0,
      use_registry := boolean(),
      io_backend := #{name := atom()}}

    The smallest allowed iov_max value according to POSIX is 16, but check your platform documentation to be sure.
socket_counters() =
    #{read_byte := integer() >= 0,
      read_fails := integer() >= 0,
      read_pkg := integer() >= 0,
      read_pkg_max := integer() >= 0,
      read_tries := integer() >= 0,
      read_waits := integer() >= 0,
      write_byte := integer() >= 0,
      write_fails := integer() >= 0,
      write_pkg := integer() >= 0,
      write_pkg_max := integer() >= 0,
      write_tries := integer() >= 0,
      write_waits := integer() >= 0,
      sendfile => integer() >= 0,
      sendfile_byte => integer() >= 0,
      sendfile_fails => integer() >= 0,
      sendfile_max => integer() >= 0,
      sendfile_pkg => integer() >= 0,
      sendfile_pkg_max => integer() >= 0,
      sendfile_tries => integer() >= 0,
      sendfile_waits => integer() >= 0,
      acc_success := integer() >= 0,
      acc_fails := integer() >= 0,
      acc_tries := integer() >= 0,
      acc_waits := integer() >= 0}
info_keys() =
    [domain | type | protocol | fd | owner | local_address |
     remote_address | recv | sent | state]

    Defines the information elements of the table(s) printed by the i/0, i/1 and i/2 functions.
socket_info() =
    #{domain := domain() | integer(),
      type := type() | integer(),
      protocol := protocol() | integer(),
      owner := pid(),
      ctype := normal | fromfd | {fromfd, integer()},
      counters := socket_counters(),
      num_readers := integer() >= 0,
      num_writers := integer() >= 0,
      num_acceptors := integer() >= 0,
      writable := boolean(),
      readable := boolean(),
      rstates := [atom()],
      wstates := [atom()]}
in_addr() = {0..255, 0..255, 0..255, 0..255}
in6_addr() =
    {0..65535,
     0..65535,
     0..65535,
     0..65535,
     0..65535,
     0..65535,
     0..65535,
     0..65535}
sockaddr() =
    sockaddr_in() |
    sockaddr_in6() |
    sockaddr_un() |
    sockaddr_ll() |
    sockaddr_dl() |
    sockaddr_unspec() |
    sockaddr_native()
sockaddr_recv() = sockaddr() | binary()
sockaddr_in() =
    #{family := inet,
      port := port_number(),
      addr := any | broadcast | loopback | in_addr()}
sockaddr_in6() =
    #{family := inet6,
      port := port_number(),
      addr := any | loopback | in6_addr(),
      flowinfo := in6_flow_info(),
      scope_id := in6_scope_id()}
sockaddr_un() = #{family := local, path := binary() | string()}

    The path element will always be a binary when returned from this module. When supplied to an API function in this module it may be a string(), which will be encoded into a binary according to the native file name encoding on the platform.

    A terminating zero character will be appended before the address path is given to the OS, and the terminating zero will be stripped before giving the address path to the caller.

    Linux's non-portable abstract socket address extension is handled by not doing any terminating zero processing in either direction, if the first byte of the address is zero.
sockaddr_ll() =
    #{family := packet,
      protocol := integer() >= 0,
      ifindex := integer(),
      pkttype := packet_type(),
      hatype := hatype(),
      addr := binary()}
sockaddr_dl() =
    #{family := link,
      index := integer() >= 0,
      type := integer() >= 0,
      nlen := integer() >= 0,
      alen := integer() >= 0,
      slen := integer() >= 0,
      data := binary()}
sockaddr_unspec() = #{family := unspec, addr := binary()}
sockaddr_native() = #{family := integer(), addr := binary()}
packet_type() =
    host | broadcast | multicast | otherhost | outgoing |
    loopback | user | kernel | fastroute |
    integer() >= 0
hatype() =
    netrom | eether | ether | ax25 | pronet | chaos | ieee802 |
    arcnet | appletlk | dlci | atm | metricom | ieee1394 | eui64 |
    infiniband | tunnel | tunnel6 | loopback | localtlk | none |
    void |
    integer() >= 0
port_number() = 0..65535
in6_flow_info() = 0..1048575
in6_scope_id() = 0..4294967295
msg_flag() =
    cmsg_cloexec | confirm | ctrunc | dontroute | eor | errqueue |
    more | oob | peek | trunc

    Flags corresponding to the message flag constants on the platform. The flags are lowercase and the constants are uppercase with the prefix MSG_.

    Some flags are only used for sending, some only for receiving, some in received control messages, and some for several of these. Not all flags are supported on all platforms. See the platform's documentation, supports(msg_flags), and is_supported(msg_flags, MsgFlag).
level() = socket | protocol()

    The OS protocol levels for, for example, socket options and control messages, with the following names in the OS header files:

        socket:
            SOL_SOCKET with options named SO_*.

        ip: IPPROTO_IP a.k.a SOL_IP with options named IP_*.

        ipv6:
            IPPROTO_IPV6 a.k.a SOL_IPV6 with options named IPV6_*.

        tcp:
            IPPROTO_TCP with options named TCP_*.

        udp:
            IPPROTO_UDP with options named UDP_*.

        sctp:
            IPPROTO_SCTP with options named SCTP_*.

    There are many other possible protocols, but the ones above are those for which this socket library implements socket options and/or control messages.

    All protocols known to the OS are enumerated when the Erlang VM is started. See the OS man page for protocols(5). The protocol level 'socket' is always implemented as SOL_SOCKET and all the others mentioned in the list above are valid, if supported by the platform, enumerated or not.

    The calls supports() and is_supported(protocols, Protocol) can be used to find out if protocols ipv6 and/or sctp are supported according to the platform's header files.
otp_socket_option() =
    debug | iow | controlling_process | rcvbuf | rcvctrlbuf |
    sndctrlbuf | meta | use_registry | fd | domain

    These are socket options for the otp protocol level, that is {otp, Name} options, above all OS protocol levels. They affect Erlang/OTP's socket implementation.

        debug:
            boolean() - Activate debug printout.

        iow:
            boolean() - Inform On Wrap of statistics counters.

        controlling_process:
            pid() - The socket "owner". Only the current controlling process can set this option.

        rcvbuf:
            BufSize :: (default | integer()>0) | {N :: integer()>0, BufSize :: (default | integer()>0)} - Receive buffer size. The value default is only valid to set. N specifies the number of read attempts to do in a tight loop before assuming no more data is pending.

        rcvctrlbuf:
            BufSize :: (default | integer()>0) - Buffer size for received ancillary messages. The value default is only valid to set.

        sndctrlbuf:
            BufSize :: (default | integer()>0) - Buffer size for sent ancillary messages. The value default is only valid to set.

        fd: integer() - Only valid to get. The OS protocol levels' socket descriptor. Functions open/1,2 can be used to create a socket according to this module from an existing OS socket descriptor.

        use_registry:
            boolean() - Only valid to get. The value is set when the socket is created with open/2 or open/4.

    Options not described here are intentionally undocumented and for Erlang/OTP internal use only.
socket_option() =
    {Level :: socket,
     Opt ::
         acceptconn | acceptfilter | bindtodevice | broadcast |
         busy_poll | debug | domain | dontroute | error |
         keepalive | linger | mark | oobinline | passcred |
         peek_off | peercred | priority | protocol | rcvbuf |
         rcvbufforce | rcvlowat | rcvtimeo | reuseaddr |
         reuseport | rxq_ovfl | setfib | sndbuf | sndbufforce |
         sndlowat | sndtimeo | timestamp | type} |
    {Level :: ip,
     Opt ::
         add_membership | add_source_membership | block_source |
         dontfrag | drop_membership | drop_source_membership |
         freebind | hdrincl | minttl | msfilter | mtu |
         mtu_discover | multicast_all | multicast_if |
         multicast_loop | multicast_ttl | nodefrag | options |
         pktinfo | recvdstaddr | recverr | recvif | recvopts |
         recvorigdstaddr | recvtos | recvttl | retopts |
         router_alert | sndsrcaddr | tos | transparent | ttl |
         unblock_source} |
    {Level :: ipv6,
     Opt ::
         addrform | add_membership | authhdr | auth_level |
         checksum | drop_membership | dstopts | esp_trans_level |
         esp_network_level | faith | flowinfo | hopopts |
         ipcomp_level | join_group | leave_group | mtu |
         mtu_discover | multicast_hops | multicast_if |
         multicast_loop | portrange | pktoptions | recverr |
         recvhoplimit | hoplimit | recvpktinfo | pktinfo |
         recvtclass | router_alert | rthdr | tclass |
         unicast_hops | use_min_mtu | v6only} |
    {Level :: tcp,
     Opt ::
         congestion | cork | info | keepcnt | keepidle |
         keepintvl | maxseg | md5sig | nodelay | noopt | nopush |
         syncnt | user_timeout} |
    {Level :: udp, Opt :: cork} |
    {Level :: sctp,
     Opt ::
         adaption_layer | associnfo | auth_active_key |
         auth_asconf | auth_chunk | auth_key | auth_delete_key |
         autoclose | context | default_send_params |
         delayed_ack_time | disable_fragments | hmac_ident |
         events | explicit_eor | fragment_interleave |
         get_peer_addr_info | initmsg | i_want_mapped_v4_addr |
         local_auth_chunks | maxseg | maxburst | nodelay |
         partial_delivery_point | peer_addr_params |
         peer_auth_chunks | primary_addr | reset_streams |
         rtoinfo | set_peer_primary_addr | status |
         use_ext_recvinfo}

    Socket option on the form {Level, Opt} where the OS protocol Level = level() and Opt is a socket option on that protocol level.

    The OS name for an options is, except where otherwise noted, the Opt atom, in capitals, with prefix according to level().

        Note:
    The IPv6 option pktoptions is a special (barf) case. It is intended for backward compatibility usage only.

    Do not use this option.

        Note:
    See the OS documentation for every socket option.

    An option below that has the value type boolean() will translate the value false to a C int with value 0, and the value true to !!0 (not (not false)).

    An option with value type integer() will be translated to a C int that may have a restricted range, for example byte: 0..255. See the OS documentation.

    The calls supports(options), supports(options, Level) and is_supported(options, {Level, Opt}) can be used to find out which socket options that are supported by the platform.

    Options for protocol level socket:

        {socket, acceptconn}:
            Value = boolean()

        {socket, bindtodevice}:
            Value = string()

        {socket, broadcast}:
            Value = boolean()

        {socket, debug}:
            Value = integer()

        {socket, domain}:
            Value = domain()

            Only valid to get.

            The socket's protocol domain. Does not work on for instance FreeBSD.

        {socket, dontroute}:
            Value = boolean()

        {socket, keepalive}:
            Value = boolean()

        {socket, linger}:
            Value = abort | linger()

            The value abort is shorthand for #{onoff => true, linger => 0}, and only valid to set.

        {socket, oobinline}:
            Value = boolean()

        {socket, passcred}:
            Value = boolean()

        {socket, peek_off}:
            Value = integer()

            Currently disabled due to a possible infinite loop when calling recv/1-4 with peek in Flags.

        {socket, priority}:
            Value = integer()

        {socket, protocol}:
            Value = protocol()

            Only valid to get.

            The socket's protocol. Does not work on for instance Darwin.

        {socket, rcvbuf}:
            Value = integer()

        {socket, rcvlowat}:
            Value = integer()

        {socket, rcvtimeo}:
            Value = timeval()

            This option is unsupported per default; OTP has to be explicitly built with the --enable-esock-rcvsndtimeo configure option for this to be available.

            Since our implementation uses nonblocking sockets, it is unknown if and how this option works, or even if it may cause malfunction. Therefore, we do not recommend setting this option.

            Instead, use the Timeout argument to, for instance, the recv/3 function.

        {socket, reuseaddr}:
            Value = boolean()

        {socket, reuseport}:
            Value = boolean()

        {socket, sndbuf}:
            Value = integer()

        {socket, sndlowat}:
            Value = integer()

        {socket, sndtimeo}:
            Value = timeval()

            This option is unsupported per default; OTP has to be explicitly built with the --enable-esock-rcvsndtimeo configure option for this to be available.

            Since our implementation uses nonblocking sockets, it is unknown if and how this option works, or even if it may cause malfunction. Therefore, we do not recommend setting this option.

            Instead, use the Timeout argument to, for instance, the send/3 function.

        {socket, timestamp}:
            Value = boolean()

        {socket, type}:
            Value = type()

            Only valid to get.

            The socket's type.

    Options for protocol level ip:

        {ip, add_membership}:
            Value = ip_mreq()

            Only valid to set.

        {ip, add_source_membership}:
            Value = ip_mreq_source()

            Only valid to set.

        {ip, block_source}:
            Value = ip_mreq_source()

            Only valid to set.

        {ip, drop_membership}:
            Value = ip_mreq()

            Only valid to set.

        {ip, drop_source_membership}:
            Value = ip_mreq_source()

            Only valid to set.

        {ip, freebind}:
            Value = boolean()

        {ip, hdrincl}:
            Value = boolean()

        {ip, minttl}:
            Value = integer()

        {ip, msfilter}:
            Value = null | ip_msfilter()

            Only valid to set.

            The value null passes a NULL pointer and size 0 to the C library call.

        {ip, mtu}:
            Value = integer()

            Only valid to get.

        {ip, mtu_discover}:
            Value = ip_pmtudisc() | integer()

            An integer() value is according to the platform's header files.

        {ip, multicast_all}:
            Value = boolean()

        {ip, multicast_if}:
            Value = any | in_addr()

        {ip, multicast_loop}:
            Value = boolean()

        {ip, multicast_ttl}:
            Value = integer()

        {ip, nodefrag}:
            Value = boolean()

        {ip, pktinfo}:
            Value = boolean()

        {ip, recvdstaddr}:
            Value = boolean()

        {ip, recverr}:
            Value = boolean()

            Warning! When this option is enabled, error messages may arrive on the socket's error queue, which should be read using the message flag errqueue, and using recvmsg/1,2,3,4,5 to get all error information in the message's ctrl field as a control message #{level := ip, type := recverr}.

            A working strategy should be to first poll the error queue using recvmsg/2,3,4 with Timeout =:= 0 and Flags containing errqueue (ignore the return value {error, timeout}) before reading the actual data to ensure that the error queue gets cleared. And read the data using one of the nowait | select_handle() recv functions: recv/3,4, recvfrom/3,4 or recvmsg/3,4,5. Otherwise you might accidentally cause a busy loop in and out of 'select' for the socket.

        {ip, recvif}:
            Value = boolean()

        {ip, recvopts}:
            Value = boolean()

        {ip, recvorigdstaddr}:
            Value = boolean()

        {ip, recvtos}:
            Value = boolean()

        {ip, recvttl}:
            Value = boolean()

        {ip, retopts}:
            Value = boolean()

        {ip, router_alert}:
            Value = integer()

        {ip, sendsrcaddr}:
            Value = boolean()

        {ip, tos}:
            Value = ip_tos() | integer()

            An integer() value is according to the platform's header files.

        {ip, transparent}:
            Value = boolean()

        {ip, ttl}:
            Value = integer()

        {ip, unblock_source}:
            Value = ip_mreq_source()

            Only valid to set.

    Options for protocol level ipv6:

        {ipv6, addrform}:
            Value = domain()

            As far as we know the only valid value is inet and it is only allowed for an IPv6 socket that is connected and bound to an IPv4-mapped IPv6 address.

        {ipv6, add_membership}:
            Value = ipv6_mreq()

            Only valid to set.

        {ipv6, authhdr}:
            Value = boolean()

        {ipv6, drop_membership}:
            Value = ipv6_mreq()

            Only valid to set.

        {ipv6, dstopts}:
            Value = boolean()

        {ipv6, flowinfo}:
            Value = boolean()

        {ipv6, hoplimit}:
            Value = boolean()

        {ipv6, hopopts}:
            Value = boolean()

        {ipv6, mtu}:
            Value = integer()

        {ipv6, mtu_discover}:
            Value = ipv6_pmtudisc() | integer()

            An integer() value is according to the platform's header files.

        {ipv6, multicast_hops}:
            Value = ipv6_hops()

        {ipv6, multicast_if}:
            Value = integer()

        {ipv6, multicast_loop}:
            Value = boolean()

        {ipv6, recverr}:
            Value = boolean()

            Warning! See the socket option {ip, recverr} regarding the socket's error queue. The same warning applies for this option.

        {ipv6, recvhoplimit}:
            Value = boolean()

        {ipv6, recvpktinfo}:
            Value = boolean()

        {ipv6, recvtclass}:
            Value = boolean()

        {ipv6, router_alert}:
            Value = integer()

        {ipv6, rthdr}:
            Value = boolean()

        {ipv6, tclass}:
            Value = boolean()

        {ipv6, unicast_hops}:
            Value = ipv6_hops()

        {ipv6, v6only}:
            Value = boolean()

    Options for protocol level sctp. See also RFC 6458.

        {sctp, associnfo}:
            Value = sctp_assocparams()

        {sctp, autoclose}:
            Value = integer()

        {sctp, disable_fragments}:
            Value = boolean()

        {sctp, events}:
            Value = sctp_event_subscribe()

            Only valid to set.

        {sctp, initmsg}:
            Value = sctp_initmsg()

        {sctp, maxseg}:
            Value = integer()

        {sctp, nodelay}:
            Value = boolean()

        {sctp, rtoinfo}:
            Value = sctp_rtoinfo()

    Options for protocol level tcp:

        {tcp, congestion}:
            Value = string()

        {tcp, cork}:
            Value = boolean()

        {tcp, maxseg}:
            Value = integer()

        {tcp, nodelay}:
            Value = boolean()

    Options for protocol level udp:

        {udp, cork}:
            Value = boolean()
linger() = #{onoff := boolean(), linger := integer() >= 0}

    Corresponds to the C struct linger for managing the socket option {socket, linger}.
timeval() = #{sec := integer(), usec := integer()}

    Corresponds to the C struct timeval. The field sec holds seconds, and usec microseconds.
ip_mreq() = #{multiaddr := in_addr(), interface := in_addr()}

    Corresponds to the C struct ip_mreq for managing multicast groups.
ip_mreq_source() =
    #{multiaddr := in_addr(),
      interface := in_addr(),
      sourceaddr := in_addr()}

    Corresponds to the C struct ip_mreq_source for managing multicast groups.
ip_msfilter() =
    #{multiaddr := in_addr(),
      interface := in_addr(),
      mode := include | exclude,
      slist := [in_addr()]}

    Corresponds to the C struct ip_msfilter for managing multicast source filtering (RFC 3376).
ip_pmtudisc() = want | dont | do | probe

    Lowercase atom() values corresponding to the C library constants IP_PMTUDISC_*. Some constant(s) may be unsupported by the platform.
ip_tos() = lowdelay | throughput | reliability | mincost

    Lowercase atom() values corresponding to the C library constants IPTOS_*. Some constant(s) may be unsupported by the platform.
ip_pktinfo() =
    #{ifindex := integer() >= 0,
      spec_dst := in_addr(),
      addr := in_addr()}
ipv6_mreq() =
    #{multiaddr := in6_addr(), interface := integer() >= 0}

    Corresponds to the C struct ipv6_mreq for managing multicast groups. See also RFC 2553.
ipv6_hops() = default | 0..255

    The value default is only valid to set and is translated to the C value -1, meaning the route default.
ipv6_pmtudisc() = want | dont | do | probe

    Lowercase atom() values corresponding to the C library constants IPV6_PMTUDISC_*. Some constant(s) may be unsupported by the platform.
ipv6_pktinfo() = #{addr := in6_addr(), ifindex := integer()}
sctp_assocparams() =
    #{assoc_id := integer(),
      asocmaxrxt := 0..65535,
      numbe_peer_destinations := 0..65535,
      peer_rwnd := 0..4294967295,
      local_rwnd := 0..4294967295,
      cookie_life := 0..4294967295}

    Corresponds to the C struct sctp_assocparams.
sctp_event_subscribe() =
    #{data_io := boolean(),
      association := boolean(),
      address := boolean(),
      send_failure := boolean(),
      peer_error := boolean(),
      shutdown := boolean(),
      partial_delivery := boolean(),
      adaptation_layer => boolean(),
      sender_dry => boolean()}

    Corresponds to the C struct sctp_event_subscribe.

    Not all fields are implemented on all platforms; unimplemented fields are ignored, but implemented fields are mandatory. Note that the '_event' suffixes have been stripped from the C struct field names, for convenience.
sctp_initmsg() =
    #{num_ostreams := 0..65535,
      max_instreams := 0..65535,
      max_attempts := 0..65535,
      max_init_timeo := 0..65535}

    Corresponds to the C struct sctp_initmsg.
sctp_rtoinfo() =
    #{assoc_id := integer(),
      initial := 0..4294967295,
      max := 0..4294967295,
      min := 0..4294967295}

    Corresponds to the C struct sctp_rtoinfo.
msg() = msg_send() | msg_recv()
msg_send() =
    #{addr => sockaddr(),
      iov := erlang:iovec(),
      ctrl =>
          [cmsg_send() |
           #{level := level() | integer(),
             type := integer(),
             data := binary()}]}

    Message sent by sendmsg/2,3,4.

    Corresponds to a C struct msghdr, see your platform documentation for sendmsg(2).

        addr:
             Optional peer address, used on unconnected sockets. Corresponds to msg_name and msg_namelen fields of a struct msghdr. If not used they are set to NULL, 0.

        iov:
             Mandatory data as a list of binaries. The msg_iov and msg_iovlen fields of a struct msghdr.

        ctrl:
             Optional list of control messages (CMSG). Corresponds to the msg_control and msg_controllen fields of a struct msghdr. If not used they are set to NULL, 0.

    The msg_flags field of the struct msghdr is set to 0.
msg_recv() =
    #{addr => sockaddr_recv(),
      iov := erlang:iovec(),
      ctrl :=
          [cmsg_recv() |
           #{level := level() | integer(),
             type := integer(),
             data := binary()}],
      flags := [msg_flag() | integer()]}

    Message returned by recvmsg/1,2,3,5.

    Corresponds to a C struct msghdr, see your platform documentation for recvmsg(2).

        addr:
             Optional peer address, used on unconnected sockets. Corresponds to msg_name and msg_namelen fields of a struct msghdr. If NULL the map key is not present.

        iov:
             Data as a list of binaries. The msg_iov and msg_iovlen fields of a struct msghdr.

        ctrl:
             A possibly empty list of control messages (CMSG). Corresponds to the msg_control and msg_controllen fields of a struct msghdr.

        flags:
             Message flags. Corresponds to the msg_flags field of a struct msghdr. Unknown flags, if any, are returned in one integer(), last in the containing list.
native_value() = integer() | boolean() | binary()
cmsg_send() =
    #{level := socket,
      type := timestamp,
      data => native_value(),
      value => timeval()} |
    #{level := socket, type := rights, data := native_value()} |
    #{level := socket,
      type := credentials,
      data := native_value()} |
    #{level := ip,
      type := tos,
      data => native_value(),
      value => ip_tos() | integer()} |
    #{level := ip,
      type := ttl,
      data => native_value(),
      value => integer()} |
    #{level := ip,
      type := hoplimit,
      data => native_value(),
      value => integer()} |
    #{level := ipv6,
      type := tclass,
      data => native_value(),
      value => integer()}

    Control messages (ancillary messages) accepted by sendmsg/2,3,4.

    A control message may for some message types have a value field with a symbolic value, or a data field with a native value, that has to be binary compatible what is defined in the platform's header files.
cmsg_recv() =
    #{level := socket,
      type := timestamp,
      data := binary(),
      value => timeval()} |
    #{level := socket, type := rights, data := binary()} |
    #{level := socket, type := credentials, data := binary()} |
    #{level := ip,
      type := tos,
      data := binary(),
      value => ip_tos() | integer()} |
    #{level := ip,
      type := recvtos,
      data := binary(),
      value := ip_tos() | integer()} |
    #{level := ip,
      type := ttl,
      data := binary(),
      value => integer()} |
    #{level := ip,
      type := recvttl,
      data := binary(),
      value := integer()} |
    #{level := ip,
      type := pktinfo,
      data := binary(),
      value => ip_pktinfo()} |
    #{level := ip,
      type := origdstaddr,
      data := binary(),
      value => sockaddr_recv()} |
    #{level := ip,
      type := recverr,
      data := binary(),
      value => extended_err()} |
    #{level := ipv6,
      type := hoplimit,
      data := binary(),
      value => integer()} |
    #{level := ipv6,
      type := pktinfo,
      data := binary(),
      value => ipv6_pktinfo()} |
    #{level := ipv6,
      type := recverr,
      data := binary(),
      value => extended_err()} |
    #{level := ipv6,
      type := tclass,
      data := binary(),
      value => integer()}

    Control messages (ancillary messages) returned by recvmsg/1,2,3,5.

    A control message has got a data field with a native (binary) value for the message data, and may also have a decoded value field if this socket library successfully decoded the data.
icmp_dest_unreach() =
    net_unreach | host_unreach | port_unreach | frag_needed |
    net_unknown | host_unknown
icmpv6_dest_unreach() =
    noroute | adm_prohibited | not_neighbour | addr_unreach |
    port_unreach | policy_fail | reject_route
ee_origin() = none | local | icmp | icmp6
extended_err() =
    #{error := posix(),
      origin := icmp,
      type := dest_unreach,
      code := icmp_dest_unreach() | 0..255,
      info := 0..4294967295,
      data := 0..4294967295,
      offender := sockaddr_recv()} |
    #{error := posix(),
      origin := icmp,
      type := time_exceeded | 0..255,
      code := 0..255,
      info := 0..4294967295,
      data := 0..4294967295,
      offender := sockaddr_recv()} |
    #{error := posix(),
      origin := icmp6,
      type := dest_unreach,
      code := icmpv6_dest_unreach() | 0..255,
      info := 0..4294967295,
      data := 0..4294967295,
      offender := sockaddr_recv()} |
    #{error := posix(),
      origin := icmp6,
      type := pkt_toobig | time_exceeded | 0..255,
      code := 0..255,
      info := 0..4294967295,
      data := 0..4294967295,
      offender := sockaddr_recv()} |
    #{error := posix(),
      origin := ee_origin() | 0..255,
      type := 0..255,
      code := 0..255,
      info := 0..4294967295,
      data := 0..4294967295,
      offender := sockaddr_recv()}
posix() = inet:posix()

    The POSIX error codes originates from the OS level socket interface.

EXPORTS

accept(ListenSocket) -> {ok, Socket} | {error, Reason}
accept(ListenSocket, Timeout :: infinity) ->
          {ok, Socket} | {error, Reason}

    Types:
        ListenSocket = Socket = socket()
        Reason =
            posix() |
            closed |
            invalid() |
            {create_accept_socket, posix()} |
            {add_socket, posix()} |
            {update_accept_context, posix()}

    Accept a connection on a socket.

    This call is used with connection oriented socket types (stream or seqpacket). It returns the first pending incoming connection for a listen socket, or waits for one to arrive, and returns the (newly) connected socket.

accept(ListenSocket, Timeout :: integer() >= 0) ->
          {ok, Socket} | {error, Reason}

    Types:
        ListenSocket = Socket = socket()
        Reason =
            posix() |
            closed |
            invalid() |
            timeout |
            {create_accept_socket, posix()} |
            {add_socket, posix()} |
            {update_accept_context, posix()}

    The same as accept/1 but returns {error, timeout} if no connection has been accepted after Timeout milliseconds.

accept(ListenSocket, Timeout :: nowait) ->
          {ok, Socket} |
          {select, SelectInfo} |
          {completion, CompletionInfo} |
          {error, Reason}
accept(ListenSocket,
       Handle :: select_handle() | completion_handle()) ->
          {ok, Socket} |
          {select, SelectInfo} |
          {completion, CompletionInfo} |
          {error, Reason}

    Types:
        ListenSocket = Socket = socket()
        SelectInfo = select_info()
        CompletionInfo = completion_info()
        Reason =
            posix() |
            closed |
            invalid() |
            {create_accept_socket, posix()} |
            {add_socket, posix()} |
            {update_accept_context, posix()}

    The same as accept/1 but returns promptly.

    When there is no pending connection to return, the function will return (on Unix ) {select, SelectInfo} or (on Windows ) {completion, CompletionInfo}, and the caller will later receive either one of these messages (depending on the platform) when the client connects:

        select message:
            {'$socket', Socket, select, SelectHandle} (with the SelectHandle contained in the SelectInfo).

            A subsequent call to accept/1,2 will then return the socket.

        completion message:
            {'$socket', Socket, completion, {CompletionHandle, CompletionStatus}} (with the CompletionHandle contained in the CompletionInfo).

            The result of the accept will be in the CompletionStatus.

    If the time-out argument is a Handle, that term will be contained in a returned SelectInfo or CompletionInfo and the corresponding select or completion message. The Handle is presumed to be unique to this call.

    If the time-out argument is nowait:

        On Unix :
            And a SelectInfo is returned, it will contain a select_handle() generated by the call.

        On Windows :
            And a CompletionInfo is returned, it will contain a completion_handle() generated by the call.

    If the caller doesn't want to wait for a connection, it must immediately call cancel/2 to cancel the operation.

bind(Socket, Addr) -> ok | {error, Reason}

    Types:
        Socket = socket()
        Addr = sockaddr() | any | broadcast | loopback
        Reason = posix() | closed | invalid()

    Bind a name to a socket.

    When a socket is created (with open), it has no address assigned to it. bind assigns the address specified by the Addr argument.

    The rules used for name binding vary between domains.

    If you bind a socket to an address in for example the 'inet' or 'inet6' address families, with an ephemeral port number (0), and want to know which port that was chosen, you can find out using something like: {ok, #{port := Port}} = socket:sockname(Socket)

cancel(Socket, SelectInfo) -> ok | {error, Reason}

    Types:
        Socket = socket()
        SelectInfo = select_info()
        Reason = closed | invalid()

    Cancel an asynchronous (select) request.

    Call this function in order to cancel a previous asynchronous call to, e.g. recv/3.

    An ongoing asynchronous operation blocks the socket until the operation has been finished in good order, or until it has been cancelled by this function.

    Any other process that tries an operation of the same basic type (accept / send / recv) will be enqueued and notified with the regular select mechanism for asynchronous operations when the current operation and all enqueued before it has been completed.

    If SelectInfo does not match an operation in progress for the calling process, this function returns {error, {invalid, SelectInfo}}.

cancel(Socket, CompletionInfo) -> ok | {error, Reason}

    Types:
        Socket = socket()
        CompletionInfo = completion_info()
        Reason = closed | invalid()

    Cancel an asynchronous (completion) request.

    Call this function in order to cancel a previous asynchronous call to, e.g. recv/3.

    An ongoing asynchronous operation blocks the socket until the operation has been finished in good order, or until it has been cancelled by this function.

    Any other process that tries an operation of the same basic type (accept / send / recv) will be enqueued and notified with the regular select mechanism for asynchronous operations when the current operation and all enqueued before it has been completed.

    If CompletionInfo does not match an operation in progress for the calling process, this function returns {error, {invalid, CompletionInfo}}.

close(Socket) -> ok | {error, Reason}

    Types:
        Socket = socket()
        Reason = posix() | closed | timeout

    Closes the socket.

        Note:
    Note that for e.g. protocol = tcp, most implementations doing a close does not guarantee that any data sent is delivered to the recipient before the close is detected at the remote side.

    One way to handle this is to use the shutdown function (socket:shutdown(Socket, write)) to signal that no more data is to be sent and then wait for the read side of the socket to be closed.

connect(Socket, SockAddr) -> ok | {error, Reason}
connect(Socket, SockAddr, Timeout :: infinity) ->
           ok | {error, Reason}

    Types:
        Socket = socket()
        SockAddr = sockaddr()
        Reason =
            posix() |
            closed |
            invalid() |
            already | not_bound |
            {add_socket, posix()} |
            {update_connect_context, posix()}

    This function connects the socket to the address specified by the SockAddr argument, and returns when the connection has been established or failed.

    If a connection attempt is already in progress (by another process), {error, already} is returned.

        Note:
    On Windows the socket has to be bound.

connect(Socket, SockAddr, Timeout :: integer() >= 0) ->
           ok | {error, Reason}

    Types:
        Socket = socket()
        SockAddr = sockaddr()
        Reason =
            posix() |
            closed |
            invalid() |
            already | not_bound | timeout |
            {add_socket, posix()} |
            {update_connect_context, posix()}

    The same as connect/2 but returns {error, timeout} if no connection has been established after Timeout milliseconds.

        Note:
    On Windows the socket has to be bound.

    Note that when this call has returned {error, timeout} the connection state of the socket is uncertain since the platform's network stack may complete the connection at any time, up to some platform specific time-out.

    Repeating a connection attempt towards the same address would be ok, but towards a different address could end up with a connection to either address.

    The safe play would be to close the socket and start over.

    Also note that all this applies to cancelling a connect call with a no-wait time-out described below.

connect(Socket, SockAddr, Timeout :: nowait) ->
           ok |
           {select, SelectInfo} |
           {completion, CompletionInfo} |
           {error, Reason}
connect(Socket, SockAddr,
        Handle :: select_handle() | completion_handle()) ->
           ok |
           {select, SelectInfo} |
           {completion, CompletionInfo} |
           {error, Reason}

    Types:
        Socket = socket()
        SockAddr = sockaddr()
        SelectInfo = select_info()
        CompletionInfo = completion_info()
        Reason =
            posix() |
            closed |
            invalid() |
            already | not_bound |
            {add_socket, posix()} |
            {update_connect_context, posix()}

    The same as connect/2 but returns promptly.

    If it is not possible to immediately establish a connection, the function will return {select, SelectInfo}, and the caller will later receive a select message, {'$socket', Socket, select, SelectHandle} ( with the SelectHandle contained in the SelectInfo ) when the connection has been completed or failed. A subsequent call to connect/1 will then finalize the connection and return the result.

    If the time-out argument is SelectHandle, that term will be contained in a returned SelectInfo and the corresponding select message. The SelectHandle is presumed to be unique to this call.

    If the time-out argument is nowait, and a SelectInfo is returned, it will contain a select_handle() generated by the call.

    If the caller doesn't want to wait for the connection to complete, it must immediately call cancel/2 to cancel the operation.

        Note:
    On Windows the socket has to be bound.

connect(Socket) -> ok | {error, Reason}

    Types:
        Socket = socket()
        Reason = posix() | closed | invalid()

    This function finalizes a connection setup on a socket, after calling connect(_, _, nowait | select_handle()) that returned {select, SelectInfo}, and receiving the select message {'$socket', Socket, select, SelectHandle}, and returns whether the connection setup was successful or not.

    Instead of calling this function, for backwards compatibility, it is allowed to call connect/2,3, but that incurs more overhead since the connect address and time-out are processed in vain.

        Note:
    Not used on Windows .

cancel_monitor(MRef) -> boolean()

    Types:
        MRef = reference()

    If MRef is a reference that the calling process obtained by calling monitor/1, this monitor is turned off. If the monitoring is already turned off, nothing happens.

    The returned value is one of the following:

        true:
            The monitor was found and removed. In this case, no 'DOWN' message corresponding to this monitor has been delivered and will not be delivered.

        false:
            The monitor was not found and could not be removed. This probably because a 'DOWN' message corresponding to this monitor has already been placed in the caller message queue.

    Failure: It is an error if MRef refers to a monitor started by another process.

getopt(X1 :: socket(),
       SocketOption :: {Level :: otp, Opt :: otp_socket_option()}) ->
          {ok, Value :: term()} | {error, invalid() | closed}

    Gets a socket option from the protocol level otp, which is this implementation's level above the OS protocol layers.

    See the type otp_socket_option() for a description of the options on this level.

getopt(X1 :: socket(), SocketOption :: socket_option()) ->
          {ok, Value :: term()} |
          {error, posix() | invalid() | closed}

    Gets a socket option from one of the OS's protocol levels. See the type socket_option() for which options that this implementation knows about, how they are related to option names in the OS, and if there are known peculiarities with any of them.

    What options are valid depends on what kind of socket it is (domain(), type() and protocol()).

    See the socket options chapter of the users guide for more info.

        Note:
    Not all options are valid, nor possible to get, on all platforms. That is, even if "we" support an option; it does not mean that the underlying OS does.

getopt(Socket, Level, Opt) -> ok | {error, Reason}

    Types:
         Socket = socket()
         Reason = inet:posix() | invalid() | closed

    Backwards compatibility function.

    The same as getopt(Socket, {Level, Opt})

getopt_native(X1 :: socket(),
              SocketOption ::
                  socket_option() |
                  {Level :: level() | (NativeLevel :: integer()),
                   NativeOpt :: integer()},
              ValueType :: integer) ->
                 {ok, Value :: integer()} |
                 {error, posix() | invalid() | closed}
getopt_native(X1 :: socket(),
              SocketOption ::
                  socket_option() |
                  {Level :: level() | (NativeLevel :: integer()),
                   NativeOpt :: integer()},
              ValueType :: boolean) ->
                 {ok, Value :: boolean()} |
                 {error, posix() | invalid() | closed}
getopt_native(X1 :: socket(),
              SocketOption ::
                  socket_option() |
                  {Level :: level() | (NativeLevel :: integer()),
                   NativeOpt :: integer()},
              ValueSize :: integer() >= 0) ->
                 {ok, Value :: binary()} |
                 {error, posix() | invalid() | closed}
getopt_native(X1 :: socket(),
              SocketOption ::
                  socket_option() |
                  {Level :: level() | (NativeLevel :: integer()),
                   NativeOpt :: integer()},
              ValueSpec :: binary()) ->
                 {ok, Value :: binary()} |
                 {error, posix() | invalid() | closed}

    Gets a socket option that may be unknown to our implementation, or that has a type not compatible with our implementation, that is; in "native mode".

    The socket option may be specified with an ordinary socket_option() tuple, with a known Level = level() and an integer NativeOpt, or with both an integer NativeLevel and NativeOpt.

    How to decode the option value has to be specified either with ValueType, by specifying the ValueSize for a binary() that will contain the fetched option value, or by specifying a binary() ValueSpec that will be copied to a buffer for the getsockopt() call to write the value in which will be returned as a new binary().

    If ValueType is integer a C type (int) will be fetched, if it is boolean a C type (int) will be fetched and converted into a boolean() according to the C implementation.

    What options are valid depends on what kind of socket it is (domain(), type() and protocol()).

    The integer values for NativeLevel and NativeOpt as well as the Value encoding has to be deduced from the header files for the running system.

i() -> ok

    Print all sockets in table format in the erlang shell.

i(InfoKeys) -> ok

    Types:
        InfoKeys = info_keys()

    Print all sockets in table format in the erlang shell. What information is included is defined by InfoKeys.

i(Domain) -> ok

    Types:
        Domain = inet | inet6 | local

    Print a selection, based on domain, of the sockets in table format in the erlang shell.

i(Proto) -> ok

    Types:
        Proto = sctp | tcp | udp

    Print a selection, based on protocol, of the sockets in table format in the erlang shell.

i(Type) -> ok

    Types:
        Type = dgram | seqpacket | stream

    Print a selection, based on type, of the sockets in table format in the erlang shell.

i(Domain, InfoKeys) -> ok

    Types:
        Domain = inet | inet6 | local
        InfoKeys = info_keys()

    Print a selection, based on domain, of the sockets in table format in the erlang shell. What information is included is defined by InfoKeys.

i(Proto, InfoKeys) -> ok

    Types:
        Proto = sctp | tcp | udp
        InfoKeys = info_keys()

    Print a selection, based on domain, of the sockets in table format in the erlang shell. What information is included is defined by InfoKeys.

i(Type, InfoKeys) -> ok

    Types:
        Type = dgram | seqpacket | stream
        InfoKeys = info_keys()

    Print a selection, based on type, of the sockets in table format in the erlang shell. What information is included is defined by InfoKeys.

info() -> info()

    Get miscellaneous info about the socket library.

    The function returns a map with each info item as a key-value binding.

        Note:
    In order to ensure data integrity, mutex'es are taken when needed. So, do not call this function often.

info(Socket) -> socket_info()

    Types:
        Socket = socket()

    Get miscellaneous info about the socket.

    The function returns a map with each info item as a key-value binding. It reflects the "current" state of the socket.

        Note:
    In order to ensure data integrity, mutex'es are taken when needed. So, do not call this function often.

ioctl(Socket, GetRequest) -> {ok, IFConf} | {error, Reason}

    Types:
        Socket = socket()
        GetRequest = gifconf
        IFConf = [#{name := string, addr := sockaddr()}]
        Reason = posix() | closed

    Retrieve socket (device) parameters.

ioctl(Socket, GetRequest, NameOrIndex) ->
         {ok, Result} | {error, Reason}

    Types:
        Socket = socket()
        GetRequest =
            gifname | gifindex | gifaddr | gifdstaddr | gifbrdaddr |
            gifnetmask | gifhwaddr | gifmtu | giftxqlen | gifflags
        NameOrIndex = string() | integer()
        Result = term()
        Reason = posix() | closed

    Retrieve socket (device) parameters. This function retrieves a specific parameter, according to GetRequest argument. The third argument is the (lookup) "key", identifying the interface (usually the name of the interface).

        gifname:
            Get the name of the interface with the specified index (integer()).

            Result, name of the interface, is a string().

        gifindex:
            Get the index of the interface with the specified name.

            Result, interface index, is a integer().

        gifaddr:
            Get the address of the interface with the specified name. Result, address of the interface, is a socket:sockaddr().

        gifdstaddr:
            Get the destination address of the point-to-point interface with the specified name.

            Result, destination address of the interface, is a socket:sockaddr().

        gifbrdaddr:
            Get the droadcast address for the interface with the specified name.

            Result, broadcast address of the interface, is a socket:sockaddr().

        gifnetmask:
            Get the network mask for the interface with the specified name.

            Result, network mask of the interface, is a socket:sockaddr().

        gifhwaddr:
            Get the hardware address for the interface with the specified name.

            Result, hardware address of the interface, is a socket:sockaddr(). The family field contains the 'ARPHRD' device type (or an integer).

        gifmtu:
            Get the MTU (Maximum Transfer Unit) for the interface with the specified name.

            Result, MTU of the interface, is an integer().

        giftxqlen:
            Get the transmit queue length of the interface with the specified name.

            Result, transmit queue length of the interface, is an integer().

        gifflags:
            Get the active flag word of the interface with the specified name.

            Result, the active flag word of the interface, is an list of socket:ioctl_device_flag() | integer().

ioctl(Socket, SetRequest, Name, Value) -> ok | {error, Reason}

    Types:
        Socket = socket()
        SetRequest =
            sifflags | sifaddr | sifdstaddr | sifbrdaddr | sifnetmask |
            sifhwaddr | gifmtu | siftxqlen
        Name = string()
        Value = term()
        Reason = posix() | closed

    Set socket (device) parameters. This function sets a specific parameter, according to SetRequest argument. The third argument is the "key", identifying the interface (usually the name of the interface), and the fourth is the "new" value.

    These are privileged operation's.

        sifflags:
            Set the the active flag word, #{Flag => boolean()}, of the interface with the specified name.

            Each flag to be changed, should be added to the value map, with the value 'true' if the flag (Flag) should be set and 'false' if the flag should be reset.

        sifaddr:
            Set the address, sockaddr(), of the interface with the specified name.

        sifdstaddr:
            Set the destination address, sockaddr(), of a point-to-point interface with the specified name.

        sifbrdaddr:
            Set the broadcast address, sockaddr(), of the interface with the specified name.

        sifnetmask:
            Set the network mask, sockaddr(), of the interface with the specified name.

        sifmtu:
            Set the MTU (Maximum Transfer Unit), integer(), for the interface with the specified name.

        siftxqlen:
            Set the transmit queue length, integer(), of the interface with the specified name.

is_supported(Key1 :: term()) -> boolean()
is_supported(Key1 :: term(), Key2 :: term()) -> boolean()

    This function retrieves information about what the platform supports, such as if SCTP is supported, or if a socket options are supported.

    For keys other than the known false is returned. Note that in a future version or on a different platform there might be more supported items.

    This functions returns a boolean corresponding to what supports/0-2 reports for the same Key1 (and Key2).

listen(Socket) -> ok | {error, Reason}
listen(Socket, Backlog) -> ok | {error, Reason}

    Types:
        Socket = socket()
        Backlog = integer()
        Reason = posix() | closed

    Listen for connections on a socket.

        Note:
    On Windows the socket has to be bound.

monitor(Socket) -> reference()

    Types:
        Socket = socket()

    Start monitor the socket Socket.

    If the monitored socket does not exist or when the monitor is triggered, a 'DOWN' message is sent that has the following pattern:

    	    {'DOWN', MonitorRef, socket, Object, Info}

    In the monitor message MonitorRef and Type are the same as described earlier, and:

        Object:
            The monitored entity, socket, which triggered the event.

        Info:
            Either the termination reason of the socket or nosock (socket Socket did not exist at the time of monitor creation).

    Making several calls to socket:monitor/1 for the same Socket is not an error; it results in as many independent monitoring instances.

number_of() -> integer() >= 0

    Returns the number of active sockets.

open(FD) -> {ok, Socket} | {error, Reason}
open(FD, Opts) -> {ok, Socket} | {error, Reason}

    Types:
        FD = integer()
        Opts =
            #{domain => domain() | integer(),
              type => type() | integer(),
              protocol => default | protocol() | integer(),
              dup => boolean(),
              debug => boolean(),
              use_registry => boolean()}
        Socket = socket()
        Reason = posix() | domain | type | protocol

    Creates an endpoint (socket) for communication based on an already existing file descriptor. The function attempts to retrieve domain, type and protocol from the system. This is however not possible on all platforms, and they should then be specified in Opts.

    The Opts argument is intended for providing extra information for the open call:

        domain:
            Which protocol domain is the descriptor of. See also open/2,3,4.

        type:
            Which protocol type type is the descriptor of.

            See also open/2,3,4.

        protocol:
            Which protocol is the descriptor of. The atom default is equivalent to the integer protocol number 0 which means the default protocol for a given domain and type.

            If the protocol can not be retrieved from the platform for the socket, and protocol is not specified, the default protocol is used, which may or may not be correct.

            See also open/2,3,4.

        dup:
            Shall the provided descriptor be duplicated (dup) or not.
            Defaults to true.

        debug:
            Enable or disable debug during the open call.
            Defaults to false.

        use_registry>:
            Enable or disable use of the socket registry for this socket. This overrides the global value.
            Defaults to the global value, see use_registry/1.

        Note:
    This function should be used with care!

    On some platforms it is necessary to provide domain, type and protocol since they cannot be retrieved from the platform.

open(Domain, Type) -> {ok, Socket} | {error, Reason}
open(Domain, Type, Opts) -> {ok, Socket} | {error, Reason}

    Types:
        Domain = domain() | integer()
        Type = type() | integer()
        Opts = map()
        Socket = socket()
        Reason = posix() | protocol

    Creates an endpoint (socket) for communication.

    The same as open(Domain, Type, default) and open(Domain, Type, default, Opts) respectively.

open(Domain, Type, Protocol) -> {ok, Socket} | {error, Reason}
open(Domain, Type, Protocol, Opts) ->
        {ok, Socket} | {error, Reason}

    Types:
        Domain = domain() | integer()
        Type = type() | integer()
        Protocol = default | protocol() | integer()
        Opts =
            #{netns => string(),
              debug => boolean(),
              use_registry => boolean()}
        Socket = socket()
        Reason = posix() | protocol

    Creates an endpoint (socket) for communication.

    Domain and Type may be integer()s, as defined in the platform's header files. The same goes for Protocol as defined in the platform's services(5) database. See also the OS man page for the library call socket(2).

        Note:
    For some combinations of Domain and Type the platform has got a default protocol that can be selected with Protocol = default, and the platform may allow or require selecting the default protocol, a specific protocol, or either.

    Examples:

        socket:open(inet, stream, tcp):
            It is common that for protocol domain and type inet,stream it is allowed to select the tcp protocol although that mostly is the default.

        socket:open(local, dgram):
            It is common that for the protocol domain local it is mandatory to not select a protocol, that is; to select the default protocol.

    The Opts argument is intended for "other" options. The supported option(s) are described below:

        netns: string():
            Used to set the network namespace during the open call. Only supported on the Linux platform.

        debug: boolean():
            Enable or disable debug during the open call.
            Defaults to false.

        use_registry: boolean():
            Enable or disable use of the socket registry for this socket. This overrides the global value.
            Defaults to the global value, see use_registry/1.

peername(Socket) -> {ok, SockAddr} | {error, Reason}

    Types:
        Socket = socket()
        SockAddr = sockaddr_recv()
        Reason = posix() | closed

    Returns the address of the peer connected to the socket.

recv(Socket) ->
        {ok, Data} | {error, Reason} | {error, {Reason, Data}}
recv(Socket, Flags) ->
        {ok, Data} | {error, Reason} | {error, {Reason, Data}}
recv(Socket, Length) ->
        {ok, Data} | {error, Reason} | {error, {Reason, Data}}
recv(Socket, Flags, Timeout :: infinity) ->
        {ok, Data} | {error, Reason} | {error, {Reason, Data}}
recv(Socket, Length, Flags) ->
        {ok, Data} | {error, Reason} | {error, {Reason, Data}}
recv(Socket, Length, Timeout :: infinity) ->
        {ok, Data} | {error, Reason} | {error, {Reason, Data}}
recv(Socket, Length, Flags, Timeout :: infinity) ->
        {ok, Data} | {error, Reason} | {error, {Reason, Data}}

    Types:
        Socket = socket()
        Length = integer() >= 0
        Flags = [msg_flag() | integer()]
        Data = binary()
        Reason = posix() | closed | invalid()

    Receives data from a socket, waiting for it to arrive.

    The argument Length specifies how many bytes to receive, with the special case 0 meaning "all available".

    For a socket of type stream this call will not return until all requested data can be delivered, or if "all available" data was requested when the first data chunk arrives.

    The message Flags may be symbolic msg_flag()s and/or integer()s, as in the platform's appropriate header files. The values of all symbolic flags and integers are or:ed together.

    When there is a socket error this function returns {error, Reason}, or if some data arrived before the error; {error, {Reason, Data}}.

recv(Socket, Flags, Timeout :: integer() >= 0) ->
        {ok, Data} | {error, Reason} | {error, {Reason, Data}}
recv(Socket, Length, Timeout :: integer() >= 0) ->
        {ok, Data} | {error, Reason} | {error, {Reason, Data}}
recv(Socket, Length, Flags, Timeout :: integer() >= 0) ->
        {ok, Data} | {error, Reason} | {error, {Reason, Data}}

    Types:
        Socket = socket()
        Length = integer() >= 0
        Flags = [msg_flag() | integer()]
        Data = binary()
        Reason = posix() | closed | invalid() | timeout

    Receives data from a socket, waiting at most Timeout milliseconds for it to arrive.

    The same as infinite time-out recv/1,2,3,4 but returns {error, timeout} or {error, {timeout, Data}} after Timeout milliseconds, if the requested data has not been delivered.

recv(Socket, Flags, Handle :: nowait) ->
        {ok, Data} |
        {select, SelectInfo} |
        {select, {SelectInfo, Data}} |
        {completion, CompletionInfo} |
        {error, Reason} |
        {error, {Reason, Data}}
recv(Socket, Flags,
     Handle :: select_handle() | completion_handle()) ->
        {ok, Data} |
        {select, SelectInfo} |
        {select, {SelectInfo, Data}} |
        {completion, CompletionInfo} |
        {error, Reason} |
        {error, {Reason, Data}}
recv(Socket, Length, Handle :: nowait) ->
        {ok, Data} |
        {select, SelectInfo} |
        {select, {SelectInfo, Data}} |
        {completion, CompletionInfo} |
        {error, Reason} |
        {error, {Reason, Data}}
recv(Socket, Length,
     Handle :: select_handle() | completion_handle()) ->
        {ok, Data} |
        {select, SelectInfo} |
        {select, {SelectInfo, Data}} |
        {completion, CompletionInfo} |
        {error, Reason} |
        {error, {Reason, Data}}
recv(Socket, Length, Flags, Handle :: nowait) ->
        {ok, Data} |
        {select, SelectInfo} |
        {select, {SelectInfo, Data}} |
        {completion, CompletionInfo} |
        {error, Reason} |
        {error, {Reason, Data}}
recv(Socket, Length, Flags,
     Handle :: select_handle() | completion_handle()) ->
        {ok, Data} |
        {select, SelectInfo} |
        {select, {SelectInfo, Data}} |
        {completion, CompletionInfo} |
        {error, Reason} |
        {error, {Reason, Data}}

    Types:
        Socket = socket()
        Length = integer() >= 0
        Flags = [msg_flag() | integer()]
        Data = binary()
        SelectInfo = select_info()
        CompletionInfo = completion_info()
        Reason = posix() | closed | invalid()

    Receives data from a socket, but returns a select or completion continuation if the data could not be returned immediately.

    The same as infinite time-out recv/1,2,3,4 but if the data can be delivered immediately, the function returns (on Unix ) {select, SelectInfo} or (on Windows ) {completion, CompletionInfo}, and the caller will then receive one of these messages:

        select message:
            {'$socket', Socket, select, SelectHandle} (with the SelectHandle that was contained in the SelectInfo) when data has arrived.

            A subsequent call to recv/1,2,3,4 will then return the data.

        completion message:
            {'$socket', Socket, completion, {CompletionHandle, CompletionStatus}} (with the CompletionHandle contained in the CompletionInfo).

            The result of the receive will be in the CompletionStatus.

    If Handle is a select_handle() or completion_handle(), that term will be contained in a returned SelectInfo or CompletionInfo and the corresponding (select or completion) message. The Handle is presumed to be unique to this call.

    If the time-out argument is nowait, and a SelectInfo or CompletionInfo is returned, it will contain a select_handle() or completion_handle() generated by the call.

    Note that for a socket of type stream (on Unix ), if Length > 0 and only part of that amount of data is available, the function will return {ok, {Data, SelectInfo}} with partial data. If the caller doesn't want to wait for more data, it must immediately call cancel/2 to cancel the operation.

recvfrom(Socket) -> {ok, {Source, Data}} | {error, Reason}
recvfrom(Socket, Flags) -> {ok, {Source, Data}} | {error, Reason}
recvfrom(Socket, BufSz) -> {ok, {Source, Data}} | {error, Reason}
recvfrom(Socket, Flags, Timeout :: infinity) ->
            {ok, {Source, Data}} | {error, Reason}
recvfrom(Socket, BufSz, Flags) ->
            {ok, {Source, Data}} | {error, Reason}
recvfrom(Socket, BufSz, Timeout :: infinity) ->
            {ok, {Source, Data}} | {error, Reason}
recvfrom(Socket, BufSz, Flags, Timeout :: infinity) ->
            {ok, {Source, Data}} | {error, Reason}

    Types:
        Socket = socket()
        BufSz = integer() >= 0
        Flags = [msg_flag() | integer()]
        Source = sockaddr_recv()
        Data = binary()
        Reason = posix() | closed | invalid()

    Receive a message from a socket, waiting for it to arrive.

    The function returns when a message is received, or when there is a socket error. Argument BufSz specifies the number of bytes for the receive buffer. If the buffer size is too small, the message will be truncated.

    If BufSz is not specified or 0, a default buffer size is used, which can be set by socket:setopt(Socket, {otp,recvbuf}, BufSz).

    If it is impossible to know the appropriate buffer size, it may be possible to use the receive message flag peek. When this flag is used, the message is not "consumed" from the underlying buffers, so another recvfrom/1,2,3,4 call is needed, possibly with an adjusted buffer size.

    The message Flags may be symbolic msg_flag()s and/or integer()s, as in the platform's appropriate header files. The values of all symbolic flags and integers are or:ed together.

recvfrom(Socket, Flags, Timeout :: integer() >= 0) ->
            {ok, {Source, Data}} | {error, Reason}
recvfrom(Socket, BufSz, Timeout :: integer() >= 0) ->
            {ok, {Source, Data}} | {error, Reason}
recvfrom(Socket, BufSz, Flags, Timeout :: integer() >= 0) ->
            {ok, {Source, Data}} | {error, Reason}

    Types:
        Socket = socket()
        BufSz = integer() >= 0
        Flags = [msg_flag() | integer()]
        Source = sockaddr_recv()
        Data = binary()
        Reason = posix() | closed | invalid() | timeout

    Receives a message from a socket, waiting at most Timeout milliseconds for it to arrive.

    The same as infinite time-out recvfrom/1,2,3,4 but returns {error, timeout} after Timeout milliseconds, if no message has been delivered.

recvfrom(Socket, Flags, Handle :: nowait) ->
            {ok, {Source, Data}} |
            {select, SelectInfo} |
            {completion, CompletionInfo} |
            {error, Reason}
recvfrom(Socket, Flags,
         Handle :: select_handle() | completion_handle()) ->
            {ok, {Source, Data}} |
            {select, SelectInfo} |
            {completion, CompletionInfo} |
            {error, Reason}
recvfrom(Socket, BufSz, Handle :: nowait) ->
            {ok, {Source, Data}} |
            {select, SelectInfo} |
            {completion, CompletionInfo} |
            {error, Reason}
recvfrom(Socket, BufSz,
         Handle :: select_handle() | completion_handle()) ->
            {ok, {Source, Data}} |
            {select, SelectInfo} |
            {completion, CompletionInfo} |
            {error, Reason}
recvfrom(Socket, BufSz, Flags, Handle :: nowait) ->
            {ok, {Source, Data}} |
            {select, SelectInfo} |
            {completion, CompletionInfo} |
            {error, Reason}
recvfrom(Socket, BufSz, Flags,
         Handle :: select_handle() | completion_handle()) ->
            {ok, {Source, Data}} |
            {select, SelectInfo} |
            {completion, CompletionInfo} |
            {error, Reason}

    Types:
        Socket = socket()
        BufSz = integer() >= 0
        Flags = [msg_flag() | integer()]
        Source = sockaddr_recv()
        Data = binary()
        SelectInfo = select_info()
        CompletionInfo = completion_info()
        Reason = posix() | closed | invalid()

    Receives a message from a socket, but returns a select continuation or a completion term if no message could be returned immediately.

    The same as infinite time-out recvfrom/1,2,3,4 but if no message can be delivered immediately, the function returns (on /Unix ) {select, SelectInfo} or (on Windows ) {completion, CompletionInfo}, and the caller will then receive one of these messages:

        select message:
            {'$socket', Socket, select, SelectHandle} (with the SelectHandle that was contained in the SelectInfo) when data has arrived.

            A subsequent call to recvfrom/1,2,3,4 will then return the message.

        completion message:
            {'$socket', Socket, completion, {CompletionHandle, CompletionStatus}} (with the CompletionHandle contained in the CompletionInfo).

            The result of the receive will be in the CompletionStatus.

    If the Handle is a select_handle() or completion_handle(), that term will be contained in a returned SelectInfo or CompletionInfo and the corresponding (select or completion) message. The Handle is presumed to be unique to this call.

    If the time-out argument is nowait, and a SelectInfo or CompletionInfo is returned, it will contain a select_handle() or completion_handle() generated by the call.

    If the caller doesn't want to wait for the data, it must immediately call cancel/2 to cancel the operation.

recvmsg(Socket) -> {ok, Msg} | {error, Reason}
recvmsg(Socket, Flags) -> {ok, Msg} | {error, Reason}
recvmsg(Socket, Timeout :: infinity) ->
           {ok, Msg} | {error, Reason}
recvmsg(Socket, Flags, Timeout :: infinity) ->
           {ok, Msg} | {error, Reason}
recvmsg(Socket, BufSz, CtrlSz) -> {ok, Msg} | {error, Reason}
recvmsg(Socket, BufSz, CtrlSz, Timeout :: infinity) ->
           {ok, Msg} | {error, Reason}
recvmsg(Socket, BufSz, CtrlSz, Flags, Timeout :: infinity) ->
           {ok, Msg} | {error, Reason}

    Types:
        Socket = socket()
        BufSz = CtrlSz = integer() >= 0
        Flags = [msg_flag() | integer()]
        Msg = msg_recv()
        Reason = posix() | closed | invalid()

    Receive a message from a socket, waiting for it to arrive.

    The function returns when a message is received, or when there is a socket error. Arguments BufSz and CtrlSz specifies the number of bytes for the receive buffer and the control message buffer. If the buffer size(s) is(are) too small, the message and/or control message list will be truncated.

    If BufSz is not specified or 0, a default buffer size is used, which can be set by socket:setopt(Socket, {otp,recvbuf}, BufSz). The same applies to CtrlSz and socket:setopt(Socket, {otp,recvctrlbuf}, CtrlSz).

    If it is impossible to know the appropriate buffer size, it may be possible to use the receive message flag peek. When this flag is used, the message is not "consumed" from the underlying buffers, so another recvfrom/1,2,3,4,5 call is needed, possibly with an adjusted buffer size.

    The message Flags may be symbolic msg_flag()s and/or integer()s, as in the platform's appropriate header files. The values of all symbolic flags and integers are or:ed together.

recvmsg(Socket, Timeout :: integer() >= 0) ->
           {ok, Msg} | {error, Reason}
recvmsg(Socket, Flags, Timeout :: integer() >= 0) ->
           {ok, Msg} | {error, Reason}
recvmsg(Socket, BufSz, CtrlSz, Timeout :: integer() >= 0) ->
           {ok, Msg} | {error, Reason}
recvmsg(Socket, BufSz, CtrlSz, Flags,
        Timeout :: integer() >= 0) ->
           {ok, Msg} | {error, Reason}

    Types:
        Socket = socket()
        BufSz = CtrlSz = integer() >= 0
        Flags = [msg_flag() | integer()]
        Msg = msg_recv()
        Reason = posix() | closed | invalid() | timeout

    Receives a message from a socket, waiting at most Timeout milliseconds for it to arrive.

    The same as recvmsg/1,2,3,4,5 but returns {error, timeout} after Timeout milliseconds, if no message has been delivered.

recvmsg(Socket, Timeout :: nowait) ->
           {ok, Msg} |
           {select, SelectInfo} |
           {completion, CompletionInfo} |
           {error, Reason}
recvmsg(Socket, Handle :: select_handle() | completion_handle()) ->
           {ok, Msg} |
           {select, SelectInfo} |
           {completion, CompletionInfo} |
           {error, Reason}
recvmsg(Socket, Flags, Timeout :: nowait) ->
           {ok, Msg} |
           {select, SelectInfo} |
           {completion, CompletionInfo} |
           {error, Reason}
recvmsg(Socket, Flags,
        Handle :: select_handle() | completion_handle()) ->
           {ok, Msg} |
           {select, SelectInfo} |
           {completion, CompletionInfo} |
           {error, Reason}
recvmsg(Socket, BufSz, CtrlSz, Timeout :: nowait) ->
           {ok, Msg} |
           {select, SelectInfo} |
           {completion, CompletionInfo} |
           {error, Reason}
recvmsg(Socket, BufSz, CtrlSz,
        Handle :: select_handle() | completion_handle()) ->
           {ok, Msg} |
           {select, SelectInfo} |
           {completion, CompletionInfo} |
           {error, Reason}
recvmsg(Socket, BufSz, CtrlSz, Flags, Timeout :: nowait) ->
           {ok, Msg} |
           {select, SelectInfo} |
           {completion, CompletionInfo} |
           {error, Reason}
recvmsg(Socket, BufSz, CtrlSz, Flags,
        Handle :: select_handle() | completion_handle()) ->
           {ok, Msg} |
           {select, SelectInfo} |
           {completion, CompletionInfo} |
           {error, Reason}

    Types:
        Socket = socket()
        BufSz = CtrlSz = integer() >= 0
        Flags = [msg_flag() | integer()]
        Msg = msg_recv()
        SelectInfo = select_info()
        CompletionInfo = completion_info()
        Reason = posix() | closed | invalid()

    Receives a message from a socket, but returns a select continuation or a completion term if no message could be returned immediately.

    The same as infinite time-out recvmsg/1,2,3,4 but if no message can delivered immediately, the function returns (on Unix ) {select, SelectInfo} or (on Windows ) {completion, CompletionInfo}, and the caller will then receive one of these messages:

        select message:
            {'$socket', Socket, select, SelectHandle} (with the SelectHandle that was contained in the SelectInfo) when data has arrived.

            A subsequent call to recvmsg/1,2,3,4,5 will then return the data.

        completion message:
            {'$socket', Socket, completion, {CompletionHandle, CompletionStatus}} (with the CompletionHandle contained in the CompletionInfo).

            The result of the receive will be in the CompletionStatus.

    If the Handle is a select_handle() or completion_handle(), that term will be contained in a returned SelectInfo or CompletionInfo and the corresponding (select or completion) message. The Handle is presumed to be unique to this call.

    If the time-out argument is nowait, and a SelectInfo or CompletionInfo is returned, it will contain a select_handle() or completion_handle() generated by the call.

    If the caller doesn't want to wait for the data, it must immediately call cancel/2 to cancel the operation.

send(Socket, Data) ->
        ok |
        {ok, RestData} |
        {error, Reason} |
        {error, {Reason, RestData}}
send(Socket, Data, Flags) ->
        ok |
        {ok, RestData} |
        {error, Reason} |
        {error, {Reason, RestData}}
send(Socket, Data, Timeout :: infinity) ->
        ok |
        {ok, RestData} |
        {error, Reason} |
        {error, {Reason, RestData}}
send(Socket, Data, Flags, Timeout :: infinity) ->
        ok |
        {ok, RestData} |
        {error, Reason} |
        {error, {Reason, RestData}}

    Types:
        Socket = socket()
        Data = iodata()
        Flags = [msg_flag() | integer()]
        RestData = binary()
        Reason = posix() | closed | invalid()

    Sends data on a connected socket, waiting for it to be sent.

    This call will not return until the Data has been accepted by the platform's network layer, or it reports an error.

    The message Flags may be symbolic msg_flag()s and/or integer()s, matching the platform's appropriate header files. The values of all symbolic flags and integers are or:ed together.

    The Data, if it is not a binary(), is copied into one before calling the platform network API, because a single buffer is required. A returned RestData is a sub binary of this data binary.

    The return value indicates the result from the platform's network layer:

        ok: All data has been accepted.

        {ok, RestData}:
            Not all data has been accepted, but no error has been reported. RestData is the tail of Data that has not been accepted.

            This cannot happen for a socket of type stream where a partially successful send is retried until the data is either accepted or there is an error.

            For a socket of type dgram this should probably also not happen since a message that cannot be passed atomically should render an error.

            It is nevertheless possible for the platform's network layer to return this.

        {error, Reason}:
            An error has been reported and no data has been accepted. The posix() Reasons are from the platform's network layer. closed means that this socket library knows that the socket is closed, and invalid() means that something about an argument is invalid.

        {error, {Reason, RestData}} :
            An error has been reported but before that some data was accepted. RestData is the tail of Data that has not been accepted. See {error, Reason} above.

            This can only happen for a socket of type stream when a partially successful send is retried until there is an error.

send(Socket, Data, Timeout :: integer() >= 0) ->
        ok |
        {ok, RestData} |
        {error, Reason | timeout} |
        {error, {Reason | timeout, RestData}}
send(Socket, Data, Flags, Timeout :: integer() >= 0) ->
        ok |
        {ok, RestData} |
        {error, Reason | timeout} |
        {error, {Reason | timeout, RestData}}

    Types:
        Socket = socket()
        Data = iodata()
        Flags = [msg_flag() | integer()]
        RestData = binary()
        Reason = posix() | closed | invalid()

    Sends data on a connected socket, waiting at most Timeout milliseconds for it to be sent.

    The same as infinite time-out send/2,3,4 but returns {error, timeout} or {error, {timeout, RestData}} after Timeout milliseconds, if no Data or only some of it was accepted by the platform's network layer.

send(Socket, Data, Handle :: nowait) ->
        ok |
        {ok, RestData} |
        {select, SelectInfo} |
        {select, {SelectInfo, RestData}} |
        {completion, CompletionInfo} |
        {error, Reason}
send(Socket, Data,
     Handle :: select_handle() | completion_handle()) ->
        ok |
        {ok, RestData} |
        {select, SelectInfo} |
        {select, {SelectInfo, RestData}} |
        {completion, CompletionInfo} |
        {error, Reason}
send(Socket, Data, Flags, Handle :: nowait) ->
        ok |
        {ok, RestData} |
        {select, SelectInfo} |
        {select, {SelectInfo, RestData}} |
        {completion, CompletionInfo} |
        {error, Reason}
send(Socket, Data, Flags,
     Handle :: select_handle() | completion_handle()) ->
        ok |
        {ok, RestData} |
        {select, SelectInfo} |
        {select, {SelectInfo, RestData}} |
        {completion, CompletionInfo} |
        {error, Reason}

    Types:
        Socket = socket()
        Data = iodata()
        Flags = [msg_flag() | integer()]
        RestData = binary()
        SelectInfo = select_info()
        CompletionInfo = completion_info()
        Reason = posix() | closed | invalid()

    Sends data on a connected socket, but returns completion or a select continuation if the data could not be sent immediately.

    The same as infinite time-out send/2,3 but if the data is not immediately accepted by the platform network layer, the function returns (on Unix ) {select, SelectInfo} or (on Windows ) {completion, CompletionInfo}, and the caller will then receive one of these messages:

        select message:
            {'$socket', Socket, select, SelectHandle} ( with the SelectHandle that was contained in the SelectInfo ) when there is room for more data.

            A subsequent call to send/2-4 will then send the data.

        completion message:
            {'$socket', Socket, completion, {CompletionHandle, CompletionStatus}} (with the CompletionHandle contained in the CompletionInfo).

            The result of the send will be in the CompletionStatus.

    If Handle is a select_handle() or completion_handle(), that term will be contained in a returned SelectInfo or CompletionInfo and the corresponding select or completion message. The Handle is presumed to be unique to this call.

    If Handle is nowait, and a SelectInfo or CompletionInfo is returned, it will contain a select_handle() or completion_handle() generated by the call.

    If some of the data was sent, the function will return {ok, {RestData, SelectInfo}, which can only happen (on Unix ) for a socket of type stream. If the caller does not want to wait to send the rest of the data, it should immediately cancel the operation with cancel/2.

send(Socket, Data, Cont) ->
        ok |
        {ok, RestData} |
        {error, Reason} |
        {error, {Reason, RestData}}
send(Socket, Data, Cont, Timeout :: infinity) ->
        ok |
        {ok, RestData} |
        {error, Reason} |
        {error, {Reason, RestData}}
send(Socket, Data, Cont, Timeout :: integer() >= 0) ->
        ok |
        {ok, RestData} |
        {error, Reason | timeout} |
        {error, {Reason | timeout, RestData}}
send(Socket, Data, Cont, SelectHandle :: nowait) ->
        ok |
        {ok, RestData} |
        {select, SelectInfo} |
        {select, {SelectInfo, RestData}} |
        {error, Reason}
send(Socket, Data, Cont, SelectHandle :: select_handle()) ->
        ok |
        {ok, RestData} |
        {select, SelectInfo} |
        {select, {SelectInfo, RestData}} |
        {error, Reason}

    Types:
        Socket = socket()
        Data = iodata()
        Cont = select_info()
        RestData = binary()
        SelectInfo = select_info()
        Reason = posix() | closed | invalid()

    Continues sending data on a connected socket, where the send operation was initiated by send/3,4 that returned a SelectInfo continuation. Otherwise like infinite time-out send/2,3,4 , limited time-out send/3,4 or nowait send/3,4 respectively.

    Cont is the SelectInfo that was returned from the previous send() call.

    If Data is not a binary(), it will be copied into one, again.

    The return value indicates the result from the platform's network layer. See send/2,3,4 and nowait send/3,4.

sendmsg(Socket, Msg) ->
           ok |
           {ok, RestData} |
           {error, Reason} |
           {error, {Reason, RestData}}
sendmsg(Socket, Msg, Flags) ->
           ok |
           {ok, RestData} |
           {error, Reason} |
           {error, {Reason, RestData}}
sendmsg(Socket, Msg, Timeout :: infinity) ->
           ok |
           {ok, RestData} |
           {error, Reason} |
           {error, {Reason, RestData}}
sendmsg(Socket, Msg, Flags, Timeout :: infinity) ->
           ok |
           {ok, RestData} |
           {error, Reason} |
           {error, {Reason, RestData}}

    Types:
        Socket = socket()
        Msg = msg_send()
        Flags = [msg_flag() | integer()]
        RestData = erlang:iovec()
        Reason = posix() | closed | invalid()

    Sends a message on a socket, waiting for it to be sent.

    The destination, if needed, that is: if the socket is not connected, is provided in Msg, which also contains the data to send as a list of binaries. Msg may also contain an list of optional control messages (depending on what the protocol and platform supports).

    For a connected socket no address field should be present in Msg, the platform may return an error or ignore one.

    The message data is given to to the platform's network layer in the form of an I/O vector without copying the content. If the number of elements in the I/O vector is larger than allowed on the platform (reported in the iov_max field from info/0), on a socket of type stream the send is iterated over all elements, but for other socket types the call fails.

    This call will not return until the data has been handed over to the platform's network layer, or when it reports an error.

    The message Flags may be symbolic msg_flag()s and/or integer()s, matching the platform's appropriate header files. The values of all symbolic flags and integers are or:ed together.

    The return value indicates the result from the platform's network layer. See send/2,3,4.

        Note:
    On Windows, this function can only be used with datagram and raw sockets.

sendmsg(Socket, Msg, Timeout :: integer() >= 0) ->
           ok |
           {ok, RestData} |
           {error, Reason | timeout} |
           {error, {Reason | timeout, RestData}}
sendmsg(Socket, Msg, Flags, Timeout :: integer() >= 0) ->
           ok |
           {ok, RestData} |
           {error, Reason | timeout} |
           {error, {Reason | timeout, RestData}}

    Types:
        Socket = socket()
        Msg = msg_send()
        Flags = [msg_flag() | integer()]
        RestData = erlang:iovec()
        Reason = posix() | closed | invalid()

    Sends a message on a socket, waiting at most Timeout milliseconds for it to be sent.

    The same as infinite time-out sendmsg/2,3,4 but returns {error, timeout} or {error, {timeout, RestData}} after Timeout milliseconds, if no data or only some of it was accepted by the platform's network layer.

        Note:
    On Windows, this function can only be used with datagram and raw sockets.

sendmsg(Socket, Msg, Timeout :: nowait) ->
           ok |
           {ok, RestData} |
           {select, SelectInfo} |
           {select, {SelectInfo, RestData}} |
           {completion, CompletionInfo} |
           {error, Reason} |
           {error, {Reason, RestData}}
sendmsg(Socket, Msg,
        Handle :: select_handle() | completion_handle()) ->
           ok |
           {ok, RestData} |
           {select, SelectInfo} |
           {select, {SelectInfo, RestData}} |
           {completion, CompletionInfo} |
           {error, Reason} |
           {error, {Reason, RestData}}
sendmsg(Socket, Msg, Flags, Timeout :: nowait) ->
           ok |
           {ok, RestData} |
           {select, SelectInfo} |
           {select, {SelectInfo, RestData}} |
           {completion, CompletionInfo} |
           {error, Reason} |
           {error, {Reason, RestData}}
sendmsg(Socket, Msg, Flags,
        Handle :: select_handle() | completion_handle()) ->
           ok |
           {ok, RestData} |
           {select, SelectInfo} |
           {select, {SelectInfo, RestData}} |
           {completion, CompletionInfo} |
           {error, Reason} |
           {error, {Reason, RestData}}

    Types:
        Socket = socket()
        Msg = msg_send()
        Flags = [msg_flag() | integer()]
        RestData = erlang:iovec()
        SelectInfo = select_info()
        CompletionInfo = completion_info()
        Reason = posix() | closed | invalid()

    Sends a message on a socket, but returns completion or a select continuation if the data could not be sent immediately.

    The same as infinity time-out sendmsg/2,3 but if the data is not immediately accepted by the platform network layer, the function returns (on Unix ) {select, SelectInfo} or (on Windows ) {completion, CompletionInfo}, and the caller will then receive one of these messages:

        select message:
            {'$socket', Socket, select, SelectHandle} ( with the SelectHandle that was contained in the SelectInfo ) when there is room for more data. A subsequent call to sendmsg/2-4 will then send the data.

        completion message:
            {'$socket', Socket, completion, {CompletionHandle, CompletionStatus}} (with the CompletionHandle contained in the CompletionInfo).

            The result of the send will be in the CompletionStatus.

    If Handle, is a select_handle() or completion_handle(), that term will be contained in a returned SelectInfo or CompletionInfo and the corresponding select or completion message. The Handle is presumed to be unique to this call.

    If Timeout is nowait, and a SelectInfo or CompletionInfo is returned, it will contain a select_handle() or completion_handle() generated by the call.

    If some of the data was sent, the function will return {ok, {RestData, SelectInfo}, which can only happen for a socket of type stream. If the caller does not want to wait to send the rest of the data, it should immediately cancel the operation with cancel/2.

        Note:
    On Windows, this function can only be used with datagram and raw sockets.

sendmsg(Socket, Data, Cont) ->
           ok |
           {ok, RestData} |
           {error, Reason} |
           {error, {Reason, RestData}}
sendmsg(Socket, Data, Cont, Timeout :: infinity) ->
           ok |
           {ok, RestData} |
           {error, Reason} |
           {error, {Reason, RestData}}
sendmsg(Socket, Data, Cont, Timeout :: integer() >= 0) ->
           ok |
           {ok, RestData} |
           {error, Reason | timeout} |
           {error, {Reason | timeout, RestData}}
sendmsg(Socket, Data, Cont, Timeout :: nowait) ->
           ok |
           {ok, RestData} |
           {select, SelectInfo} |
           {select, {SelectInfo, RestData}} |
           {completion, CompletionInfo} |
           {error, Reason} |
           {error, {Reason, RestData}}
sendmsg(Socket, Data, Cont, SelectHandle :: select_handle()) ->
           ok |
           {ok, RestData} |
           {select, SelectInfo} |
           {select, {SelectInfo, RestData}} |
           {error, Reason} |
           {error, {Reason, RestData}}

    Types:
        Socket = socket()
        Data = msg_send() | erlang:iovec()
        Cont = select_info()
        RestData = erlang:iovec()
        SelectInfo = select_info()
        Reason = posix() | closed | invalid()

    Continues sending a message data on a socket, where the send operation was initiated by sendmsg/3,4 that returned a SelectInfo continuation. Otherwise like infinite time-out sendmsg/2,3,4 , limited time-out sendmsg/3,4 or nowait sendmsg/3,4 respectively.

    Cont is the SelectInfo that was returned from the previous sendmsg() call.

    The return value indicates the result from the platform's network layer. See send/2,3,4 and nowait sendmsg/3,4.

sendto(Socket, Data, Dest) ->
          ok |
          {ok, RestData} |
          {error, Reason} |
          {error, {Reason, RestData}}
sendto(Socket, Data, Dest, Flags) ->
          ok |
          {ok, RestData} |
          {error, Reason} |
          {error, {Reason, RestData}}
sendto(Socket, Data, Dest, Timeout :: infinity) ->
          ok |
          {ok, RestData} |
          {error, Reason} |
          {error, {Reason, RestData}}
sendto(Socket, Data, Dest, Flags, Timeout :: infinity) ->
          ok |
          {ok, RestData} |
          {error, Reason} |
          {error, {Reason, RestData}}

    Types:
        Socket = socket()
        Data = iodata()
        Dest = sockaddr()
        Flags = [msg_flag() | integer()]
        RestData = binary()
        Reason = posix() | closed | invalid()

    Sends data on a socket, to the specified destination, waiting for it to be sent.

    This call will not return until the data has been accepted by the platform's network layer, or it reports an error.

    If this call is used on a connection mode socket or on a connected socket, the platforms's network layer may return an error or ignore the destination address.

    The message Flags may be symbolic msg_flag()s and/or integer()s, matching the platform's appropriate header files. The values of all symbolic flags and integers are or:ed together.

    The return value indicates the result from the platform's network layer. See send/2,3,4.

sendto(Socket, Data, Dest, Timeout :: integer() >= 0) ->
          ok |
          {ok, RestData} |
          {error, Reason | timeout} |
          {error, {Reason | timeout, RestData}}
sendto(Socket, Data, Dest, Flags, Timeout :: integer() >= 0) ->
          ok |
          {ok, RestData} |
          {error, Reason | timeout} |
          {error, {Reason | timeout, RestData}}

    Types:
        Socket = socket()
        Data = iodata()
        Dest = sockaddr()
        Flags = [msg_flag() | integer()]
        RestData = binary()
        Reason = posix() | closed | invalid()

    Sends data on a socket, waiting at most Timeout milliseconds for it to be sent.

    The same as infinite time-out sendto/3,4,5 but returns {error, timeout} or {error, {timeout, RestData}} after Timeout milliseconds, if no Data or only some of it was accepted by the platform's network layer.

sendto(Socket, Data, Dest, Handle :: nowait) ->
          ok |
          {ok, RestData} |
          {select, SelectInfo} |
          {select, {SelectInfo, RestData}} |
          {completion, CompletionInfo} |
          {error, Reason}
sendto(Socket, Data, Dest,
       Handle :: select_handle() | completion_handle()) ->
          ok |
          {ok, RestData} |
          {select, SelectInfo} |
          {select, {SelectInfo, RestData}} |
          {completion, CompletionInfo} |
          {error, Reason}
sendto(Socket, Data, Dest, Flags, Handle :: nowait) ->
          ok |
          {ok, RestData} |
          {select, SelectInfo} |
          {select, {SelectInfo, RestData}} |
          {completion, CompletionInfo} |
          {error, Reason}
sendto(Socket, Data, Dest, Flags,
       Handle :: select_handle() | completion_handle()) ->
          ok |
          {ok, RestData} |
          {select, SelectInfo} |
          {select, {SelectInfo, RestData}} |
          {completion, CompletionInfo} |
          {error, Reason}

    Types:
        Socket = socket()
        Data = iodata()
        Dest = sockaddr()
        Flags = [msg_flag() | integer()]
        RestData = binary()
        SelectInfo = select_info()
        CompletionInfo = completion_info()
        Reason = posix() | closed | invalid()

    Sends data on a socket, but returns completion or a select continuation if the data could not be sent immediately.

    The same as infinity time-out sendto/3,4 but if the data is not immediately accepted by the platform network layer, the function returns (on Unix ) {select, SelectInfo} or (on Windows ) {completion, CompletionInfo}, and the caller will then receive one of these messages:

        select message:
            {'$socket', Socket, select, SelectHandle} ( with the SelectHandle that was contained in the SelectInfo ) when there is room for more data.

            A subsequent call to send/2-4 will then send the data.

        completion message:
            {'$socket', Socket, completion, {CompletionHandle, CompletionStatus}} (with the CompletionHandle contained in the CompletionInfo).

            The result of the send will be in the CompletionStatus.

    If Handle is a select_handle() or completion_handle(), that term will be contained in a returned SelectInfo or CompletionInfo and the corresponding select or completion message. The Handle is presumed to be unique to this call.

    If Handle is nowait, and a SelectInfo or CompletionInfo is returned, it will contain a select_handle() or completion_handle() generated by the call.

    If some of the data was sent, the function will return {ok, {RestData, SelectInfo}, which can only happen (on Unix ) for a socket of type stream. If the caller does not want to wait to send the rest of the data, it should immediately cancel the operation with cancel/2.

sendto(Socket, Data, Cont) ->
          ok |
          {ok, RestData} |
          {error, Reason} |
          {error, {Reason, RestData}}
sendto(Socket, Data, Cont, Timeout :: infinity) ->
          ok |
          {ok, RestData} |
          {error, Reason} |
          {error, {Reason, RestData}}
sendto(Socket, Data, Cont, Timeout :: integer() >= 0) ->
          ok |
          {ok, RestData} |
          {error, Reason | timeout} |
          {error, {Reason | timeout, RestData}}
sendto(Socket, Data, Cont, SelectHandle :: nowait) ->
          ok |
          {ok, RestData} |
          {select, SelectInfo} |
          {select, {SelectInfo, RestData}} |
          {error, Reason}
sendto(Socket, Data, Cont, SelectHandle :: select_handle()) ->
          ok |
          {ok, RestData} |
          {select, SelectInfo} |
          {select, {SelectInfo, RestData}} |
          {error, Reason}

    Types:
        Socket = socket()
        Data = iodata()
        Cont = select_info()
        RestData = binary()
        SelectInfo = select_info()
        Reason = posix() | closed | invalid()

    Continues sending data on a socket, where the send operation was initiated by sendto/4,5 that returned a SelectInfo continuation. Otherwise like infinite time-out sendto/3,4,5 , limited time-out sendto/4,5 or nowait sendto/4,5 respectively.

    Cont is the SelectInfo that was returned from the previous sendto() call.

    If Data is not a binary(), it will be copied into one, again.

    The return value indicates the result from the platform's network layer. See send/2,3,4 and nowait sendto/4,5.

sendfile(Socket, FileHandle, Offset, Count, Timeout :: infinity) ->
            {ok, BytesSent} |
            {error, Reason} |
            {error, {Reason, BytesSent}}

    Types:
        Socket = socket()
        FileHandle = file:fd()
        Offset = integer()
        Count = BytesSent = integer() >= 0
        Reason = posix() | closed | invalid()

    Sends file data on a socket, to the specified destination, waiting for it to be sent ("infinite" time-out).

    The FileHandle must refer to an open raw file as described in file:open/2.

    This call will not return until the data has been accepted by the platform's network layer, or it reports an error.

    The Offset argument is the file offset to start reading from. The default value is 0.

    The Count argument is the number of bytes to transfer from FileHandle to Socket. If Count =:= 0 (the default) the transfer stops at the end of file.

    The return value indicates the result from the platform's network layer:

        {ok, BytesSent}:
            The transfer completed successfully after BytesSent bytes of data.

        {error, Reason}:
            An error has been reported and no data has been transferred. The posix() Reasons are from the platform's network layer. closed means that this socket library knows that the socket is closed, and invalid() means that something about an argument is invalid.

        {error, {Reason, BytesSent}} :
            An error has been reported but before that some data was transferred. See {error, Reason} and {ok, BytesSent} above.

sendfile(Socket, FileHandle, Offset, Count,
         Timeout :: integer() >= 0) ->
            {ok, BytesSent} |
            {error, Reason | timeout} |
            {error, {Reason | timeout, BytesSent}}

    Types:
        Socket = socket()
        FileHandle = file:fd()
        Offset = integer()
        Count = BytesSent = integer() >= 0
        Reason = posix() | closed | invalid()

    Sends file data on a socket, waiting at most Timeout milliseconds for it to be sent (limited time-out).

    The same as "infinite" time-out sendfile/5 but returns {error, timeout} or {error, {timeout, BytesSent}} after Timeout milliseconds, if not all file data was transferred by the platform's network layer.

sendfile(Socket, FileHandle, Offset, Count,
         SelectHandle :: nowait) ->
            {ok, BytesSent} |
            {select, SelectInfo} |
            {select, {SelectInfo, BytesSent}} |
            {error, Reason}
sendfile(Socket, FileHandle, Offset, Count,
         SelectHandle :: select_handle()) ->
            {ok, BytesSent} |
            {select, SelectInfo} |
            {select, {SelectInfo, BytesSent}} |
            {error, Reason}

    Types:
        Socket = socket()
        FileHandle = file:fd()
        Offset = integer()
        Count = BytesSent = integer() >= 0
        SelectInfo = select_info()
        Reason = posix() | closed | invalid()

    Sends file data on a socket, but returns a select continuation if the data could not be sent immediately (nowait).

    The same as "infinite" time-out sendfile/5 but if the data is not immediately accepted by the platform network layer, the function returns {select, SelectInfo}, and the caller will then receive a select message, {'$socket', Socket, select, SelectHandle} ( with the SelectHandle that was contained in the SelectInfo ) when there is room for more data. Then a call to sendfile/3 with SelectInfo as the second argument will continue the data transfer.

    If SelectHandle is a select_handle(), that term will be contained in a returned SelectInfo and the corresponding select message. The SelectHandle is presumed to be unique to this call.

    If SelectHandle is nowait, and a SelectInfo is returned, it will contain a select_handle() generated by the call.

    If some file data was sent, the function will return {ok, {BytesSent, SelectInfo}. If the caller does not want to wait to send the rest of the data, it should immediately cancel the operation with cancel/2.

sendfile(Socket, Cont, Offset, Count, Timeout :: infinity) ->
            {ok, BytesSent} |
            {error, Reason} |
            {error, {Reason, BytesSent}}
sendfile(Socket, Cont, Offset, Count,
         Timeout :: integer() >= 0) ->
            {ok, BytesSent} |
            {error, Reason | timeout} |
            {error, {Reason | timeout, BytesSent}}
sendfile(Socket, Cont, Offset, Count, SelectHandle :: nowait) ->
            {ok, BytesSent} |
            {select, SelectInfo} |
            {select, {SelectInfo, BytesSent}} |
            {error, Reason}
sendfile(Socket, Cont, Offset, Count,
         SelectHandle :: select_handle()) ->
            {ok, BytesSent} |
            {select, SelectInfo} |
            {select, {SelectInfo, BytesSent}} |
            {error, Reason}

    Types:
        Socket = socket()
        Cont = select_info()
        Offset = integer()
        Count = BytesSent = integer() >= 0
        SelectInfo = select_info()
        Reason = posix() | closed | invalid()

    Continues sending file data on a socket, where the send operation was initiated by sendfile/3,5 that returned a SelectInfo continuation. Otherwise like "infinite" time-out sendfile/5 , limited time-out sendfile/5 or nowait sendfile/5 respectively.

    Cont is the SelectInfo that was returned from the previous sendfile() call.

    The return value indicates the result from the platform's network layer. See "infinite" time-out sendfile/5.

sendfile(Socket, FileHandle, Offset, Count) -> Result

    Types:
        Socket = socket()
        FileHandle = file:fd()
        Offset = integer()
        Count = integer() >= 0

    The same as sendfile(Socket, FileHandle, Offset, Count, infinity), that is: send the file data at Offset and Count to the socket, without time-out other than from the platform's network stack.

sendfile(Socket, FileHandle, Timeout) -> Result

    Types:
        Socket = socket()
        FileHandle = file:fd()
         Timeout = timeout() | 'nowait' | select_handle()

    Depending on the Timeout argument; the same as sendfile(Socket, FileHandle, 0, 0, infinity), sendfile(Socket, FileHandle, 0, 0, Timeout), or sendfile(Socket, FileHandle, 0, 0, SelectHandle), that is: send all data in the file to the socket, with the given Timeout.

sendfile(Socket, FileHandle) -> Result

    Types:
        Socket = socket()
        FileHandle = file:fd()

    The same as sendfile(Socket, FileHandle, 0, 0, infinity), that is: send all data in the file to the socket, without time-out other than from the platform's network stack.

setopt(Socket :: socket(),
       SocketOption :: {Level :: otp, Opt :: otp_socket_option()},
       Value :: term()) ->
          ok | {error, invalid() | closed}

    Sets a socket option in the protocol level otp, which is this implementation's level above the OS protocol layers.

    See the type otp_socket_option() for a description of the options on this level.

setopt(Socket :: socket(),
       SocketOption :: socket_option(),
       Value :: term()) ->
          ok | {error, posix() | invalid() | closed}

    Set a socket option in one of the OS's protocol levels. See the type socket_option() for which options that this implementation knows about, how they are related to option names in the OS, and if there are known peculiarities with any of them.

    What options are valid depends on what kind of socket it is (domain(), type() and protocol()).

    See the socket options chapter of the users guide for more info.

        Note:
    Not all options are valid, nor possible to set, on all platforms. That is, even if "we" support an option; it does not mean that the underlying OS does.

setopt(Socket, Level, Opt, Value) -> ok | {error, Reason}

    Types:
         Socket = socket()
         Value = term()
         Reason = inet:posix() | invalid() | closed

    Backwards compatibility function.

    The same as setopt(Socket, {Level, Opt}, Value)

setopt_native(Socket :: socket(),
              SocketOption ::
                  socket_option() |
                  {Level :: level() | (NativeLevel :: integer()),
                   NativeOpt :: integer()},
              Value :: native_value()) ->
                 ok | {error, posix() | invalid() | closed}

    Sets a socket option that may be unknown to our implementation, or that has a type not compatible with our implementation, that is; in "native mode".

    If Value is an integer() it will be used as a C type (int), if it is a boolean() it will be used as a C type (int) with the C implementations values for false or true, and if it is a binary() its content and size will be used as the option value.

    The socket option may be specified with an ordinary socket_option() tuple, with a known Level = level() and an integer NativeOpt, or with both an integer NativeLevel and NativeOpt.

    What options are valid depends on what kind of socket it is (domain(), type() and protocol()).

    The integer values for NativeLevel and NativeOpt as well as the encoding of Value has to be deduced from the header files for the running system.

shutdown(Socket, How) -> ok | {error, Reason}

    Types:
        Socket = socket()
        How = read | write | read_write
        Reason = posix() | closed

    Shut down all or part of a full-duplex connection.

sockname(Socket) -> {ok, SockAddr} | {error, Reason}

    Types:
        Socket = socket()
        SockAddr = sockaddr_recv()
        Reason = posix() | closed

    Returns the current address to which the socket is bound.

supports() ->
            [{Key1 :: term(),
              boolean() |
              [{Key2 :: term(),
                boolean() | [{Key3 :: term(), boolean()}]}]}]
supports(Key1 :: term()) ->
            [{Key2 :: term(),
              boolean() | [{Key3 :: term(), boolean()}]}]
supports(Key1 :: term(), Key2 :: term()) ->
            [{Key3 :: term(), boolean()}]

    These functions function retrieves information about what the platform supports, such which platform features or which socket options, are supported.

    For keys other than the known the empty list is returned, Note that in a future version or on a different platform there might be more supported items.

        supports():
            Returns a list of {Key1, supports(Key1)} tuples for every Key1 described in supports/1 and {Key1, boolean()} tuples for each of the following keys:

            sctp:
                SCTP support

            ipv6:
                IPv6 support

            local:
                 Unix Domain sockets support (AF_UNIX | AF_LOCAL)

            netns:
                 Network Namespaces support (Linux, setns(2))

            sendfile:
                 Sendfile support (sendfile(2))

        supports(msg_flags = Key1):
            Returns a list of {Flag, boolean()} tuples for every Flag in msg_flag() with the boolean() indicating if the flag is supported on this platform.

        supports(protocols = Key1):
            Returns a list of {Name :: atom(), boolean()} tuples for every Name in protocol() with the boolean() indicating if the protocol is supported on this platform.

        supports(options = Key1):
            Returns a list of {SocketOption, boolean()} tuples for every SocketOption in socket_option() with the boolean() indicating if the socket option is supported on this platform.

        supports(options = Key1, Key2) :
            For a Key2 in level() returns a list of {Opt, boolean()} tuples for all known socket options Opt on that Level =:= Key2, and the boolean() indicating if the socket option is supported on this platform. See setopt/3 and getopt/2.

use_registry(D :: boolean()) -> ok

    Globally change if the socket registry is to be used or not. Note that its still possible to override this explicitly when creating an individual sockets, see open/2 or open/4 for more info (use the Extra argument).

which_sockets() -> [socket()]
which_sockets(FilterRule) -> [socket()]

    Types:
        FilterRule =
            inet | inet6 | local | stream | dgram | seqpacket | sctp |
            tcp | udp |
            pid() |
            fun((socket_info()) -> boolean())

    Returns a list of all sockets, according to the filter rule.

    There are several pre-made filter rule(s) and one general:

        inet | inet6:
            Selection based on the domain of the socket.
            Only a subset is valid.

        stream | dgram | seqpacket:
            Selection based on the type of the socket.
            Only a subset is valid.

        sctp | tcp | udp:
            Selection based on the protocol of the socket.
            Only a subset is valid.

        pid():
            Selection base on which sockets has this pid as Controlling Process.

        fun((socket_info()) -> boolean()):
            The general filter rule.
            A fun that takes the socket info and returns a boolean() (true if the socket could be included and false if should not).

EXAMPLES

client(SAddr, SPort) ->
   {ok, Sock} = socket:open(inet, stream, tcp),
   ok = socket:connect(Sock, #{family => inet,
                               addr   => SAddr,
                               port   => SPort}),
   Msg = <<"hello">>,
   ok = socket:send(Sock, Msg),
   ok = socket:shutdown(Sock, write),
   {ok, Msg} = socket:recv(Sock),
   ok = socket:close(Sock).
server(Addr, Port) ->
   {ok, LSock} = socket:open(inet, stream, tcp),
   ok = socket:bind(LSock, #{family => inet,
                             port   => Port,
                             addr   => Addr}),
   ok = socket:listen(LSock),
   {ok, Sock} = socket:accept(LSock),
   {ok, Msg} = socket:recv(Sock),
   ok = socket:send(Sock, Msg),
   ok = socket:close(Sock),
   ok = socket:close(LSock).