Search documentation of kernel

# socket (kernel v10.3)

View Source

Socket interface.

This module provides an API for network sockets. 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 additions are that some of the functions, e.g. `recv/3`, have a time-out argument, and that `recv/*` for a `stream` socket iterates until the requested amount of data has been received.

#### Note

#### Asynchronous Calls

Some functions feature asynchronous calls. This is achieved by setting the `Timeout` argument to `nowait` or to a `Handle ::` `reference/0`. See the respective function's type specification.

This module has two different implementations of asynchronous calls. One on the Unix family of operating systems: `select` - based on the standard socket interface's `select(3)`/`poll(3)` calls, and one on Windows: `completion` - based on asynchronous I/O Completion Ports. The difference shows in the return values and message formats because they have slightly different semantics.

#### The `completion` and `select` Return Values

For instance, the call `recv(Socket, 0, nowait)`, when there is no data available for reading, will, depending on the operating system, return one of:

- `{completion,``CompletionInfo``}`
- `{select,``SelectInfo``}`

Where `CompletionInfo` is `{completion_info, _,``CompletionHandle``}` and `SelectInfo` is `{select_info, _,``SelectHandle``}`. Both the `CompletionHandle` and the `SelectHandle` are of type `reference/0`.

When the operation can continue, a `completion` message containing the `CompletionHandle` or a `select` message containing the `SelectHandle` is sent to the calling process.

On `select` systems, `recv/2,3,4` may also return:

- `{select, {``SelectInfo``, Data}`

This may happen for sockets of type `stream` when `Length > 1` since the OS may split a data stream at any point and deliver just the first part of the requested data. For the next `recv/2,3,4` call; the `Length` to receive will probably have to be adjusted due to the already delivered data in this return value.

On `select` systems, when the `{otp, select_read}` option is `true`, the asynchronous `recv/3,4`, `recvfrom/3,4`, and `recvmsg/3,4,5` functions may also return:

- `{select_read, {``SelectInfo``, Data}`

This indicates that the receive operation was completed; all requested data has been delivered, and that the calling process will get a `select` message when there is data available for the next receive operation.

#### The `completion` and `select` Messages

The `completion` message has the format:

- `{'$socket',``Socket``, completion, {``CompletionHandle``, CompletionStatus}}`

The `select` message has the format:

- `{'$socket',``Socket``, select,``SelectHandle``}`

When a `completion` message is received (which contains the result of the operation), it means that the operation has been completed and `CompletionStatus` is the return value for the operation, which is what the function that initiated the operation could have returned, with the `nowait` argument, except for the `completion` and `select` return values.

When a `select` message is received, it only means that the operation may now continue, by retrying the operation (which may return a new `{select, _}` value). Some operations are retried by repeating the same function call, and some have a dedicated function variant to be used for the retry. See the respective function's documentation.

#### Operation Queuing on `select` Systems

On `select` systems, all other processes are locked out until the current process has completed the operation as in a continuation call has returned a value indicating success or failure (not a `select` or `select_read` return). Other processes are queued and get a `select` return which makes them wait for their turn.

Note that receiving data from parallel processes is only suitable for some protocols. For a `stream` socket it is in general a recipe for disaster.

#### Cancelling an operation

An operation that is in progress (not completed) may be canceled using `cancel/2` both on `completion` and `select` systems.

Cancelling an operation ensures that there is no `completion`, `select`, nor `abort` message in the inbox after the `cancel/2` call.

#### Using a `Handle`

If creating a `reference/0` with `make_ref()` and using that as the `Timeout | Handle` argument, the same `Handle` will then be the `SelectHandle` in the returned `select_info/0` and the received `select` message, or be the `CompletionHandle` in the returned `completion_info/0` and the received `completion` message.

The compiler may then optimize a following `receive` statement to only scan the messages that arrive after the `reference/0` is created. If the message queue is large this is a big optimization.

It is not possible to have more than one operation in progress with the same `reference/0`.

#### Repeating an Operation on a `select` Systems

On`select` systems, if a call would be repeated before the `select` message has been received it replaces the operation in progress:

        {select, {select_info, Handle}} = socket:accept(LSock, nowait),
        {ok, Socket} = socket:accept(LSock, 1000),
        :

Above, `Handle` is no longer valid once the second `accept/2`, call has been made (the first call is automatically canceled). After the second `accept/2` call returns, the accept operation has completed.

Note that there is a race here; there is no way to know if the call is repeated before the `select` message is sent since it may have been sent just before the repeated call. So now there might be a `select` message containing `Handle` in the inbox.

#### The `abort` Message

Another message the user must be prepared for (when using `nowait | Handle`) is the `abort` message:

- `{'$socket',``Socket``, abort, Info}`

This message indicates that the operation in progress has been aborted. For instance, if the socket has been closed by another process; `Info` will be `{Handle, closed}`.

#### Note

Support for IPv6 has been implemented but not fully tested.

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

This module was introduced in OTP 22.0, as experimental code.

- In OTP 22.1, the `nowait` argument was added for many functions, and the `cancel/2` and `info/1` functions were also added.
- In OTP 22.3, the `number_of/0` function was added.
- In OTP 23.0, the functions `is_supported/1,2` and the `open/1,2` functions with an `FD` argument were added.
- In OTP 23.1, the `use_registry/1` function was added.
- In OTP 24.0, the `select_handle/0` argument was added for many functions, the `cancel/1`, `cancel_monitor/1`, `getopt/3` with tuple options, `getopt_native/3`, `info/0`, `monitor/1`, `open/3` with an option list, many variants of the `recv/*`, `recvfrom/*`, `recvmsg/*`, `send/*`, `sendmsg/*`, the `sendto/*` functions, the `sendfile/*` functions, and the `setopt/3`, `setopt_native/3` functions, were added.
- In OTP 24.1, the `i/*` functions were added.
- In OTP 24.2, the `ioctl/*` functions were added.
- In OTP 26.0, the `completion_handle/0` argument was added for many functions, and the `cancel/2` function with `completion_info/0` argument was added. That is: support for Windows asynchronous I/O Completion Ports was added. The Unix-ish flavored (select handle) API features could be considered no longer experimental.
- In OTP 27.0, the Windows flavored (completion handle) API features could be considered no longer experimental.

## 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).

# Summary

## Types

cmsg()

Control messages (ancillary messages).

cmsg_recv()

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

cmsg_send()

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

completion_handle()

Completion operation handle.

completion_info()

Completion operation info.

completion_tag()

Completion operation tag.

domain()

Protocol domain a.k.a address family.

ee_origin()
eei()

Extended Error Information.

extended_err()
hatype()
icmp_dest_unreach()
icmpv6_dest_unreach()
in6_addr()
in6_flow_info()
in6_scope_id()
in_addr()
info()

Platform dependent information items.

info_keys()

Information element designators for the `i/1` and `i/2` functions.

interface_type()

The interface type (of the datalink). We only translate a few values to atoms, the rest are left as (unsigned) integer values.

invalid()
ioctl_device_flag()
ioctl_device_map()
ip_mreq()

C: `struct ip_mreq`

ip_mreq_source()

C: `struct ip_mreq_source`

ip_msfilter()

C: `struct ip_msfilter`

ip_pktinfo()

C: `struct ip_pktinfo`

ip_pmtudisc()

C: `IP_PMTUDISC_*` values.

ip_tos()

C: `IPTOS_*` values.

ipv6_hops()

IPv6 hop limit value.

ipv6_mreq()

C: `struct ipv6_mreq`

ipv6_pktinfo()

C: `struct in6_pktinfo`

ipv6_pmtudisc()

C: `IPV6_PMTUDISC_*` values

level()

Protocol level.

linger()

C: `struct linger`

msg()

C: `struct msghdr`

msg_flag()

Platform dependent message flags.

msg_recv()

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

msg_send()

Message sent by `sendmsg/2,3,4`.

native_value()
otp_socket_option()

Protocol level `otp` socket option.

packet_type()
port_number()
posix()

Posix error codes.

protocol()

Protocol name.

sctp_assocparams()

C: `struct sctp_assocparams`

sctp_event_subscribe()

C: `struct sctp_event_subscribe`.

sctp_initmsg()

C: `struct sctp_initmsg`.

sctp_rtoinfo()

C: `struct sctp_rtoinfo`.

select_handle()

Select operation handle.

select_info()

Select operation info.

select_tag()

Select operation tag.

sockaddr()
sockaddr_dl()

C: `struct sockaddr_dl`

sockaddr_in6()

C: `struct sockaddr_in6`

sockaddr_in()

C: `struct sockaddr_in`

sockaddr_ll()

C: `struct sockaddr_ll`

sockaddr_native()

C: `struct sockaddr`

sockaddr_recv()
sockaddr_un()

C: `struct sockaddr_un`.

sockaddr_unspec()

C: `struct sockaddr` of `AF_UNSPEC`

socket()

A socket, according to this module.

socket_counters()

A `map/0` of `Name := Counter` associations.

socket_handle()

Opaque socket handle unique for the socket.

socket_info()
socket_option()

Socket option.

timeval()

C: `struct timeval`

type()

Protocol type.

## Functions

accept(ListenSocket)

Equivalent to `accept(ListenSocket, infinity)`.

accept/2

Accept a connection on a listening socket.

bind(Socket, Addr)

Bind a name to a socket.

cancel/2

Cancel an asynchronous call in progress.

cancel_monitor(MRef)

Cancel a socket monitor.

close(Socket)

Close a socket.

connect(Socket)

Finalize a `connect/3` operation.

connect(Socket, SockAddr)

Equivalent to `connect(Socket, SockAddr, infinity)`.

connect/3

Connect the socket to the given address.

getopt/2

Get the value of a socket option.

getopt(Socket, Level, Opt)

Get a socket option (backwards compatibility function).

getopt_native/3

Get a "native" socket option.

i()

Print information to the erlang shell in table format for all sockets.

i/1

Print information to the erlang shell in table format for all sockets.

i/2

Print information to the erlang shell in table format for a selection of sockets.

info()

Get miscellaneous information about this `socket` library.

info(Socket)

Get miscellaneous info about a socket.

ioctl/2

Set socket (device) parameters.

ioctl/3

Get or set socket (device) parameters.

ioctl(Socket, SetRequest, Name, Value)

Set socket (device) parameters.

is_supported(Key1)

Check if a socket feature is supported.

is_supported(Key1, Key2)

Check if a socket feature is supported.

listen(Socket)

Make a socket listen for connections.

listen(Socket, Backlog)

Make a socket listen for connections.

monitor(Socket)

Start a socket monitor.

number_of()

Return the number of active sockets.

open(FD)

Equivalent to `open(FD, #{})`.

open/2

Create a socket.

open/3

Create a socket.

open(Domain, Type, Protocol, Opts)

Create a socket.

peername(Socket)

Return the remote address of a socket.

recv(Socket)

Equivalent to `recv(Socket, 0, [], infinity)`.

recv/2

Receive data on a connected socket.

recv/3

Receive data on a connected socket.

recv/4

Receive data on a connected socket.

recvfrom(Socket)

Equivalent to `recvfrom(Socket, 0, [], infinity)`.

recvfrom/2

Receive a message on a socket.

recvfrom/3

Receive a message on a socket.

recvfrom/4

Receive a message on a socket.

recvmsg(Socket)

Equivalent to `recvmsg(Socket, 0, 0, [], infinity)`.

recvmsg/2

Receive a message on a socket.

recvmsg/3

Receive a message on a socket.

recvmsg(Socket, BufSz, CtrlSz, TimeoutOrHandle)

Equivalent to `recvmsg(Socket, BufSz, CtrlSz, [], TimeoutOrHandle)`.

recvmsg/5

Receive a message on a socket.

send(Socket, Data)

Equivalent to `send(Socket, Data, [], infinity)`.

send/3

Send data on a connected socket.

send/4

Send data on a connected socket.

sendfile(Socket, FileHandle_Cont)

Send a file on a socket.

sendfile(Socket, FileHandle_Cont, Timeout_Handle)

Send a file on a socket.

sendfile(Socket, FileHandle_Cont, Offset, Count)

Send a file on a socket.

sendfile/5

Send a file on a socket.

sendmsg(Socket, Msg)

Equivalent to `sendmsg(Socket, Msg, [], infinity)`.

sendmsg/3

Send data and control messages on a socket.

sendmsg/4

Send data and control messages on a socket.

sendto/3

Send data on a socket.

sendto/4

Send data on a socket.

sendto/5

Send data on a socket.

sendv(Socket, IOV)

Equivalent to `sendv(Socket, IOV, infinity)`.

sendv/3

Send `erlang:iovec/0` data on a connected socket.

sendv/4

Send data on a connected socket, continuation.

setopt/3

Set a socket option.

setopt/4

Set a socket option (backwards compatibility function).

setopt_native(Socket, Option, Value)

Set a "native" socket option.

shutdown(Socket, How)

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

sockname(Socket)

Get the socket's address.

supports()

Retrieve information about what socket features the module and the platform supports.

supports(Key1)

Retrieve information about what socket features the module and the platform supports.

supports(Key1, Key2)

Retrieve information about what socket features the module and the platform supports.

use_registry(D)

Set the global `use_registry` option default value.

which_sockets()

Return a list of all known sockets.

which_sockets(FilterRule)

Return a filtered list of known sockets.

# Types

# cmsg()

(since OTP 22.0)

    -type cmsg() :: cmsg_recv() | cmsg_send().

Control messages (ancillary messages).

# cmsg_recv()

(since OTP 22.0)

    -type 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.

# cmsg_send()

(since OTP 22.0)

    -type 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.

# completion_handle()

(since OTP 26.0)

    -type completion_handle() :: reference().

Completion operation handle.

A `reference/0` that uniquely identifies the (completion) operation, contained in the returned `completion_info/0`.

# completion_info()

(since OTP 26.0)

    -type completion_info() ::
              {completion_info, CompletionTag :: completion_tag(), CompletionHandle :: completion_handle()}.

Completion operation info.

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`.

# completion_tag()

(since OTP 26.0)

    -type completion_tag() ::
              accept | connect | recv | recvfrom | recvmsg | send | sendv | sendto | sendmsg | sendfile.

Completion operation tag.

A tag that describes the ongoing (completion) operation (= function name), contained in the returned `completion_info/0`.

# domain()

(since OTP 22.0)

    -type domain() :: inet | inet6 | local | unspec.

Protocol domain a.k.a address family.

A lowercase `atom/0` representing a protocol domain on the platform named `AF_*` (or `PF_*`). For example `inet` corresponds to `AF_INET`.

`is_supported(ipv6)` tells if the IPv6 protocol, protocol domain `inet6`, is supported.

`is_supported(local)` tells if the protocol domain `local` is supported.

`supports/0` reports both values, but also many more, with a single call.

# ee_origin()

(since OTP 22.0)

    -type ee_origin() :: none | local | icmp | icmp6.

# eei()

(since OTP 22.0)

    -type eei() ::
              #{info := econnreset | econnaborted | netname_deleted | too_many_cmds | atom(),
                raw_info := term()}.

Extended Error Information.

A term containing additional (error) information if the socket NIF has been configured to produce it.

# extended_err()

(since OTP 22.0)

    -type 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()}.

# hatype()

(not exported) (since OTP 22.0)

    -type hatype() ::
              netrom | eether | ether | ax25 | pronet | chaos | ieee802 | arcnet | appletlk | dlci | atm |
              metricom | ieee1394 | eui64 | infiniband | tunnel | tunnel6 | loopback | localtlk | none |
              void |
              non_neg_integer().

# icmp_dest_unreach()

(since OTP 22.0)

    -type icmp_dest_unreach() ::
              net_unreach | host_unreach | port_unreach | frag_needed | net_unknown | host_unknown.

# icmpv6_dest_unreach()

(since OTP 22.0)

    -type icmpv6_dest_unreach() ::
              noroute | adm_prohibited | not_neighbour | addr_unreach | port_unreach | policy_fail |
              reject_route.

# in6_addr()

(since OTP 22.0)

    -type in6_addr() :: {0..65535, 0..65535, 0..65535, 0..65535, 0..65535, 0..65535, 0..65535, 0..65535}.

# in6_flow_info()

(not exported) (since OTP 22.0)

    -type in6_flow_info() :: 0..1048575.

# in6_scope_id()

(not exported) (since OTP 22.0)

    -type in6_scope_id() :: 0..4294967295.

# in_addr()

(since OTP 22.0)

    -type in_addr() :: {0..255, 0..255, 0..255, 0..255}.

# info()

(not exported) (since OTP 22.0)

    -type info() ::
              #{counters := #{atom() := non_neg_integer()},
                iov_max := non_neg_integer(),
                use_registry := boolean(),
                io_backend := #{name := atom()}}.

Platform dependent information items.

The value of `iov_max` is the value of the `IOV_MAX` constant in the system headers, which is the largest allowed I/O vector. See also `sendmsg/4` regarding the `iov` key of `msg_send/0`. The smallest allowed `IOV_MAX` value according to POSIX is `16`, but check your platform documentation to be sure.

About the `use_registry` key, see `use_registry/1` and the `otp_socket_option/0` with the same name.

# info_keys()

(since OTP 22.0)

    -type info_keys() ::
              [domain | type | protocol | fd | owner | local_address | remote_address | recv | sent | state].

Information element designators for the `i/1` and `i/2` functions.

# interface_type()

(since OTP 22.0)

    -type interface_type() ::
              other | hdh1822 | x25ddh | x25 | ether | ppp | loop | ipv4 | ipv6 | '6to4' | gif | faith |
              stf | bridge | cellular |
              non_neg_integer().

The interface type (of the datalink). We only translate a few values to atoms, the rest are left as (unsigned) integer values.

# invalid()

(since OTP 22.0)

    -type invalid() :: {invalid, What :: term()}.

# ioctl_device_flag()

(since OTP 22.0)

    -type ioctl_device_flag() ::
              up | broadcast | debug | loopback | pointopoint | notrailers | knowsepoch | running | noarp |
              promisc | allmulti | master | oactive | slave | simplex | link0 | link1 | link2 | multicast |
              portsel | automedia | cantconfig | ppromisc | dynamic | monitor | staticarp | dying |
              renaming | nogroup | lower_up | dormant | echo.

# ioctl_device_map()

(since OTP 22.0)

    -type ioctl_device_map() ::
              #{mem_start := non_neg_integer(),
                mem_end := non_neg_integer(),
                base_addr := non_neg_integer(),
                irq := non_neg_integer(),
                dma := non_neg_integer(),
                port := non_neg_integer()}.

# ip_mreq()

(since OTP 22.0)

    -type ip_mreq() :: #{multiaddr := in_addr(), interface := in_addr()}.

C: `struct ip_mreq`

Corresponds to the C `struct ip_mreq` for managing multicast groups.

# ip_mreq_source()

(since OTP 22.0)

    -type ip_mreq_source() :: #{multiaddr := in_addr(), interface := in_addr(), sourceaddr := in_addr()}.

C: `struct ip_mreq_source`

Corresponds to the C `struct ip_mreq_source` for managing multicast groups.

# ip_msfilter()

(since OTP 22.0)

    -type ip_msfilter() ::
              #{multiaddr := in_addr(),
                interface := in_addr(),
                mode := include | exclude,
                slist := [in_addr()]}.

C: `struct ip_msfilter`

Corresponds to the C `struct ip_msfilter` for managing multicast source filtering (RFC 3376).

# ip_pktinfo()

(since OTP 22.0)

    -type ip_pktinfo() :: #{ifindex := non_neg_integer(), spec_dst := in_addr(), addr := in_addr()}.

C: `struct ip_pktinfo`

# ip_pmtudisc()

(since OTP 22.0)

    -type ip_pmtudisc() :: want | dont | do | probe.

C: `IP_PMTUDISC_*` values.

Lowercase `atom/0` values corresponding to the C library constants `IP_PMTUDISC_*`. Some constant(s) may be unsupported by the platform.

# ip_tos()

(since OTP 22.0)

    -type ip_tos() :: lowdelay | throughput | reliability | mincost.

C: `IPTOS_*` values.

Lowercase `atom/0` values corresponding to the C library constants `IPTOS_*`. Some constant(s) may be unsupported by the platform.

# ipv6_hops()

(since OTP 22.0)

    -type ipv6_hops() :: default | 0..255.

IPv6 hop limit value.

The value `default` is only valid to set and is translated to the C value `-1`, meaning the route default.

# ipv6_mreq()

(since OTP 22.0)

    -type ipv6_mreq() :: #{multiaddr := in6_addr(), interface := non_neg_integer()}.

C: `struct ipv6_mreq`

Corresponds to the C `struct ipv6_mreq` for managing multicast groups. See also RFC 2553.

# ipv6_pktinfo()

(since OTP 22.0)

    -type ipv6_pktinfo() :: #{addr := in6_addr(), ifindex := integer()}.

C: `struct in6_pktinfo`

# ipv6_pmtudisc()

(since OTP 22.0)

    -type ipv6_pmtudisc() :: want | dont | do | probe.

C: `IPV6_PMTUDISC_*` values

Lowercase `atom/0` values corresponding to the C library constants `IPV6_PMTUDISC_*`. Some constant(s) may be unsupported by the platform.

# level()

(since OTP 22.0)

    -type level() :: socket | protocol().

Protocol level.

A lowercase `atom/0` OS protocol level, that is: `socket` or a `protocol/0` name.

`socket` is the `SOL_SOCKET` protocol level in the OS header files, with options named `SO_`* .

# linger()

(since OTP 22.0)

    -type linger() :: #{onoff := boolean(), linger := non_neg_integer()}.

C: `struct linger`

Corresponds to the C `struct linger` for managing the socket option `{socket, linger}`.

# msg()

(since OTP 22.0)

    -type msg() :: msg_send() | msg_recv().

C: `struct msghdr`

# msg_flag()

(since OTP 22.0)

    -type msg_flag() ::
              cmsg_cloexec | confirm | ctrunc | dontroute | eor | errqueue | more | oob | peek | trunc.

Platform dependent message flags.

Translates to/from message flag constants on the platform. These flags are lowercase while the constants are uppercase with prefix `MSG_`; for example `oob` translates to `MSG_OOB`.

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)`.

# msg_recv()

(since OTP 22.0)

    -type 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/0`, last in the containing list.

# msg_send()

(since OTP 22.0)

    -type 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`.

# native_value()

(not exported) (since OTP 22.0)

    -type native_value() :: integer() | boolean() | binary().

# otp_socket_option()

(since OTP 22.0)

    -type otp_socket_option() ::
              debug | iow | controlling_process | rcvbuf | rcvctrlbuf | sndctrlbuf | select_read | meta |
              use_registry | fd | domain.

Protocol level `otp` socket option.

Socket options for the `otp` pseudo protocol level, that is: `{otp, Name}` options.

This protocol level is the Erlang/OTP's socket implementation layer, hence above all OS protocol levels.

- `debug` - `boolean/0` - Activate debug logging.

- `iow` - `boolean/0` - Inform On Wrap of statistics counters.

- `controlling_process` - `pid/0` - 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.

  This is the allocation size for the receive buffer used when calling the OS protocol stack's receive API, when no specific size (size 0) is requested. When the receive function returns the receive buffer is reallocated to the actually received size. If the data is copied or shrunk in place is up to the allocator, and can to some extent be configured in the Erlang VM.

  The similar socket option; `{socket,rcvbuf}` is a related option for the OS' protocol stack that on Unix corresponds to `SOL_SOCKET,SO_RCVBUF`.

- `rcvctrlbuf` - `BufSize :: (default | integer()>0)`- Allocation size for the ancillary data buffer used when calling the OS protocol stack's receive API.

  The value `default` is only valid to set.

- `sndctrlbuf` - `BufSize :: (default | integer()>0)`- Allocation size for the ancillary data buffer used when calling the OS protocol stack's sendmsg API.

  The value `default` is only valid to set.

  It is the user's responsibility to set a buffer size that has room for the encoded ancillary data in the message to send.

  See sendmsg and also the `ctrl` field of the `msg_send/0` type.

- `select_read` - `boolean/0` - On `select` implementations, see Asynchronous Calls, automatically activate select after a completed read.

  Instead of `{ok, Data}` the receive operation returns `{select_read, {SelectInfo, Data}}`, and the calling process can wait for a `select` message containing `SelectInfo` when there is data available again.

  Setting this option locks out other processes from receiving any data since the current process continues its operation, so it effectively disables receive operation queuing.

- `fd` - `integer/0` - 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/0` - 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.

# packet_type()

(not exported) (since OTP 22.0)

    -type packet_type() ::
              host | broadcast | multicast | otherhost | outgoing | loopback | user | kernel | fastroute |
              non_neg_integer().

# port_number()

(since OTP 22.0)

    -type port_number() :: 0..65535.

# posix()

(not exported) (since OTP 22.0)

    -type posix() :: inet:posix().

Posix error codes.

Local alias for `inet:posix/0`, a set of `atom/0`s.

# protocol()

(since OTP 22.0)

    -type protocol() :: atom().

Protocol name.

A lowercase `atom/0` representing an OS protocol name. To be used for example in `socket_option/0` in control messages.

They have the following names in the OS header files:

- `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, through the `C` library call `getprotoent()`. See the OS man page for protocols(5). Those in the list above are valid if supported by the platform, even if they aren't enumerated.

The calls `is_supported(ipv6)` and `is_supported(sctp)` can be used to find out if the protocols `ipv6` and `sctp` are supported on the platform as in appropriate header file and library exists.

The call `is_supported(protocols, Protocol)` can only be used to find out if the platform knows the protocol number for a named `Protocol`.

See `open/2,3,4`

# sctp_assocparams()

(since OTP 22.0)

    -type 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}.

C: `struct sctp_assocparams`

# sctp_event_subscribe()

(since OTP 22.0)

    -type 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()}.

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()

(since OTP 22.0)

    -type sctp_initmsg() ::
              #{num_ostreams := 0..65535,
                max_instreams := 0..65535,
                max_attempts := 0..65535,
                max_init_timeo := 0..65535}.

C: `struct sctp_initmsg`.

# sctp_rtoinfo()

(since OTP 22.0)

    -type sctp_rtoinfo() ::
              #{assoc_id := integer(), initial := 0..4294967295, max := 0..4294967295, min := 0..4294967295}.

C: `struct sctp_rtoinfo`.

# select_handle()

(since OTP 22.0)

    -type select_handle() :: reference().

Select operation handle.

A `reference/0` that uniquely identifies the (select) operation, contained in the returned `select_info/0`.

# select_info()

(since OTP 22.0)

    -type select_info() :: {select_info, SelectTag :: select_tag(), SelectHandle :: select_handle()}.

Select operation info.

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

On `select` systems, if the option `{otp, select_read}` is set, `{select_read, {select_info(), _}}` is returned instead of `{ok, _}` to indicate that a new asynchronous receive operation has been initiated and the caller should wait for a `select` message containing the `SelectHandle`.

# select_tag()

(since OTP 22.0)

    -type select_tag() ::
              accept | connect | recv | recvfrom | recvmsg | send | sendv | sendto | sendmsg | sendfile |
              {recv | recvfrom | recvmsg | send | sendv | sendto | sendmsg | sendfile, ContData :: term()}.

Select operation tag.

A tag that describes the (select) operation (= function name), contained in the returned `select_info/0`.

# sockaddr()

(since OTP 22.0)

    -type sockaddr() ::
              sockaddr_in() |
              sockaddr_in6() |
              sockaddr_un() |
              sockaddr_ll() |
              sockaddr_dl() |
              sockaddr_unspec() |
              sockaddr_native().

# sockaddr_dl()

(since OTP 22.0)

    -type sockaddr_dl() ::
              #{family := link,
                index := non_neg_integer(),
                type := interface_type(),
                nlen := non_neg_integer(),
                alen := non_neg_integer(),
                slen := non_neg_integer(),
                data := binary()}.

C: `struct sockaddr_dl`

Link level address (PF_LINK) on BSD:s.

# sockaddr_in6()

(since OTP 22.0)

    -type sockaddr_in6() ::
              #{family := inet6,
                port := port_number(),
                addr := any | loopback | in6_addr(),
                flowinfo := in6_flow_info(),
                scope_id := in6_scope_id()}.

C: `struct sockaddr_in6`

Domain `inet6` (IPv6) address.

# sockaddr_in()

(since OTP 22.0)

    -type sockaddr_in() ::
              #{family := inet, port := port_number(), addr := any | broadcast | loopback | in_addr()}.

C: `struct sockaddr_in`

Domain `inet` (IPv4) address.

# sockaddr_ll()

(since OTP 22.0)

    -type sockaddr_ll() ::
              #{family := packet,
                protocol := non_neg_integer(),
                ifindex := integer(),
                pkttype := packet_type(),
                hatype := hatype(),
                addr := binary()}.

C: `struct sockaddr_ll`

Domain `packet`, type `raw` (link level) address.

# sockaddr_native()

(since OTP 22.0)

    -type sockaddr_native() :: #{family := integer(), addr := binary()}.

C: `struct sockaddr`

In C, a `struct sockaddr` with the integer value of `sa_family` in the `map/0` key `family`, and the content of `sa_data` in the `map/0` key `addr`.

# sockaddr_recv()

(since OTP 22.0)

    -type sockaddr_recv() :: sockaddr() | binary().

# sockaddr_un()

(since OTP 22.0)

    -type sockaddr_un() :: #{family := local, path := binary() | string()}.

C: `struct sockaddr_un`.

A Unix Domain socket address, a.k.a local address (`AF_LOCAL`).

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/0`, 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_unspec()

(since OTP 22.0)

    -type sockaddr_unspec() :: #{family := unspec, addr := binary()}.

C: `struct sockaddr` of `AF_UNSPEC`

In C, a `struct sockaddr` with `sa_family = AF_UNSPEC` and the content of `sa_data` in the `map/0` key `addr`.

# socket()

(since OTP 22.0)

    -type socket() :: {'$socket', socket_handle()}.

A socket, according to this module.

Created and returned by `open/1,2,3,4` and `accept/1,2`.

# socket_counters()

(since OTP 22.0)

    -type socket_counters() ::
              #{read_byte := non_neg_integer(),
                read_fails := non_neg_integer(),
                read_pkg := non_neg_integer(),
                read_pkg_max := non_neg_integer(),
                read_tries := non_neg_integer(),
                read_waits := non_neg_integer(),
                write_byte := non_neg_integer(),
                write_fails := non_neg_integer(),
                write_pkg := non_neg_integer(),
                write_pkg_max := non_neg_integer(),
                write_tries := non_neg_integer(),
                write_waits := non_neg_integer(),
                sendfile => non_neg_integer(),
                sendfile_byte => non_neg_integer(),
                sendfile_fails => non_neg_integer(),
                sendfile_max => non_neg_integer(),
                sendfile_pkg => non_neg_integer(),
                sendfile_pkg_max => non_neg_integer(),
                sendfile_tries => non_neg_integer(),
                sendfile_waits => non_neg_integer(),
                acc_success := non_neg_integer(),
                acc_fails := non_neg_integer(),
                acc_tries := non_neg_integer(),
                acc_waits := non_neg_integer()}.

A `map/0` of `Name := Counter` associations.

# socket_handle()

(since OTP 22.0)

    -opaque socket_handle()

Opaque socket handle unique for the socket.

# socket_info()

(since OTP 22.0)

    -type socket_info() ::
              #{domain := domain() | integer(),
                type := type() | integer(),
                protocol := protocol() | integer(),
                owner := pid(),
                ctype := normal | fromfd | {fromfd, integer()},
                counters := socket_counters(),
                num_readers := non_neg_integer(),
                num_writers := non_neg_integer(),
                num_acceptors := non_neg_integer(),
                writable := boolean(),
                readable := boolean(),
                rstates := [atom()],
                wstates := [atom()]}.

# socket_option()

(since OTP 22.0)

    -type socket_option() ::
              {Level :: socket,
               Opt ::
                   acceptconn | acceptfilter | bindtodevice | broadcast | bsp_state | busy_poll | debug |
                   domain | dontroute | error | exclusiveaddruse | keepalive | linger | mark | maxdg |
                   max_msg_size | 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.

Socket options of the form `{Level, Opt}` where the OS protocol `Level` = `level/0` 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/0`.

#### 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/0` 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/0` 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/0`

  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/0`

  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/0`

  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/0`

  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 non-blocking 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/0`

  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 non-blocking 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/0`

  Only valid to get.

  The socket's type.

Options for protocol level `ip`:

- `{ip, add_membership}` - `Value =` `ip_mreq/0`

  Only valid to set.

- `{ip, add_source_membership}` - `Value =` `ip_mreq_source/0`

  Only valid to set.

- `{ip, block_source}` - `Value =` `ip_mreq_source/0`

  Only valid to set.

- `{ip, drop_membership}` - `Value =` `ip_mreq/0`

  Only valid to set.

- `{ip, drop_source_membership}` - `Value =` `ip_mreq_source/0`

  Only valid to set.

- `{ip, freebind}` - `Value = boolean()`

- `{ip, hdrincl}` - `Value = boolean()`

- `{ip, minttl}` - `Value = integer()`

- `{ip, msfilter}` - `Value =` `null |` `ip_msfilter/0`

  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/0` value according to the platform's header files.

- `{ip, multicast_all}` - `Value = boolean()`

- `{ip, multicast_if}` - `Value =` `any |` `in_addr/0`

- `{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()`

  Enable extended reliable error message passing.

  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/0` value is according to the platform's header files.

- `{ip, transparent}` - `Value = boolean()`

- `{ip, ttl}` - `Value = integer()`

- `{ip, unblock_source}` - `Value =` `ip_mreq_source/0`

  Only valid to set.

Options for protocol level `ipv6`:

- `{ipv6, addrform}` - `Value =` `domain/0`

  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/0`

  Only valid to set.

- `{ipv6, authhdr}` - `Value = boolean()`

- `{ipv6, drop_membership}` - `Value =` `ipv6_mreq/0`

  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/0` value is according to the platform's header files.

- `{ipv6, multicast_hops}` - `Value =` `ipv6_hops/0`

- `{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/0`

- `{ipv6, v6only}` - `Value = boolean()`

Options for protocol level `sctp`. See also RFC 6458.

- `{sctp, associnfo}` - `Value =` `sctp_assocparams/0`

- `{sctp, autoclose}` - `Value = integer()`

- `{sctp, disable_fragments}` - `Value = boolean()`

- `{sctp, events}` - `Value =` `sctp_event_subscribe/0`

  Only valid to set.

- `{sctp, initmsg}` - `Value =` `sctp_initmsg/0`

- `{sctp, maxseg}` - `Value = integer()`

- `{sctp, nodelay}` - `Value = boolean()`

- `{sctp, rtoinfo}` - `Value =` `sctp_rtoinfo/0`

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()`

# timeval()

(since OTP 22.0)

    -type timeval() :: #{sec := integer(), usec := integer()}.

C: `struct timeval`

Corresponds to the C `struct timeval`. The field `sec` holds seconds, and `usec` microseconds.

# type()

(since OTP 22.0)

    -type type() :: stream | dgram | raw | rdm | seqpacket.

Protocol type.

A lowercase `atom/0` representing a protocol type on the platform named `SOCK_*`. For example `stream` corresponds to `SOCK_STREAM`.

# Functions

# accept(ListenSocket)

(since OTP 22.0)

    -spec accept(ListenSocket) -> Result
                    when
                        Result :: {ok, Socket} | {error, Reason},
                        ListenSocket :: socket(),
                        Socket :: socket(),
                        Reason :: dynamic().

Equivalent to `accept(ListenSocket, infinity)`.

# accept/2

(since OTP 22.0)

    -spec accept(ListenSocket, Timeout :: infinity) -> {ok, Socket} | {error, Reason}
                    when
                        ListenSocket :: socket(),
                        Socket :: socket(),
                        Reason ::
                            posix() |
                            closed |
                            invalid() |
                            {create_accept_socket, posix()} |
                            {add_socket, posix()} |
                            {update_accept_context, posix()};
                (ListenSocket, Timeout :: non_neg_integer()) -> {ok, Socket} | {error, Reason}
                    when
                        ListenSocket :: socket(),
                        Socket :: socket(),
                        Reason ::
                            posix() |
                            closed |
                            invalid() |
                            timeout |
                            {create_accept_socket, posix()} |
                            {add_socket, posix()} |
                            {update_accept_context, posix()};
                (ListenSocket, nowait | (Handle :: select_handle() | completion_handle())) ->
                    {ok, Socket} | {select, SelectInfo} | {completion, CompletionInfo} | {error, Reason}
                    when
                        ListenSocket :: socket(),
                        Socket :: socket(),
                        SelectInfo :: select_info(),
                        CompletionInfo :: completion_info(),
                        Reason ::
                            posix() |
                            closed |
                            invalid() |
                            {create_accept_socket, posix()} |
                            {add_accept_socket, posix()} |
                            {update_accept_context, posix()}.

Accept a connection on a listening socket.

`ListenSocket` has to be of a connection oriented type (types `stream` or `seqpacket`, see `open/1`), and set to listen (see `listen/1`).

If the `Timeout` argument is `infinity`; accepts the first pending incoming connection for the listen socket or wait for one to arrive, and return the new connection socket.

If the `Timeout` argument is a time-out value (`non_neg_integer/0`); returns `{error, timeout}` if no connection has arrived after `Timeout` milliseconds.

If the `Handle` argument `nowait` (since OTP 22.1), starts an asynchronous call if the operation couldn't be completed immediately.

If the `Handle` argument is a `select_handle/0`, (since OTP 24.0), or on Windows, the equivalent `completion_handle/0` (since OTP 26.0), starts an asynchronous call like for `nowait`.

See the note Asynchronous Calls at the start of this module reference manual page.

# bind(Socket, Addr)

(since OTP 22.0)

    -spec bind(Socket, Addr) -> ok | {error, Reason}
                  when
                      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/2

(since OTP 22.1)

    -spec cancel(Socket, SelectInfo | CompletionInfo) -> ok | {error, Reason}
                    when
                        Socket :: socket(),
                        SelectInfo :: select_info(),
                        CompletionInfo :: completion_info(),
                        Reason :: closed | invalid().

Cancel an asynchronous call in progress.

Call this function to cancel an asynchronous call in progress, that is; it returned a value containing a `completion_info/0` or `select_info/0`.

See the note Asynchronous Calls at the start of this module reference manual page.

If another process tries an operation of the same basic type (`accept/1` | `send/2` | `recv/2`) it will be enqueued and notified through a `select` or `completion` message when the current operation and all enqueued before it has been completed. If the current operation is canceled by this function it is treated as a completed operation; the process first in queue is notified.

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

# cancel_monitor(MRef)

(since OTP 24.0)

    -spec cancel_monitor(MRef :: reference()) -> boolean().

Cancel a socket monitor.

If `MRef` is a reference that the calling process obtained by calling `monitor/1`, this monitor is removed. If there is no such monitor for the calling process (or MRef doesn't correspond to a monitor), 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 so it couldn't be removed. This might be because the monitor has already triggered and there is a `'DOWN'` message from this monitor in the caller message queue.

# close(Socket)

(since OTP 22.0)

    -spec close(Socket) -> ok | {error, Reason}
                   when Socket :: socket(), Reason :: posix() | closed | timeout.

Close a socket.

#### Note

Note that for `Protocol = tcp` (see `open/3`), although TCP guarantees that when the other side sees the stream close all data that we sent before closing has been delivered, there is no way for us to know that the other side got all data and the stream close. All kinds of network and OS issues may obliterate that.

To get such a guarantee we need to implement an in-band acknowledge protocol on the connection, or we can use the `shutdown` function to signal that no more data will be sent and then wait for the other end to close the socket. Then we will see our read side getting a socket close. In this way we implement a small acknowledge protocol using `shutdown/2`. The other side cannot know that we ever saw the socket close, but in a client/server scenario that is often not relevant.

# connect(Socket)

(since OTP 24.0)

    -spec connect(Socket :: socket()) -> ok | {error, Reason} when Reason :: posix() | closed | invalid().

Finalize a `connect/3` operation.

See the note Asynchronous Calls at the start of this module reference manual page.

On `select` systems this function finalizes a connection setup on a socket, after receiving a `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` again, but that incurs more overhead since the connect address and time-out argument are processed in vain.

The call that completes the connect operation, the second call, cannot return a `select` return value.

# connect(Socket, SockAddr)

(since OTP 22.0)

    -spec connect(Socket :: socket(), SockAddr :: sockaddr()) -> ok | {error, Reason :: dynamic()}.

Equivalent to `connect(Socket, SockAddr, infinity)`.

# connect/3

(since OTP 22.0)

    -spec connect(Socket, SockAddr, Timeout :: infinity) -> ok | {error, Reason}
                     when
                         Socket :: socket(),
                         SockAddr :: sockaddr(),
                         Reason ::
                             posix() |
                             closed |
                             invalid() |
                             already | not_bound |
                             {add_socket, posix()} |
                             {update_connect_context, posix()};
                 (Socket, SockAddr, Timeout :: non_neg_integer()) -> ok | {error, Reason}
                     when
                         Socket :: socket(),
                         SockAddr :: sockaddr(),
                         Reason ::
                             posix() |
                             closed |
                             invalid() |
                             already | not_bound | timeout |
                             {add_socket, posix()} |
                             {update_connect_context, posix()};
                 (Socket, SockAddr, nowait | Handle) ->
                     ok | {select, SelectInfo} | {completion, CompletionInfo} | {error, Reason}
                     when
                         Socket :: socket(),
                         SockAddr :: sockaddr(),
                         Handle :: select_handle() | completion_handle(),
                         SelectInfo :: select_info(),
                         CompletionInfo :: completion_info(),
                         Reason ::
                             posix() |
                             closed |
                             invalid() |
                             already | not_bound |
                             {add_socket, posix()} |
                             {update_connect_context, posix()}.

Connect the socket to the given address.

This function connects the socket to the address specified by the `SockAddr` argument.

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

#### Note

On Windows the socket has to be bound.

If the time-out argument (argument 3) is `infinity` it is up to the OS implementation to decide when the connection attempt failed and then what to return; probably `{error, etimedout}`. The OS time-out may be very long.

If the time-out argument (argument 3) is a time-out value (`non_neg_integer/0`); return `{error, timeout}` if the connection hasn't been established within `Timeout` milliseconds.

#### Note

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 is to close the socket and start over.

Also note that this applies to cancelling a `nowait` connect call described below.

If the time-out argument (argument 2) is `nowait` (since OTP 22.1), start an asynchronous call if the operation couldn't be completed immediately.

If the time-out argument (argument 2) is a `Handle ::` `select_handle/0`, (since OTP 24.0), or on Windows, the equivalent `Handle ::` `completion_handle/0` (since OTP 26.0), start an asynchronous call like for `nowait`.

See the note Asynchronous Calls at the start of this module reference manual page.

After receiving a `select` message; call `connect/1` to complete the operation.

If cancelling the operation with `cancel/2` see the note above about connection time-out.

# getopt/2

(since OTP 24.0)

    -spec getopt(socket(), SocketOption :: {Level :: otp, Opt :: otp_socket_option()}) ->
                    {ok, Value :: term()} | {error, invalid() | closed};
                (socket(), SocketOption :: socket_option()) ->
                    {ok, Value :: term()} | {error, posix() | invalid() | closed}.

Get the value of a socket option.

Gets the value of an OS protocol level socket option, or from the `otp` pseudo protocol level, which is this module's implementation level above the OS protocol levels.

See the type otp_socket_option() for a description of the `otp` protocol level.

See the type `socket_option/0` for which OS protocol level options that this implementation knows about, how they are related to OS option names, and if there are known peculiarities with any of them.

What options that are valid depends on the OS, and on the kind of socket (`domain/0`,`type/0` and `protocol/0`). See the type `t:socket_option()` and the socket options chapter in the User's Guide for more info.

#### Note

Not all options are valid, nor possible to get, on all platforms. That is, even if this `socket` implementation support an option; it doesn't mean that the underlying OS does.

# getopt(Socket, Level, Opt)

(since OTP 22.0)

    -spec getopt(Socket :: term(), Level :: term(), Opt :: term()) -> _.

Get a socket option (backwards compatibility function).

Equivalent to `getopt(Socket, {Level, Opt})`, or as a special case if `Opt = {NativeOpt ::``integer/0``, ValueSpec}` equivalent to `getopt_native(Socket, {Level, NativeOpt}, ValueSpec)`.

Use `getopt/2` or `getopt_native/3` instead to handle the option level and name as a single term, and to make the difference between known options and native options clear.

# getopt_native/3

(since OTP 24.0)

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

Get a "native" socket option.

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/0` that will contain the fetched option value, or by specifying a `binary/0` `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/0`.

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/0` according to the `C` implementation's notion about true and false.

If an option is valid depends both on the platform and on what kind of socket it is (`domain/0`, `type/0` and `protocol/0`).

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()

(since OTP 24.1)

    -spec i() -> ok.

Print information to the erlang shell in table format for all sockets.

The information printed for each socket is specified by the default set of `info_keys/0` (all keys).

The sockets that are printed are all sockets created by this `socket` module's implementation.

# i/1

(since OTP 24.1)

    -spec i(InfoKeys :: info_keys()) -> ok;
           (Domain :: inet | inet6 | local) -> ok;
           (Proto :: sctp | tcp | udp) -> ok;
           (Type :: dgram | seqpacket | stream) -> ok.

Print information to the erlang shell in table format for all sockets.

If the argument is a list of `info_keys/0`, print the specified information for all sockets. See `i/0`.

Otherwise the same as `i/2` with the same first argument and the default information (see `i/0`).

# i/2

(since OTP 24.1)

    -spec i(Domain :: inet | inet6 | local, InfoKeys) -> ok when InfoKeys :: info_keys();
           (Proto :: sctp | tcp | udp, InfoKeys) -> ok when InfoKeys :: info_keys();
           (Type :: dgram | seqpacket | stream, InfoKeys) -> ok when InfoKeys :: info_keys().

Print information to the erlang shell in table format for a selection of sockets.

The argument `InfoKeys` specifies which information is printed for each socket.

If the first argument is `Domain` print information for all sockets of that specific `domain/0`.

If the first argument is `Proto` print information for all sockets of that specific `protocol/0`.

If the first argument is `Type` print information for all sockets of that specific `type/0`.

# info()

(since OTP 24.0)

    -spec info() -> info().

Get miscellaneous information about this `socket` library.

The function returns a map with each information item as a key-value pair.

#### Note

In order to ensure data integrity, mutexes are taken when needed. So, don't call this function often.

# info(Socket)

(since OTP 22.1)

    -spec info(Socket) -> socket_info() when Socket :: socket().

Get miscellaneous info about a socket.

The function returns a map with each information item as a key-value pair reflecting the "current" state of the socket.

#### Note

In order to ensure data integrity, mutexes are taken when needed. So, don't call this function often.

# ioctl/2

(since OTP 24.2)

    -spec ioctl(Socket, GetRequest :: gifconf) ->
                   {ok, IFConf :: [#{name := string, addr := sockaddr()}]} | {error, Reason}
                   when Socket :: socket(), Reason :: posix() | closed;
               (Socket, GetRequest :: nread | nwrite | nspace) ->
                   {ok, NumBytes :: non_neg_integer()} | {error, Reason}
                   when Socket :: socket(), Reason :: posix() | closed;
               (Socket, GetRequest :: atmark) -> {ok, Available :: boolean()} | {error, Reason}
                   when Socket :: socket(), Reason :: posix() | closed;
               (Socket, GetRequest :: tcp_info) -> {ok, Info :: map()} | {error, Reason}
                   when Socket :: socket(), Reason :: posix() | closed.

Set socket (device) parameters.

This function retrieves a specific parameter, according to the `GetRequest` argument.

- `gifconf` - Get a list of interface (transport layer) addresses.

  Result; a list of `map/0`s, one for each interface, with its name and address.

- `nread` - Get the number of bytes immediately available for reading (since OTP 26.1).

  Result; the number of bytes, `integer/0`.

- `nwrite` - Get the number of bytes in the send queue (since OTP 26.1).

  Result; the number of bytes, `integer/0`.

- `nspace` - Get the free space in the send queue (since OTP 26.1).

  Result; the number of bytes, `integer/0`.

- `atmark` - Test if there is OOB (out-of-bound) data waiting to be read (since OTP 26.1).

  Result; a `boolean/0`.

- `tcp_info` - Get miscellaneous TCP related information for a connected socket (since OTP 26.1).

  Result; a `map/0` with information items as key-value pairs.

#### Note

Not all requests are supported by all platforms. To see if a ioctl request is supported on the current platform:

          Request = nread,
          true = socket:is_supported(ioctl_requests, Request),
          :

# ioctl/3

(since OTP 24.2)

    -spec ioctl(Socket, GetRequest, NameOrIndex) -> {ok, Result} | {error, Reason}
                   when
                       Socket :: socket(),
                       GetRequest ::
                           gifname | gifindex | gifaddr | gifdstaddr | gifbrdaddr | gifnetmask | gifhwaddr |
                           genaddr | gifmtu | giftxqlen | gifflags | tcp_info,
                       NameOrIndex :: string() | integer(),
                       Result :: dynamic(),
                       Reason :: posix() | closed;
               (Socket, SetRequest, Value) -> ok | {error, Reason}
                   when
                       Socket :: socket(),
                       SetRequest :: rcvall,
                       Value :: off | on | iplevel,
                       Reason :: posix() | closed;
               (Socket, SetRequest, Value) -> ok | {error, Reason}
                   when
                       Socket :: socket(),
                       SetRequest :: rcvall_igmpmcast | rcvall_mcast,
                       Value :: off | on,
                       Reason :: posix() | closed.

Get or set socket (device) parameters.

This function retrieves a specific parameter, according to one of the following `GetRequest` arguments. The third argument is the (lookup) "key", identifying the interface, for most requests the name of the interface as a `string/0`. Also, see the note above.

- `gifname` - Get the name of the interface with the specified index (`integer/0`).

  Result; the name of the interface, `string/0`.

- `gifindex` - Get the index of the interface with the specified name.

  Result; the interface index, `integer/0`.

- `gifaddr` - Get the address of the interface with the specified name.

  Result; the address of the interface, `sockaddr/0`.

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

  Result; the destination address of the interface, `sockaddr/0`.

- `gifbrdaddr` - Get the broadcast address of the interface with the specified name.

  Result; broadcast address of the interface, `sockaddr/0`.

- `gifnetmask` - Get the network mask of the interface with the specified name.

  Result; the network mask of the interface, `sockaddr/0`.

- `gifhwaddr` | `genaddr` - Get the hardware address for the interface with the specified name.

  Result; the hardware address of the interface, `sockaddr/0` | `binary/0`. 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, `integer/0`.

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

  Result; transmit queue length of the interface, `integer/0`.

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

  Result; the active flag word of the interface, is a list of `ioctl_device_flag/0` `|` `t:integer( )`.

With the following `SetRequest` argument this function sets the `Value` for the request parameter (since OTP 26.1).

- `rcvall` - Enables (or disables) a socket to receive all IPv4 or IPv6 packages passing through a network interface.

  The `Socket` has to be one of:

  - An IPv4 socket - Created with the address domain `inet`, socket type `raw` and protocol `ip`.

  - An IPv6 socket - Created with the address domain `inet6`, socket type `raw` and protocol `ipv6`.

  The socket must also be bound to an (explicit) local IPv4 or IPv6 interface (`any` isn't allowed).

  Setting this IOCTL requires elevated privileges.

With the following `SetRequest` arguments this function sets the `Value` for the request parameter (since OTP 26.1).

- `rcvall_igmpmcall` - Enables (or disables) a socket to receive IGMP multicast IP traffic, without receiving any other IP traffic.

  The socket has to be created with the address domain `inet`, socket type `raw` and protocol `igmp`.

  The socket must also be bound to an (explicit) local interface (`any` isn't allowed).

  The receive buffer must be sufficiently large.

  Setting this IOCTL requires elevated privileges.

- `rcvall_mcall` - Enables (or disables) a socket to receive all multicast IP traffic (as in; all IP packets destined for IP addresses in the range 224.0.0.0 to 239.255.255.255).

  The socket has to be created with the address domain `inet`, socket type `raw` and protocol `udp`.

  The socket must also be bound to an (explicit) local interface (`any` isn't allowed), And bound to port `0`.

  The receive buffer must be sufficiently large.

  Setting this IOCTL requires elevated privileges.

# ioctl(Socket, SetRequest, Name, Value)

(since OTP 24.2)

    -spec ioctl(Socket, SetRequest, Name, Value) -> ok | {error, Reason}
                   when
                       Socket :: socket(),
                       SetRequest ::
                           sifflags | sifaddr | sifdstaddr | sifbrdaddr | sifnetmask | sifhwaddr | sifmtu |
                           siftxqlen,
                       Name :: string(),
                       Value :: dynamic(),
                       Reason :: posix() | closed.

Set socket (device) parameters.

This function sets a specific parameter, according to the `SetRequest` argument. The `Name` argument is the name of the interface, and the `Value` argument is the value to set.

These operations require elevated privileges.

- `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/0`, with the value `true` if the `Flag` should be set and `false` if the flag should be cleared.

- `sifaddr` - Set the address, `sockaddr/0`, of the interface with the specified name.

- `sifdstaddr` - Set the destination address, `sockaddr/0`, of a point-to-point interface with the specified name.

- `sifbrdaddr` - Set the broadcast address, `sockaddr/0`, of the interface with the specified name.

- `sifnetmask` - Set the network mask, `sockaddr/0`, of the interface with the specified name.

- `sifhwaddr` - Set the hardware address, `sockaddr/0`, of the interface with the specified name.

- `sifmtu` - Set the MTU (Maximum Transfer Unit), `integer/0`, for the interface with the specified name.

- `siftxqlen` - Set the transmit queue length, `integer/0`, of the interface with the specified name.

# is_supported(Key1)

(since OTP 23.0)

    -spec is_supported(Key1 :: term()) -> boolean().

Check if a socket feature is supported.

Returns `true` if `supports/0` has a `{Key1, true}` tuple or a `{Key1, list()}` tuple in its returned list, otherwise `false` (also for unknown keys).

Example:

    true = socket:is_supported(local),

# is_supported(Key1, Key2)

(since OTP 23.0)

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

Check if a socket feature is supported.

Returns `true` if `supports(Key1)` has a `{Key2, true}` tuple in its returned list, otherwise `false` (also for unknown keys).

Example:

    true = socket:is_supported(msg_flags, errqueue),

# listen(Socket)

(since OTP 22.0)

    -spec listen(Socket :: socket()) -> ok | {error, Reason :: posix() | closed}.

Make a socket listen for connections.

Equivalent to `listen(Socket, Backlog)` with a default value for `Backlog` (currently `5`).

# listen(Socket, Backlog)

(since OTP 22.0)

    -spec listen(Socket :: socket(), Backlog :: integer()) -> ok | {error, Reason :: posix() | closed}.

Make a socket listen for connections.

The `Backlog` argument states the length of the queue for incoming not yet accepted connections. Exactly how that number is interpreted is up to the OS' protocol stack, but the resulting effective queue length will most probably be perceived as at least that long.

#### Note

On Windows the socket has to be bound.

# monitor(Socket)

(since OTP 24.0)

    -spec monitor(Socket :: socket()) -> MonitorRef :: reference().

Start a socket monitor.

If the `Socket` doesn't exist or when later the monitor is triggered, a `'DOWN'` message is sent to the process that called `monitor/1` with the following pattern:

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

`Info` is the termination reason of the socket or `nosock` if `Socket` did not exist when the monitor was started.

Making several calls to `socket:monitor/1` for the same `Socket` is not an error; each call creates an independent monitor instance.

# number_of()

(since OTP 22.3)

    -spec number_of() -> non_neg_integer().

Return the number of active sockets.

# open(FD)

(since OTP 23.0)

    -spec open(FD :: integer()) -> dynamic().

Equivalent to `open(FD, #{})`.

# open/2

(since OTP 22.0)

    -spec open(FD, Opts) -> {ok, Socket} | {error, Reason}
                  when
                      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;
              (Domain, Type) -> {ok, Socket} | {error, Reason}
                  when
                      Domain :: domain(),
                      Type :: type() | integer(),
                      Socket :: socket(),
                      Reason :: posix() | protocol.

Create a socket.

#### With arguments `Domain` and `Type`

Equivalent to `open(Domain, Type, default, #{})`.

#### With arguments `FD` and `Opts` (since OTP 23.0)

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

The `Opts` argument can provide extra information:

- `domain` - The file descriptor's communication domain. See also

  `open/2,3,4`.

- `type` - The file descriptor's socket type.

  See also `open/2,3,4`.

- `protocol` - The file descriptor's protocol. 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` - If `false` don't duplicate the provided file descriptor.

  Defaults to `true`; do duplicate the file descriptor.

- `debug` - If `true` enable socket debug logging.

  Defaults to `false`; don't enable socket debug logging.

- `use_registry` - Enable or disable use of the socket registry for this socket. This overrides the global setting.

  Defaults to the global setting, 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.

On some platforms it is not easy to get hold of a file descriptor to use in this function.

# open/3

(since OTP 22.0)

    -spec open(Domain, Type, Opts | Protocol) -> {ok, Socket} | {error, Reason}
                  when
                      Domain :: domain() | integer(),
                      Type :: type() | integer(),
                      Opts :: map(),
                      Protocol :: default | protocol() | integer(),
                      Socket :: socket(),
                      Reason :: posix() | protocol.

Create a socket.

#### With arguments `Domain`, `Type` and `Protocol`

Equivalent to `open(Domain, Type, Protocol, #{})`.

#### With arguments `Domain`, `Type` and `Opts` (since OTP 24.0)

Equivalent to `open(Domain, Type, default, #{})`.

# open(Domain, Type, Protocol, Opts)

(since OTP 22.0)

    -spec open(Domain, Type, Protocol, Opts) -> {ok, Socket} | {error, Reason}
                  when
                      Domain :: domain() | integer(),
                      Type :: type() | integer(),
                      Protocol :: default | protocol() | integer(),
                      Opts :: #{netns => string(), debug => boolean(), use_registry => boolean()},
                      Socket :: socket(),
                      Reason :: posix() | protocol.

Create a socket.

Creates an endpoint for communication (socket).

`Domain` and `Type` may be `integer/0`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, or a specific protocol.

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 Linux.

- `debug: boolean()` - Enable or disable debug logging.

  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)

(since OTP 22.0)

    -spec peername(Socket :: socket()) -> {ok, SockAddr} | {error, Reason}
                      when SockAddr :: sockaddr_recv(), Reason :: posix() | closed.

Return the remote address of a socket.

Returns the address of the connected peer, that is, the remote end of the socket.

# recv(Socket)

(since OTP 22.0)

    -spec recv(Socket :: socket()) -> dynamic().

Equivalent to `recv(Socket, 0, [], infinity)`.

# recv/2

(since OTP 22.0)

    -spec recv(Socket :: socket(), Flags :: [msg_flag() | integer()]) -> dynamic();
              (Socket :: socket(), Length :: non_neg_integer()) -> dynamic().

Receive data on a connected socket.

With argument `Length`; equivalent to `recv(Socket, Length, [], infinity)`.

With argument `Flags`; equivalent to `recv(Socket, 0, Flags, infinity)` (since OTP 24.0).

# recv/3

(since OTP 22.0)

    -spec recv(Socket, Flags, TimeoutOrHandle) -> dynamic()
                  when
                      Socket :: socket(),
                      Flags :: [msg_flag() | integer()],
                      TimeoutOrHandle :: timeout() | nowait | select_handle() | completion_handle();
              (Socket :: socket(), Length :: non_neg_integer(), Flags :: [msg_flag() | integer()]) ->
                  dynamic();
              (Socket :: socket(), Length :: non_neg_integer(), TimeoutOrHandle) -> dynamic()
                  when TimeoutOrHandle :: timeout() | nowait | select_handle() | completion_handle().

Receive data on a connected socket.

With arguments `Length` and `Flags`; equivalent to `recv(Socket, Length, Flags, infinity)`.

With arguments `Length` and `TimeoutOrHandle`; equivalent to `recv(Socket, Length, [], TimeoutOrHandle)`. `TimeoutOrHandle :: nowait` has been allowed since OTP 22.1. `TimeoutOrHandle :: Handle` has been allowed since OTP 24.0.

With arguments `Flags` and `TimeoutOrHandle`; equivalent to `recv(Socket, 0, Flags, TimeoutOrHandle)` (since OTP 24.0).

# recv/4

(since OTP 22.0)

    -spec recv(Socket, Length, Flags, Timeout :: infinity) ->
                  {ok, Data} | {error, Reason} | {error, {Reason, Data}}
                  when
                      Socket :: socket(),
                      Length :: non_neg_integer(),
                      Flags :: [msg_flag() | integer()],
                      Data :: binary(),
                      Reason :: posix() | closed | invalid();
              (Socket, Length, Flags, Timeout :: non_neg_integer()) ->
                  {ok, Data} | {error, Reason} | {error, {Reason, Data}}
                  when
                      Socket :: socket(),
                      Length :: non_neg_integer(),
                      Flags :: [msg_flag() | integer()],
                      Data :: binary(),
                      Reason :: posix() | closed | invalid() | timeout;
              (Socket, Length, Flags, nowait | Handle) ->
                  {ok, Data} |
                  {select, SelectInfo} |
                  {select, {SelectInfo, Data}} |
                  {select_read, {SelectInfo, Data}} |
                  {completion, CompletionInfo} |
                  {error, Reason}
                  when
                      Socket :: socket(),
                      Length :: non_neg_integer(),
                      Flags :: [msg_flag() | integer()],
                      Handle :: select_handle() | completion_handle(),
                      Data :: binary(),
                      SelectInfo :: select_info(),
                      CompletionInfo :: completion_info(),
                      Reason :: posix() | closed | invalid().

Receive data on a connected socket.

The argument `Length` specifies the size of the receive buffer. Packet oriented sockets truncate the packet if the size is too small.

If `Length == 0`; a default buffer size is used, which can be set by `socket:setopt(Socket, {otp,recvbuf}, BufSz)`.

For a socket of type `stream`, when a `Timeout` argument is used, the operation iterates until `Length` bytes has been received, or the operation times out. If `Length == 0` all readily available data is returned.

On a `select` system, when the default receive buffer size option `{otp,recvbuf}` special value `{N,BufSize}` is used, `N` limits how many `BufSize` buffers that may be received in a tight loop before the receive operation returns. The option value `{1,BufSize}` is equivalent to just specifying a size value `BufSize`.

The message `Flags` may be symbolic `msg_flag/0`s and/or `integer/0`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}}` (can only happen for a socket of type `stream`).

If the `Timeout` argument is `infinity`; waits for the data to arrive. For a socket of type `stream` this call won't return until all requested data can be delivered, or if "all available" was requested when the first data chunk arrives, or if the OS reports an error for the operation.

If the `Timeout` argument is a time-out value (`non_neg_integer/0`); return `{error, timeout}` if no data has arrived after `Timeout` milliseconds, or `{error, {timeout, Data}}` if some but not enough data has been received on a socket of type `stream` with Length > 0. It can also return directly with `{ok, Data}` (type `dgram`).

`Timeout = 0` only polls the OS receive call and doesn't engage the Asynchronous Calls mechanisms. If no data is immediately available `{error, timeout} is returned.`On a socket of type `stream`, `{error, {timeout, Data}}` is returned if there is an insufficient amount of data immediately available.

If the `Handle` argument is `nowait` (since OTP 22.1), starts an asynchronous call if the operation couldn't be completed immediately.

If the `Handle` argument is a `select_handle/0`, (since OTP 24.0), or on Windows, the equivalent `completion_handle/0` (since OTP 26.0), starts an asynchronous call like for `nowait`.

See the note Asynchronous Calls at the start of this module reference manual page.

On `select` systems, for a socket of type `stream`, if `Length > 0` and there is some but not enough data available, this function will return `{select, {SelectInfo, Data}}` with partial `Data`. A repeated call to complete the operation may need an updated `Length` argument.

On `select` systems, if the option `{otp, select_read}` is set, `{select_read, {SelectInfo, Data}}` is returned instead of `{ok, Data}` and a new asynchronous receive operation has been initiated, which can be seen as an automatic nowait call whenever a receive operation is completed.

# recvfrom(Socket)

(since OTP 22.0)

    -spec recvfrom(Socket :: socket()) -> dynamic().

Equivalent to `recvfrom(Socket, 0, [], infinity)`.

# recvfrom/2

(since OTP 22.0)

    -spec recvfrom(Socket :: socket(), Flags :: list()) -> dynamic();
                  (Socket :: socket(), BufSz :: non_neg_integer()) -> dynamic().

Receive a message on a socket.

With argument `BufSz`; equivalent to `recvfrom(Socket, BufSz, [], infinity)`.

With argument `Flags`; equivalent to `recvfrom(Socket, 0, Flags, infinity)` (since OTP 24.0).

# recvfrom/3

(since OTP 22.0)

    -spec recvfrom(Socket :: socket(), Flags :: [msg_flag() | integer()], TimeoutOrHandle :: dynamic()) ->
                      dynamic();
                  (Socket :: socket(), BufSz :: non_neg_integer(), Flags :: [msg_flag() | integer()]) ->
                      dynamic();
                  (Socket :: socket(), BufSz :: non_neg_integer(), TimeoutOrHandle) -> dynamic()
                      when TimeoutOrHandle :: timeout() | nowait | select_handle() | completion_handle().

Receive a message on a socket.

With arguments `BufSz` and `Flags`; equivalent to `recvfrom(Socket, BufSz, Flags, infinity)`.

With arguments `BufSz` and `TimeoutOrHandle`; equivalent to `recvfrom(Socket, BufSz, [], TimeoutOrHandle)`.

With arguments `Flags` and `TimeoutOrHandle`; equivalent to `recvfrom(Socket, 0, Flags, TimeoutOrHandle)`

`TimeoutOrHandle :: 'nowait'` has been allowed since OTP 22.1.

`TimeoutOrHandle :: Handle` has been allowed since OTP 24.0.

# recvfrom/4

(since OTP 22.0)

    -spec recvfrom(Socket, BufSz, Flags, Timeout :: infinity) -> {ok, {Source, Data}} | {error, Reason}
                      when
                          Socket :: socket(),
                          BufSz :: non_neg_integer(),
                          Flags :: [msg_flag() | integer()],
                          Source :: sockaddr_recv(),
                          Data :: binary(),
                          Reason :: posix() | closed | invalid();
                  (Socket, BufSz, Flags, Timeout :: non_neg_integer()) ->
                      {ok, {Source, Data}} | {error, Reason}
                      when
                          Socket :: socket(),
                          BufSz :: non_neg_integer(),
                          Flags :: [msg_flag() | integer()],
                          Source :: sockaddr_recv(),
                          Data :: binary(),
                          Reason :: posix() | closed | invalid() | timeout;
                  (Socket, BufSz, Flags, nowait | Handle) ->
                      {ok, {Source, Data}} |
                      {select, SelectInfo} |
                      {select_read, {SelectInfo, {Source, Data}}} |
                      {completion, CompletionInfo} |
                      {error, Reason}
                      when
                          Socket :: socket(),
                          BufSz :: non_neg_integer(),
                          Flags :: [msg_flag() | integer()],
                          Handle :: select_handle() | completion_handle(),
                          Source :: sockaddr_recv(),
                          Data :: binary(),
                          SelectInfo :: select_info(),
                          CompletionInfo :: completion_info(),
                          Reason :: posix() | closed | invalid().

Receive a message on a socket.

This function is intended primarily for sockets that are not connection oriented such as type `dgram` or `seqpacket` where messages may arrive from different source addresses.

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 `0`, a default buffer size is used, which can be set by `socket:setopt(Socket, {otp,recvbuf}, BufSz)`.

If there is no known 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/0`s and/or `integer/0`s, as in the platform's appropriate header files. The values of all symbolic flags and integers are or:ed together.

If the `Timeout` argument is `infinity`; waits for a message to arrive, or for a socket error.

If the `Timeout` argument is a time-out value (`non_neg_integer/0`); returns `{error, timeout}` if no message has arrived after `Timeout` milliseconds.

`Timeout = 0` only polls the OS receive call and doesn't engage the Asynchronous Calls mechanisms. If no message is immediately available `{error, timeout}` is returned.

If the `Handle` argument is `nowait` (since OTP 22.1), starts an asynchronous call if the operation couldn't be completed immediately.

If the 'Handle' argument is a `select_handle/0`, (since OTP 24.0), or on Windows, the equivalent `completion_handle/0` (since OTP 26.0), starts an asynchronous call like for `nowait`.

See the note Asynchronous Calls at the start of this module reference manual page.

# recvmsg(Socket)

(since OTP 22.0)

    -spec recvmsg(Socket :: socket()) -> dynamic().

Equivalent to `recvmsg(Socket, 0, 0, [], infinity)`.

# recvmsg/2

(since OTP 22.0)

    -spec recvmsg(Socket :: socket(), Flags :: list()) -> dynamic();
                 (Socket :: socket(), TimeoutOrHandle) -> dynamic()
                     when
                         TimeoutOrHandle :: timeout() | nowait | Handle,
                         Handle :: select_handle() | completion_handle().

Receive a message on a socket.

With argument `Flags`; equivalent to `recvmsg(Socket, 0, 0, Flags, infinity)`.

With argument `TimeoutOrHandle`; equivalent to `recvmsg(Socket, 0, 0, [], TimeoutOrHandle)`.

`TimeoutOrHandle :: nowait` has been allowed since OTP 22.1.

`TimeoutOrHandle :: Handle` has been allowed since OTP 24.0.

# recvmsg/3

(since OTP 22.0)

    -spec recvmsg(Socket :: dynamic(), Flags :: list(), TimeoutOrHandle :: dynamic()) -> dynamic();
                 (Socket :: dynamic(), BufSz :: integer(), CtrlSz :: integer()) -> dynamic().

Receive a message on a socket.

With arguments `Flags`; equivalent to `recvmsg(Socket, 0, 0, Flags, infinity)`.

With argument `TimeoutOrHandle`; equivalent to `recvmsg(Socket, 0, 0, [], TimeoutOrHandle)`.

`TimeoutOrHandle :: nowait` has been allowed since OTP 22.1.

`TimeoutOrHandle :: Handle` has been allowed since OTP 24.0.

# recvmsg(Socket, BufSz, CtrlSz, TimeoutOrHandle)

(since OTP 24.0)

    -spec recvmsg(Socket :: socket(),
                  BufSz :: non_neg_integer(),
                  CtrlSz :: non_neg_integer(),
                  TimeoutOrHandle :: dynamic()) ->
                     dynamic().

Equivalent to `recvmsg(Socket, BufSz, CtrlSz, [], TimeoutOrHandle)`.

# recvmsg/5

(since OTP 22.0)

    -spec recvmsg(Socket, BufSz, CtrlSz, Flags, Timeout :: infinity) -> {ok, Msg} | {error, Reason}
                     when
                         Socket :: socket(),
                         BufSz :: non_neg_integer(),
                         CtrlSz :: non_neg_integer(),
                         Flags :: [msg_flag() | integer()],
                         Msg :: msg_recv(),
                         Reason :: posix() | closed | invalid();
                 (Socket, BufSz, CtrlSz, Flags, Timeout :: non_neg_integer()) -> {ok, Msg} | {error, Reason}
                     when
                         Socket :: socket(),
                         BufSz :: non_neg_integer(),
                         CtrlSz :: non_neg_integer(),
                         Flags :: [msg_flag() | integer()],
                         Msg :: msg_recv(),
                         Reason :: posix() | closed | invalid() | timeout;
                 (Socket, BufSz, CtrlSz, Flags, nowait | Handle) ->
                     {ok, Msg} |
                     {select, SelectInfo} |
                     {select_read, {SelectInfo, Msg}} |
                     {completion, CompletionInfo} |
                     {error, Reason}
                     when
                         Socket :: socket(),
                         BufSz :: non_neg_integer(),
                         CtrlSz :: non_neg_integer(),
                         Handle :: select_handle() | completion_handle(),
                         Flags :: [msg_flag() | integer()],
                         Msg :: msg_recv(),
                         SelectInfo :: select_info(),
                         CompletionInfo :: completion_info(),
                         Reason :: posix() | closed | invalid().

Receive a message on a socket.

This function receives a data message with control messages as well as its source address.

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 `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 there is no known 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/0`s and/or `integer/0`s, as in the platform's appropriate header files. The values of all symbolic flags and integers are or:ed together.

If the `Timeout` argument is `infinity`; waits for the message to arrive, or for a socket error.

If the `Timeout` argument is a time-out value (`non_neg_integer/0`); return `{error, timeout}` if no message has arrived after `Timeout` milliseconds.

`Timeout = 0` only polls the OS receive call and doesn't engage the Asynchronous Calls mechanisms. If no message is immediately available `{error, timeout}` is returned.

If the `Handle` argument is `nowait` (since OTP 22.1), starts an asynchronous call if the operation couldn't be completed immediately.

If the 'Handle' argument is a `select_handle/0`, (since OTP 24.0), or on Windows, the equivalent `completion_handle/0` (since OTP 26.0), starts an asynchronous call like for `nowait`.

See the note Asynchronous Calls at the start of this module reference manual page.

# send(Socket, Data)

(since OTP 22.0)

    -spec send(Socket, Data) -> Result
                  when
                      Socket :: socket(),
                      Data :: iodata(),
                      Result ::
                          ok |
                          {ok, RestData :: binary()} |
                          {select, SelectInfo :: dynamic()} |
                          {completion, CompletionInfo :: dynamic()} |
                          {error, Reason :: dynamic()}.

Equivalent to `send(Socket, Data, [], infinity)`.

# send/3

(since OTP 22.0)

    -spec send(Socket :: term(), Data :: term(), Cont :: tuple()) -> _;
              (Socket :: term(), Data :: term(), Flags :: list()) -> _;
              (Socket :: term(), Data :: term(), Timeout :: timeout()) -> _.

Send data on a connected socket.

With argument `Timeout`; equivalent to `send(Socket, Data, [], Timeout)`.

With argument `Flags`; equivalent to `send(Socket, Data, Flags, infinity)`.

With argument `Cont`; equivalent to `send(Socket, Data, Cont, infinity)` (since OTP 24.0).

# send/4

(since OTP 22.0)

    -spec send(Socket, Data, Flags | Cont, Timeout :: infinity) ->
                  ok | {ok, RestData} | {error, Reason} | {error, {Reason, RestData}}
                  when
                      Socket :: socket(),
                      Data :: iodata(),
                      Flags :: [msg_flag() | integer()],
                      Cont :: select_info(),
                      RestData :: binary(),
                      Reason :: posix() | closed | invalid() | netname_deleted | too_many_cmds | eei();
              (Socket, Data, Flags | Cont, Timeout :: non_neg_integer()) ->
                  ok | {ok, RestData} | {error, Reason | timeout} | {error, {Reason | timeout, RestData}}
                  when
                      Socket :: socket(),
                      Data :: iodata(),
                      Flags :: [msg_flag() | integer()],
                      Cont :: select_info(),
                      RestData :: binary(),
                      Reason :: posix() | closed | invalid() | netname_deleted | too_many_cmds | eei();
              (Socket, Data, Flags | Cont, nowait | Handle) ->
                  ok |
                  {ok, RestData} |
                  {select, SelectInfo} |
                  {select, {SelectInfo, RestData}} |
                  {completion, CompletionInfo} |
                  {error, Reason}
                  when
                      Socket :: socket(),
                      Data :: iodata(),
                      Flags :: [msg_flag() | integer()],
                      Cont :: select_info(),
                      Handle :: select_handle() | completion_handle(),
                      RestData :: binary(),
                      SelectInfo :: select_info(),
                      CompletionInfo :: completion_info(),
                      Reason :: posix() | closed | invalid() | netname_deleted | too_many_cmds | eei().

Send data on a connected socket.

The message `Flags` may be symbolic `msg_flag/0`s and/or `integer/0`s as in 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/0`, is copied into one before calling the platform network API, because a single buffer is required. A returned `RestData` is a sub binary of it.

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

- `ok` - All data was accepted by the OS for delivery

- `{ok, RestData}` - Some but not all data was accepted, but no error was reported (partially successful send). `RestData` is the tail of `Data` that wasn't accepted.

  This cannot happen for a socket of type `stream` where such a partially successful send is retried until the data is either accepted for delivery 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, surely more possible for a socket of type `seqpacket`.

- `{error, Reason}` - An error has been reported and no data was accepted for delivery. `Reason :: posix/0` is what the platform's network layer reported. `closed` means that this socket library was informed that the socket was closed, and `invalid/0` means that this socket library found an argument to be invalid.

- `{error, {Reason, RestData}}` - An error was reported but before that some data was accepted for delivery. `RestData` is the tail of `Data` that wasn't 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.

If the `Timeout` argument is `infinity`; wait for the OS to complete the send operation (take responsibility for the data), or return an error.

If the `Timeout` argument is a time-out value (`non_neg_integer/0`); return `{error, timeout}` if no data has been sent within `Timeout` millisecond, or `{error, {timeout, RestData}}` if some data was sent (accepted by the OS for delivery). `RestData` is the tail of the data that hasn't been sent.

If the `Handle` argument is `nowait` (since OTP 22.1), starts an asynchronous call if the operation couldn't be completed immediately.

If the `Handle` argument is a `select_handle/0`, (since OTP 24.0), or on Windows, the equivalent `completion_handle/0` (since OTP 26.0), starts an asynchronous call like for `nowait`.

See the note Asynchronous Calls at the start of this module reference manual page.

If the function is called with a `Cont` argument, that is; the `SelectInfo` from the previous `send/3,4` call; the send is continued with preprocessed send parameters in the `SelectInfo`. Using this argument variant avoids for example having to validate and encode message flags in every call but the first.

# sendfile(Socket, FileHandle_Cont)

(since OTP 24.0)

    -spec sendfile(Socket, FileHandle | Continuation) -> dynamic()
                      when Socket :: socket(), FileHandle :: file:fd(), Continuation :: select_info().

Send a file on a socket.

Equivalent to `sendfile(Socket, FileHandle_or_Continuation, 0, 0, infinity)`.

# sendfile(Socket, FileHandle_Cont, Timeout_Handle)

(since OTP 24.0)

    -spec sendfile(Socket, FileHandle | Continuation, Timeout | Handle) -> dynamic()
                      when
                          Socket :: socket(),
                          FileHandle :: file:fd(),
                          Continuation :: select_info(),
                          Timeout :: infinity | non_neg_integer(),
                          Handle :: nowait | select_handle().

Send a file on a socket.

Equivalent to `sendfile(Socket, FileHandle_or_Continuation, 0, 0, Timeout_or_Handle)`.

# sendfile(Socket, FileHandle_Cont, Offset, Count)

(since OTP 24.0)

    -spec sendfile(Socket, FileHandle | Continuation, Offset, Count) -> dynamic()
                      when
                          Socket :: socket(),
                          FileHandle :: file:fd(),
                          Continuation :: select_info(),
                          Offset :: integer(),
                          Count :: non_neg_integer().

Send a file on a socket.

Equivalent to `sendfile(Socket, FileHandle_or_Continuation, Offset, Count, infinity)`.

# sendfile/5

(since OTP 24.0)

    -spec sendfile(Socket, FileHandle | Continuation, Offset, Count, Timeout :: infinity) ->
                      {ok, BytesSent} | {error, Reason} | {error, {Reason, BytesSent}}
                      when
                          Socket :: socket(),
                          FileHandle :: file:fd(),
                          Continuation :: select_info(),
                          Offset :: integer(),
                          Count :: non_neg_integer(),
                          BytesSent :: non_neg_integer(),
                          Reason :: posix() | closed | invalid();
                  (Socket, FileHandle | Continuation, Offset, Count, Timeout :: non_neg_integer()) ->
                      {ok, BytesSent} | {error, Reason} | {error, {Reason, BytesSent}}
                      when
                          Socket :: socket(),
                          FileHandle :: file:fd(),
                          Continuation :: select_info(),
                          Offset :: integer(),
                          Count :: non_neg_integer(),
                          BytesSent :: non_neg_integer(),
                          Reason :: posix() | closed | invalid() | timeout;
                  (Socket,
                   FileHandle | Continuation,
                   Offset, Count,
                   nowait | (SelectHandle :: select_handle())) ->
                      {ok, BytesSent} |
                      {select, SelectInfo} |
                      {select, {SelectInfo, BytesSent}} |
                      {error, Reason}
                      when
                          Socket :: socket(),
                          FileHandle :: file:fd(),
                          Continuation :: select_info(),
                          Offset :: integer(),
                          Count :: non_neg_integer(),
                          BytesSent :: non_neg_integer(),
                          SelectInfo :: select_info(),
                          Reason :: posix() | closed | invalid().

Send a file on a socket.

#### Note

This function unsupported on Windows.

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

The `Offset` argument is the file offset to start reading from. The default offset 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 was transferred. `Reason :: posix/0` is what the platform's network layer reported. `closed` means that this socket library was informed that the socket was closed, and `invalid/0` means that this socket library found an argument to be invalid.

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

If the `Timeout` argument is `infinity`; wait for the OS to complete the send operation (take responsibility for the data), or return an error.

If the `Timeout` argument is a time-out value (`non_neg_integer/0`); return `{error, timeout}` if no data has been sent within `Timeout` millisecond, or `{error, {timeout, BytesSent}}` if some but not all data was sent (accepted by the OS for delivery).

If the `Handle` argument is `nowait`, starts an asynchronous call if the operation couldn't be completed immediately.

If the `Handle` argument is a `select_handle/0`, starts an asynchronous call like for `nowait`.

See the note Asynchronous Calls at the start of this module reference manual page.

After receiving a `select` message; call `sendfile/2,3,4,5` with `SelectInfo` as the `Continuation` argument, to complete the operation.

If the function is called with a `Continuation` argument, that is; the `SelectInfo` from the previous `sendfile/5` call; the transfer is continued with preprocessed parameters in the `SelectInfo`.

The `Offset` and maybe `Count` arguments will probably need to be updated between continuation calls.

# sendmsg(Socket, Msg)

(since OTP 22.0)

    -spec sendmsg(Socket, Msg) -> Result
                     when Socket :: socket(), Msg :: msg_send() | erlang:iovec(), Result :: dynamic().

Equivalent to `sendmsg(Socket, Msg, [], infinity)`.

# sendmsg/3

(since OTP 22.0)

    -spec sendmsg(Socket :: socket(), Msg :: msg_send(), Flags :: list()) -> dynamic();
                 (Socket :: socket(), Data :: msg_send() | erlang:iovec(), Cont :: select_info()) ->
                     dynamic();
                 (Socket :: socket(), Msg :: msg_send(), Timeout :: infinity) -> dynamic().

Send data and control messages on a socket.

With arguments `Msg` and `Timeout`; equivalent to `sendmsg(Socket, Msg, [], Timeout)`.

With arguments `Msg` and `Flags`; equivalent to `sendmsg(Socket, Msg, Flags, infinity)`.

With arguments `Data` and `Cont`; equivalent to `sendmsg(Socket, Data, Cont, infinity)` since OTP 24.0.

# sendmsg/4

(since OTP 22.0)

    -spec sendmsg(Socket, Msg, Flags, Timeout :: infinity) ->
                     ok | {ok, RestData} | {error, Reason} | {error, {Reason, RestData}}
                     when
                         Socket :: socket(),
                         Msg :: msg_send(),
                         Flags :: [msg_flag() | integer()],
                         RestData :: erlang:iovec(),
                         Reason :: posix() | closed | invalid();
                 (Socket, Msg, Flags, Timeout :: non_neg_integer()) ->
                     ok | {ok, RestData} | {error, Reason | timeout} | {error, {Reason | timeout, RestData}}
                     when
                         Socket :: socket(),
                         Msg :: msg_send(),
                         Flags :: [msg_flag() | integer()],
                         RestData :: erlang:iovec(),
                         Reason :: posix() | closed | invalid();
                 (Socket, Msg, Flags, nowait | Handle) ->
                     ok |
                     {ok, RestData} |
                     {select, SelectInfo} |
                     {select, {SelectInfo, RestData}} |
                     {completion, CompletionInfo} |
                     {error, Reason} |
                     {error, {Reason, RestData}}
                     when
                         Socket :: socket(),
                         Msg :: msg_send(),
                         Flags :: [msg_flag() | integer()],
                         Handle :: select_handle() | completion_handle(),
                         RestData :: erlang:iovec(),
                         SelectInfo :: select_info(),
                         CompletionInfo :: completion_info(),
                         Reason :: posix() | closed | invalid();
                 (Socket, Data, Cont, Timeout :: infinity) ->
                     ok | {ok, RestData} | {error, Reason} | {error, {Reason, RestData}}
                     when
                         Socket :: socket(),
                         Data :: msg_send() | erlang:iovec(),
                         Cont :: select_info(),
                         RestData :: erlang:iovec(),
                         Reason :: posix() | closed | invalid();
                 (Socket, Data, Cont, Timeout :: non_neg_integer()) ->
                     ok | {ok, RestData} | {error, Reason | timeout} | {error, {Reason | timeout, RestData}}
                     when
                         Socket :: socket(),
                         Data :: msg_send() | erlang:iovec(),
                         Cont :: select_info(),
                         RestData :: erlang:iovec(),
                         Reason :: posix() | closed | invalid();
                 (Socket, Data, Cont, nowait | Handle) ->
                     ok |
                     {ok, RestData} |
                     {select, SelectInfo} |
                     {select, {SelectInfo, RestData}} |
                     {completion, CompletionInfo} |
                     {error, Reason} |
                     {error, {Reason, RestData}}
                     when
                         Socket :: socket(),
                         Data :: msg_send() | erlang:iovec(),
                         Cont :: select_info(),
                         Handle :: select_handle(),
                         RestData :: erlang:iovec(),
                         SelectInfo :: select_info(),
                         CompletionInfo :: completion_info(),
                         Reason :: posix() | closed | invalid().

Send data and control messages on a socket.

The argument `Msg` is a map that contains the data to be sent under the key `iov` as an`erlang:iovec/0` (list of `binary/0`). It may also contain the destination address under the key `addr`, which is mandatory if the socket isn't connected. If the socket is connected it is best to not have an `addr` key since the platform may regard that as an error (or ignore it). Under the key `ctrl` there may be a list of protocol and platform dependent control messages (a.k.a ancillary data, a.k.a control information) to send.

The message data is given to the platform's network layer as 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.

See `send/4` for a description of the `Flags` argument and the return values.

#### Note

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

If the `Timeout` argument is `infinity`; wait for the OS to complete the send operation (take responsibility for the data), or return an error.

If the `Timeout` argument is a time-out value (`non_neg_integer/0`); return `{error, timeout}` if no data has been sent within `Timeout` millisecond, or `{error, {timeout, RestData}}` if some data was sent (accepted by the OS for delivery). `RestData` is the tail of the data that hasn't been sent.

If the `Handle` argument is `nowait` (since OTP 22.1), starts an asynchronous call if the operation couldn't be completed immediately.

If the `Handle` argument is a `select_handle/0`, (since OTP 24.0), or on Windows, the equivalent `completion_handle/0` (since OTP 26.0), starts an asynchronous call like for `nowait`.

See the note Asynchronous Calls at the start of this module reference manual page.

After receiving a `select` message; call `sendmsg/3,4` with `SelectInfo` as the `Cont` argument, to complete the operation.

With the arguments `Data` and `Cont`, continues the send operation. `Cont` should be the `SelectInfo` returned from the previous `sendmsg/2,3,4` call.

`Data` can be a `Msg` `map/0` where only the key `iov` is used, or an `erlang:iovec/0`.

# sendto/3

(since OTP 22.0)

    -spec sendto(Socket :: socket(), Data :: iodata(), Cont | Dest) -> Result
                    when
                        Cont :: select_info(),
                        Dest :: sockaddr(),
                        Result :: ok | {ok, RestData} | {error, Reason} | {error, {Reason, RestData}},
                        RestData :: binary(),
                        Reason :: posix() | closed | invalid().

Send data on a socket.

With argument `Dest`; equivalent to `sendto(Socket, Data, Dest, [], infinity)`.

With argument `Cont`; equivalent to `sendto(Socket, Data, Cont, infinity)` since OTP 24.0.

# sendto/4

(since OTP 22.0)

    -spec sendto(Socket :: socket(),
                 Data :: iodata(),
                 Dest :: sockaddr(),
                 Flags :: [msg_flag() | integer()]) ->
                    dynamic();
                (Socket :: socket(),
                 Data :: iodata(),
                 Cont :: select_info(),
                 Timeout :: timeout() | nowait | (Handle :: select_handle())) ->
                    dynamic();
                (Socket :: socket(),
                 Data :: iodata(),
                 Dest :: sockaddr(),
                 Timeout :: timeout() | nowait | (Handle :: select_handle() | completion_handle())) ->
                    dynamic().

Send data on a socket.

With arguments `Dest` and `TimeoutOrHandle`; equivalent to `sendto(Socket, Data, Dest, [], TimeoutOrHandle)`.

With arguments `Dest` and `Flags`; equivalent to `sendto(Socket, Data, Dest, Flags, infinity)`.

With arguments `Cont` and `TimeoutOrHandle`; `Cont` must be the `SelectInfo` from the previous `sendto/3,4,5` call and the send is continued with preprocessed send parameters in the `SelectInfo`. Using this argument variant avoids for example having o validate and encode message flags in every call but the first. (Since OTP 24.0)

See the last argument (argument 5) of `sendto/5` for an explanation of `TimeoutOrHandle`.

# sendto/5

(since OTP 22.0)

    -spec sendto(Socket, Data, Dest, Flags, Timeout :: infinity) ->
                    ok | {ok, RestData} | {error, Reason} | {error, {Reason, RestData}}
                    when
                        Socket :: socket(),
                        Data :: iodata(),
                        Dest :: sockaddr(),
                        Flags :: [msg_flag() | integer()],
                        RestData :: binary(),
                        Reason :: posix() | closed | invalid();
                (Socket, Data, Dest, Flags, Timeout :: non_neg_integer()) ->
                    ok | {ok, RestData} | {error, Reason | timeout} | {error, {Reason | timeout, RestData}}
                    when
                        Socket :: socket(),
                        Data :: iodata(),
                        Dest :: sockaddr(),
                        Flags :: [msg_flag() | integer()],
                        RestData :: binary(),
                        Reason :: posix() | closed | invalid();
                (Socket, Data, Dest, Flags, nowait | Handle) ->
                    ok |
                    {ok, RestData} |
                    {select, SelectInfo} |
                    {select, {SelectInfo, RestData}} |
                    {completion, CompletionInfo} |
                    {error, Reason}
                    when
                        Socket :: socket(),
                        Data :: iodata(),
                        Dest :: sockaddr(),
                        Flags :: [msg_flag() | integer()],
                        Handle :: select_handle() | completion_handle(),
                        RestData :: binary(),
                        SelectInfo :: select_info(),
                        CompletionInfo :: completion_info(),
                        Reason :: posix() | closed | invalid().

Send data on a socket.

The `To` argument is the destination address where to send the data. For a connected socket this argument is still passed to the OS call that may ignore the address or return an error.

See `send/4` for a description of the `Flags` and `Data` arguments, and the return values.

If the `Timeout` argument is `infinity`; wait for the OS to complete the send operation (take responsibility for the data), or return an error.

If the `Timeout` argument is a time-out value (`non_neg_integer/0`); return `{error, timeout}` if no data has been sent within `Timeout` millisecond, or `{error, {timeout, RestData}}` if some data was sent (accepted by the OS for delivery). `RestData` is the tail of the data that hasn't been sent.

If the `Handle` argument is `nowait` (since OTP 22.1), starts an asynchronous call if the operation couldn't be completed immediately.

If the `Handle` argument is a `select_handle/0`, (since OTP 24.0), or on Windows, the equivalent `completion_handle/0` (since OTP 26.0), starts an asynchronous call like for `nowait`.

See the note Asynchronous Calls at the start of this module reference manual page.

After receiving a `select` message; call `sendto/3,4` with `SelectInfo` as the `Cont` argument, to complete the operation.

# sendv(Socket, IOV)

(since OTP 27.0)

    -spec sendv(Socket, IOV) -> ok | {ok, RestIOV} | {error, Reason} | {error, {Reason, RestIOV}}
                   when
                       Socket :: socket(),
                       IOV :: erlang:iovec(),
                       RestIOV :: erlang:iovec(),
                       Reason :: posix() | closed | invalid().

Equivalent to `sendv(Socket, IOV, infinity)`.

# sendv/3

(since OTP 27.0)

    -spec sendv(Socket, IOV, Timeout :: infinity) ->
                   ok | {ok, RestIOV} | {error, Reason} | {error, {Reason, RestIOV}}
                   when
                       Socket :: socket(),
                       IOV :: erlang:iovec(),
                       RestIOV :: erlang:iovec(),
                       Reason :: posix() | closed | invalid();
               (Socket, IOV, Timeout :: non_neg_integer()) ->
                   ok | {ok, RestIOV} | {error, Reason} | {error, {Reason, RestIOV}}
                   when
                       Socket :: socket(),
                       IOV :: erlang:iovec(),
                       RestIOV :: erlang:iovec(),
                       Reason :: posix() | closed | invalid() | timeout;
               (Socket, IOV, nowait | Handle) ->
                   ok |
                   {ok, RestIOV} |
                   {select, SelectInfo} |
                   {select, {SelectInfo, RestIOV}} |
                   {completion, CompletionInfo} |
                   {error, Reason} |
                   {error, {Reason, RestIOV}}
                   when
                       Socket :: socket(),
                       IOV :: erlang:iovec(),
                       Handle :: select_handle() | completion_handle(),
                       RestIOV :: erlang:iovec(),
                       SelectInfo :: select_info(),
                       CompletionInfo :: completion_info(),
                       Reason :: posix() | closed | invalid();
               (Socket, IOV, Cont) -> ok | {ok, RestIOV} | {error, Reason} | {error, {Reason, RestIOV}}
                   when
                       Socket :: socket(),
                       IOV :: erlang:iovec(),
                       Cont :: select_info(),
                       RestIOV :: erlang:iovec(),
                       Reason :: posix() | closed | invalid().

Send `erlang:iovec/0` data on a connected socket.

See `sendmsg/4` about how the `IOV` data is handled towards the platform's network layer.

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

- `ok` - All data has been accepted by the OS for delivery.

- `{ok, RestIOV}` - Some but not all data was accepted, but no error was reported (partially successful send). `RestIOV` is the tail of `IOV` that wasn't accepted.

- `{error, Reason}` - An error has been reported and no data was accepted for delivery. `Reason :: posix/0` is what the platform's network layer reported. `closed` means that this socket library was informed that the socket was closed, and `invalid/0` means that this socket library found an argument to be invalid.

- `{error, {Reason, RestIOV}}` - - An error was reported but before that some data was accepted for delivery. `RestIOV` is the tail of `IOV` that wasn't accepted. See `{error, Reason}` above.

If the `Timeout` argument is `infinity`; wait for the OS to complete the send operation (take responsibility for the data), or return an error.

If the `Timeout` argument is a time-out value (`non_neg_integer/0`); return `{error, timeout}` if no data has been sent within `Timeout` millisecond, or `{error, {timeout, RestIOV}}` if some data was sent (accepted by the OS for delivery). `RestIOV` is the tail of the data that hasn't been sent.

If the `Handle` argument is `nowait`, starts an asynchronous call if the operation couldn't be completed immediately.

If the `Handle` argument is a `select_handle/0`, or on Windows, the equivalent `completion_handle/0`, starts an asynchronous call like for `nowait`.

See the note Asynchronous Calls at the start of this module reference manual page.

With the argument `Cont`, equivalent to `sendv(Socket, IOV, Cont, infinity)`.

# sendv/4

(since OTP 27.0)

    -spec sendv(Socket, IOV, Cont, Timeout :: infinity) ->
                   ok | {ok, RestIOV} | {error, Reason} | {error, {Reason, RestIOV}}
                   when
                       Socket :: socket(),
                       IOV :: erlang:iovec(),
                       Cont :: select_info(),
                       RestIOV :: erlang:iovec(),
                       Reason :: posix() | closed | invalid();
               (Socket, IOV, Cont, Timeout :: non_neg_integer()) ->
                   ok | {ok, RestIOV} | {error, Reason} | {error, {Reason | RestIOV}}
                   when
                       Socket :: socket(),
                       IOV :: erlang:iovec(),
                       Cont :: select_info(),
                       RestIOV :: erlang:iovec(),
                       Reason :: posix() | closed | invalid() | timeout;
               (Socket, IOV, Cont, nowait | SelectHandle) ->
                   ok |
                   {ok, RestIOV} |
                   {select, SelectInfo} |
                   {select, {SelectInfo, RestIOV}} |
                   {error, Reason} |
                   {error, {Reason, RestIOV}}
                   when
                       Socket :: socket(),
                       IOV :: erlang:iovec(),
                       Cont :: select_info(),
                       SelectHandle :: select_handle(),
                       RestIOV :: erlang:iovec(),
                       SelectInfo :: select_info(),
                       Reason :: posix() | closed | invalid().

Send data on a connected socket, continuation.

Continues sending data on a connected socket. `Cont` is the `SelectInfo` returned from the previous `sendv/2,3` call. `IOV` should be the rest data that wasn't sent.

See asynchronous calls about continuing unfinished calls.

See `sendv/3` about the return values.

# setopt/3

(since OTP 24.0)

    -spec setopt(Socket, SocketOption, Value) -> ok | {error, invalid() | closed}
                    when
                        Socket :: socket(),
                        SocketOption :: {Level :: otp, Opt :: otp_socket_option()},
                        Value :: dynamic();
                (Socket, SocketOption, Value) -> ok | {error, posix() | invalid() | closed}
                    when Socket :: socket(), SocketOption :: socket_option(), Value :: dynamic().

Set a socket option.

Set an OS protocol level option, or an `otp` pseudo protocol level option. The latter level is this module's implementation level above the OS protocol levels.

See the type otp_socket_option() for a description of the `otp` protocol level.

See the type `socket_option/0` for which OS protocol level options that this implementation knows about, how they are related to OS option names, and if there are known peculiarities with any of them.

What options that are valid depends on the OS, and on the kind of socket (`domain/0`,`type/0` and `protocol/0`). See the type `t:socket_option()` and the socket options chapter in the User's Guide for more info.

#### Note

Not all options are valid, nor possible to set, on all platforms. That is, even if this `socket` implementation support an option; it doesn't mean that the underlying OS does.

# setopt/4

(since OTP 22.0)

    -spec setopt(socket(), Level :: term(), Opt :: term(), Value :: term()) -> _.

Set a socket option (backwards compatibility function).

Equivalent to `setopt(Socket, {Level, Opt}, Value)`, or as a special case if `Opt = NativeOpt ::` `integer/0` and `Value =` `binary/0` equivalent to `setopt_native(Socket, {Level, NativeOpt}, ValueSpec)`.

Use `setopt/3` or `setopt_native/3` instead to handle the option level and name as a single term, and to make the difference between known options and native options clear.

# setopt_native(Socket, Option, Value)

(since OTP 24.0)

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

Set a "native" socket option.

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/0` it will be used as a `C` type `(int)`, if it is a `boolean/0` it will be used as a `C` type `(int)` with the `C` implementations values for `false` or `true`, and if it is a `binary/0` its content and size will be used as the option value.

The socket option may be specified with an ordinary `socket_option/0` tuple, with a symbolic `Level` as `{``Level :: level/0``,``NativeOpt :: integer/0``}`, or with integers for both `NativeLevel` and `NativeOpt` as `{``NativeLevel :: integer/0``,``NativeOpt :: integer/0``}`.

If an option is valid depends both on the platform and on what kind of socket it is (`domain/0`, `type/0` and `protocol/0`).

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.

# shutdown(Socket, How)

(since OTP 22.0)

    -spec shutdown(Socket, How) -> ok | {error, Reason}
                      when Socket :: socket(), How :: read | write | read_write, Reason :: posix() | closed.

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

# sockname(Socket)

(since OTP 22.0)

    -spec sockname(Socket :: socket()) -> {ok, SockAddr} | {error, Reason}
                      when SockAddr :: sockaddr_recv(), Reason :: posix() | closed.

Get the socket's address.

Returns the address to which the socket is currently bound. If the bind address had the wildcard port `0`, the address returned by this function contains the ephemeral port selected by the OS.

# supports()

(since OTP 22.0)

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

Retrieve information about what socket features the module and the platform supports.

Returns a list of, in no particular order, `{Key1,``supports(Key1)``}` tuples for every `Key1` described in `supports/1`, and `{Key, 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(Key1)

(since OTP 22.0)

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

Retrieve information about what socket features the module and the platform supports.

If `Key1 = msg_flags` returns a list of `{Flag, boolean()}` tuples for every `Flag` in `msg_flag/0` with the `boolean/0` indicating if the flag is supported on this platform.

If `Key1 = protocols` returns a list of `{Name, boolean()}` tuples for every `Name` in`protocol/0` with the `boolean/0` indicating if the protocol is supported on this platform.

If `Key1 = options` returns a list of `{SocketOption, boolean()}` tuples for every `SocketOption` in `socket_option/0` with the `boolean/0` indicating if the socket option is supported on this platform.

There is no particular order of any of the returned lists.

For other values of `Key1` returns `[]`. Note that in future versions of this module or on different platforms, there might be more supported keys.

# supports(Key1, Key2)

(since OTP 22.0)

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

Retrieve information about what socket features the module and the platform supports.

If `Key1 = options`, for a `Key2` in `level/0` returns a list of `{Opt, boolean()}` tuples for all known socket options `Opt` on that `Level = Key2` with the `boolean/0` indicating if the socket option is supported on this platform. See `setopt/3` and `getopt/2`.

There is no particular order of any of the returned lists.

For other values of `Key1` or `Key2` returns `[]`. Note that in future versions of this module or on different platforms, there might be more supported keys.

# use_registry(D)

(since OTP 23.1)

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

Set the global `use_registry` option default value.

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/2,3,4` for more info (the `Opts :: map/0`).

# which_sockets()

(since OTP 22.3)

    -spec which_sockets() -> [socket()].

Return a list of all known sockets.

Equivalent to `which_sockets(fun (_) -> true end)`.

# which_sockets(FilterRule)

(since OTP 22.3)

    -spec which_sockets(FilterRule) -> [socket()]
                           when
                               FilterRule ::
                                   inet | inet6 | local | stream | dgram | seqpacket | sctp | tcp | udp |
                                   pid() |
                                   fun((socket_info()) -> boolean()).

Return a filtered list of known sockets.

There are several predefined `FilterRule`s and one general:

- `inet | inet6` - Only the sockets with matching `domain/0` are returned.

- `stream | dgram | seqpacket` - Only the sockets with matching `type/0` are returned.

- `sctp | tcp | udp` - Only the sockets with matching `protocol/0` are returned.

- `pid/0` - Only the sockets with matching Controlling Process are returned. See the OTP socket option `controlling_process`.

- `fun((socket_info()) -> boolean())` - The general filter rule. A fun that takes the socket info and returns a `boolean/0` indicating if the socket should be returned or not.