6.5 Socket predicate reference

[det]socket_create(-SocketId, +Options)

Create a socket according to Options. Supported Options are:

domain(+Domain)

One of inet (default), inet6, unix or local (same as unix)

type(+Type)

One of stream (default) to create a TCP connection or dgram to create a UDP socket.

This predicate subsumes tcp_socket/1, udp_socket/1 and unix_domain_socket/1.

[det]tcp_socket(-SocketId)

Equivalent to socket_create(SocketId, []) or, explicit, socket_create(SocketId, [domain(inet), type(stream)]).

[det]unix_domain_socket(-SocketId)

Equivalent to socket_create(SocketId, [domain(unix)]) or, explicit, socket_create(SocketId, [domain(unix), type(stream)])

Unix domain socket affect tcp_connect/2 (for clients) and tcp_bind/2 and tcp_accept/3 (for servers). The address is an atom or string that is handled as a file name. On most systems the length of this file name is limited to 128 bytes (including null terminator), but according to the Linux documentation (unix(7)), portable applications must keep the address below 92 bytes. Note that these lengths are in bytes. Non-ascii characters may be represented as multiple bytes. If the length limit is exceeded a representation_error(af_unix_name) exception is raised.

[det]tcp_close_socket(+SocketId)

Closes the indicated socket, making SocketId invalid. Normally, sockets are closed by closing both stream handles returned by open_socket/3. There are two cases where tcp_close_socket/1 is used because there are no stream-handles:

• If, after tcp_accept/3, the server uses fork/1 to handle the client in a sub-process. In this case the accepted socket is not longer needed from the main server and must be discarded using tcp_close_socket/1.
• If, after discovering the connecting client with tcp_accept/3, the server does not want to accept the connection, it should discard the accepted socket immediately using tcp_close_socket/1.

[det]tcp_open_socket(+SocketId, -StreamPair)

Create streams to communicate to SocketId. If SocketId is a master socket (see tcp_bind/2), StreamPair should be used for tcp_accept/3. If SocketId is a connected (see tcp_connect/2) or accepted socket (see tcp_accept/3), StreamPair is unified to a stream pair (see stream_pair/3) that can be used for reading and writing. The stream or pair must be closed with close/1, which also closes SocketId.

[det]tcp_open_socket(+SocketId, -InStream, -OutStream)

Similar to tcp_open_socket/2, but creates two separate sockets where tcp_open_socket/2 would have created a stream pair.

deprecated

New code should use tcp_open_socket/2 because closing a stream pair is much easier to perform safely.

[det]tcp_bind(SocketId, ?Address)

Bind the socket to Address on the current machine. This operation, together with tcp_listen/2 and tcp_accept/3 implement the server-side of the socket interface. Address is either an plain Port or a term HostPort. The first form binds the socket to the given port on all interfaces, while the second only binds to the matching interface. A typical example is below, causing the socket to listen only on port 8080 on the local machine's network.

  tcp_bind(Socket, localhost:8080)

If Port is unbound, the system picks an arbitrary free port and unifies Port with the selected port number. Port is either an integer or the name of a registered service. See also tcp_connect/4.

[det]tcp_listen(+SocketId, +BackLog)

Tells, after tcp_bind/2, the socket to listen for incoming requests for connections. Backlog indicates how many pending connection requests are allowed. Pending requests are requests that are not yet acknowledged using tcp_accept/3. If the indicated number is exceeded, the requesting client will be signalled that the service is currently not available. A commonly used default value for Backlog is 5.

[det]tcp_accept(+Socket, -Slave, -Peer)

This predicate waits on a server socket for a connection request by a client. On success, it creates a new socket for the client and binds the identifier to Slave. Peer is bound to the IP-address of the client or the atom af_unix if Socket is an AF_UNIX socket (see unix_domain_socket/1).

[det]tcp_connect(+SocketId, +Address)

Connect SocketId. After successful completion, tcp_open_socket/3 can be used to create I/O-Streams to the remote socket. This predicate is part of the low level client API. A connection to a particular host and port is realised using these steps:

    tcp_socket(Socket),
    tcp_connect(Socket, Host:Port),
    tcp_open_socket(Socket, StreamPair)

Typical client applications should use the high level interface provided by tcp_connect/3 which avoids resource leaking if a step in the process fails, and can be hooked to support proxies. For example:

    setup_call_cleanup(
        tcp_connect(Host:Port, StreamPair, []),
        talk(StreamPair),
        close(StreamPair))

If SocketId is an AF_UNIX socket (see unix_domain_socket/1), Address is an atom or string denoting a file name.

[nondet,multifile]rewrite_host(+HostIn, -HostOut, +Socket)

Allow rewriting the host for tcp_connect/2 and therefore all other predicates to connect a socket.

This hook is currently defined in Windows to map localhost to ip(127,0,0,1) as resolving localhost on Windows is often very slow. Note that we do not want to do that in general as a system may prefer to map localhost to‘::1`, i.e., the IPv6 loopback address.

[det]tcp_connect(+Socket, +Address, -Read, -Write)

Connect a (client) socket to Address and return a bi-directional connection through the stream-handles Read and Write. This predicate may be hooked by defining socket:tcp_connect_hook/4 with the same signature. Hooking can be used to deal with proxy connections. E.g.,

:- multifile socket:tcp_connect_hook/4.

socket:tcp_connect_hook(Socket, Address, Read, Write) :-
    proxy(ProxyAdress),
    tcp_connect(Socket, ProxyAdress),
    tcp_open_socket(Socket, Read, Write),
    proxy_connect(Address, Read, Write).

deprecated

New code should use tcp_connect/3 called as tcp_connect(+Address, -StreamPair, +Options).

[det]tcp_connect(+Address, -StreamPair, +Options)

[det]tcp_connect(+Socket, +Address, -StreamPair)

Establish a TCP communication as a client. The +,-,+ mode is the preferred way for a client to establish a connection. This predicate can be hooked to support network proxies. To use a proxy, the hook proxy_for_url/3 must be defined. Permitted options are:

bypass_proxy(+Boolean)

Defaults to false. If true, do not attempt to use any proxies to obtain the connection

nodelay(+Boolean)

Defaults to false. If true, set nodelay on the resulting socket using tcp_setopt(Socket, nodelay)

domain(+Domain)

One of‘inet’or inet6. When omitted we use host_address/2 with type(stream) and try the returned addresses in order.

The +,+,- mode is deprecated and does not support proxies. It behaves like tcp_connect/4, but creates a stream pair (see stream_pair/3).

Address

is either a Host:Port term or a file name (atom or string). The latter connects to an AF_UNIX socket and requires unix_domain_socket/1.

Errors

proxy_error(tried(ResultList)) is raised by mode (+,-,+) if proxies are defines by proxy_for_url/3 but no proxy can establsh the connection. ResultList contains one or more terms of the form false(Proxy) for a hook that simply failed or error(Proxy, ErrorTerm) for a hook that raised an exception.

See also

library(http/http_proxy) defines a hook that allows to connect through HTTP proxies that support the CONNECT method.

tcp_select(+ListOfStreams, -ReadyList, +TimeOut)

Same as the built-in wait_for_input/3. Used to allow for interrupts and timeouts on Windows. A redesign of the Windows socket interface makes it impossible to do better than Windows select() call underlying wait_for_input/3. As input multiplexing typically happens in a background thread anyway we accept the loss of timeouts and interrupts.

deprecated

Use wait_for_input/3

[semidet,multifile]try_proxy(+Proxy, +TargetAddress, -Socket, -StreamPair)

Attempt a socket-level connection via the given proxy to TargetAddress. The Proxy argument must match the output argument of proxy_for_url/3. The predicate tcp_connect/3 (and http_open/3 from the library(http/http_open)) collect the results of failed proxies and raise an exception no proxy is capable of realizing the connection.

The default implementation recognises the values for Proxy described below. The library(http/http_proxy) adds proxy(Host,Port) which allows for HTTP proxies using the CONNECT method.

direct

Do not use any proxy

socks(Host, Port)

Use a SOCKS5 proxy

[nondet,multifile]proxy_for_url(+URL, +Hostname, -Proxy)

This hook can be implemented to return a proxy to try when connecting to URL. Returned proxies are tried in the order in which they are returned by the multifile hook try_proxy/4. Pre-defined proxy methods are:

direct

connect directly to the resource

proxy(Host, Port)

Connect to the resource using an HTTP proxy. If the resource is not an HTTP URL, then try to connect using the CONNECT verb, otherwise, use the GET verb.

socks(Host, Port)

Connect to the resource via a SOCKS5 proxy

These correspond to the proxy methods defined by PAC Proxy auto-config. Additional methods can be returned if suitable clauses for http:http_connection_over_proxy/6 or try_proxy/4 are defined.

[det]udp_socket(-SocketId)

Equivalent to socket_create(SocketId, [type(dgram)]) or, explicit, socket_create(SocketId, [domain(inet), type(dgram)]).

[det]udp_receive(+Socket, -Data, -From, +Options)

Wait for and return the next datagram. The Data is returned as a Prolog term depending on Options. From is a term of the format Ip:Port indicating the sender of the message. Here, Ip is either an ip4 or ip6 structure. Socket can be waited for using wait_for_input/3. Defined Options:

as(+Type)

Defines the type for Data. Possible values are atom, codes, string (default) or term (parse as Prolog term).

encoding(+Encoding)

Specify the encoding used to interpret the message. It is one of octet. iso_latin_1, text or utf8.

max_message_size(+Size)

Specify the maximum number of bytes to read from a UDP datagram. Size must be within the range 0-65535. If unspecified, a maximum of 4096 bytes will be read.

For example:

receive(Port) :-
    udp_socket(Socket),
    tcp_bind(Socket, Port),
    repeat,
        udp_receive(Socket, Data, From, [as(atom)]),
        format('Got ~q from ~q~n', [Data, From]),
        fail.

[det]udp_send(+Socket, +Data, +To, +Options)

Send a UDP message. Data is a string, atom or code-list providing the data. To is an address of the form Host:Port where Host is either the hostname or an IP address. Defined Options are:

encoding(+Encoding)

Specifies the encoding to use for the string. See udp_receive/4 for details

as(+Type)

This uses the same values for Type as the as(Type) option of udp_receive/4. The are interpreted differently though. No Type corresponds to CVT_ALL of PL_get_chars(). Using atom corresponds to CVT_ATOM and any of string or codes is mapped to CVT_STRING|CVT_LIST, allowing for a SWI-Prolog string object, list of character codes or list of characters. Finally, term maps to CVT_WRITE_CANONICAL. This implies that arbitrary Prolog terms can be sent reliably using the option list‘[as(term),encoding(utf8)])`, using the same option list for udp_receive/4.

For example

send(Host, Port, Message) :-
    udp_socket(S),
    udp_send(S, Message, Host:Port, []),
    tcp_close_socket(S).

A broadcast is achieved by using tcp_setopt(Socket, broadcast) prior to sending the datagram and using the local network broadcast address as a ip/4 term.

[det]tcp_setopt(+SocketId, +Option)

Set options on the socket. Defined options are:

reuseaddr

Allow servers to reuse a port without the system being completely sure the port is no longer in use.

bindtodevice(+Device)

Bind the socket to Device (an atom). For example, the code below binds the socket to the loopback device that is typically used to realise the localhost. See the manual pages for setsockopt() and the socket interface (e.g., socket(7) on Linux) for details.

tcp_socket(Socket),
tcp_setopt(Socket, bindtodevice(lo))

nodelay

nodelay(true)

If true, disable the Nagle optimization on this socket, which is enabled by default on almost all modern TCP/IP stacks. The Nagle optimization joins small packages, which is generally desirable, but sometimes not. Please note that the underlying TCP_NODELAY setting to setsockopt() is not available on all platforms and systems may require additional privileges to change this option. If the option is not supported, tcp_setopt/2 raises a domain_error exception. See Wikipedia for details.

broadcast

UDP sockets only: broadcast the package to all addresses matching the address. The address is normally the address of the local subnet (i.e. 192.168.1.255). See udp_send/4.

ip_add_membership(+MultiCastGroup)

ip_add_membership(+MultiCastGroup, +LocalInterface)

ip_add_membership(+MultiCastGroup, +LocalInterface, +InterfaceIndex)

ip_drop_membership(+MultiCastGroup)

ip_drop_membership(+MultiCastGroup, +LocalInterface)

ip_drop_membership(+MultiCastGroup, +LocalInterface, +InterfaceIndex)

Join/leave a multicast group. Calls setsockopt() with the corresponding arguments.

dispatch(+Boolean)

In GUI environments (using XPCE or the Windows swipl-win.exe executable) this flags defines whether or not any events are dispatched on behalf of the user interface. Default is true. Only very specific situations require setting this to false.

sndbuf(+Integer)

Sets the send buffer size to Integer (bytes). On Windows this defaults (now) to 64kb. Higher latency links may benefit from increasing this further since the maximum theoretical throughput on a link is given by buffer-size / latency. See https://support.microsoft.com/en-gb/help/823764/slow-performance-occurs-when-you-copy-data-to-a-tcp-server-by-using-a for Microsoft's discussion

[det]tcp_fcntl(+Stream, +Action, ?Argument)

Interface to the fcntl() call. Currently only suitable to deal switch stream to non-blocking mode using:

  tcp_fcntl(Stream, setfl, nonblock),

An attempt to read from a non-blocking stream while there is no data available returns -1 (or end_of_file for read/1), but at_end_of_stream/1 fails. On actual end-of-input, at_end_of_stream/1 succeeds.

[semidet]tcp_getopt(+Socket, ?Option)

Get information about Socket. Defined properties are below. Requesting an unknown option results in a domain_error exception.

file_no(-File)

Get the OS file handle as an integer. This may be used for debugging and integration.

[nondet]host_address(+HostName, -Address, +Options)

[det]host_address(-HostName, +Address, +Options)

Translate between a machines host-name and it's (IP-)address. Supported options:

domain(+Domain)

One of inet or inet6 to limit the results to the given family.

type(+Type)

One of stream or dgram.

canonname(+Boolean)

If true (default false), return the canonical host name in the frist answer

In mode (+,-,+) Address is unified to a dict with the following keys:

address

A Prolog terms describing the ip address.

domain

One of inet or inet6. The underlying getaddrinfo() calls this family. We use domain for consistency with socket_create/2.

type

Currently one of stream or dgram.

host

Available if canonname(true) is specified on the first returned address. Holds the official canonical host name.

[det]tcp_host_to_address(?HostName, ?Address)

Translate between a machines host-name and it's (IP-)address. If HostName is an atom, it is resolved using getaddrinfo() and the IP-number is unified to Address using a term of the format ip(Byte1,Byte2,Byte3,Byte4). Otherwise, if Address is bound to an ip(Byte1,Byte2,Byte3,Byte4) term, it is resolved by gethostbyaddr() and the canonical hostname is unified with HostName.

deprecated

New code should use host_address/3. This version is bootstrapped from host_address/3 and only searches for IP4 addresses that support TCP connections.

[det]gethostname(-Hostname)

Return the canonical fully qualified name of this host. This is achieved by calling gethostname() and return the canonical name returned by getaddrinfo().

[det]ip_name(?IP, ?Name)

Translate between the textual representation of an IP address and the Prolog data structure. Prolog represents ip4 addresses as ip(A,B,C,D) and ip6 addresses as ip(A,B,C,D,E,F,H). For example:

?- ip_name(ip(1,2,3,4), Name)
Name = '1.2.3.4'.
?- ip_name(IP, '::').
IP = ip(0,0,0,0,0,0,0,0).
?- ip_name(IP, '1:2::3').
IP = ip(1,2,0,0,0,0,0,3).

[det]negotiate_socks_connection(+DesiredEndpoint, +StreamPair)

Negotiate a connection to DesiredEndpoint over StreamPair. DesiredEndpoint should be in the form of either:

• hostname : port
• ip(A,B,C,D) : port

Errors

socks_error(Details) if the SOCKS negotiation failed.