The library(socket) provides TCP and UDP inet-domain sockets from
SWI-Prolog, both client and server-side communication. The interface of
this library is very close to the Unix socket interface, also supported
by the MS-Windows winsock API. SWI-Prolog applications that wish to
communicate with multiple sources have two options:
- Use I/O multiplexing based on wait_for_input/3. On Windows
systems this can only be used for sockets, not for general
(device-) file handles.
- Use multiple threads, handling either a single blocking socket
or a pool using I/O multiplexing as above.
Using this library to establish a TCP connection to a server is as
simple as opening a file. See also http_open/3.
tcp_connect('www.swi-prolog.org':http, Stream, ),
'GET / HTTP/1.1~n\c
Connection: close~n~n', ),
To deal with timeouts and multiple connections, threads,
wait_for_input/3 and/or non-blocking streams (see tcp_fcntl/3) can be
The typical sequence for generating a server application is given below.
To close the server, use close/1 on AcceptFd.
tcp_open_socket(Socket, AcceptFd, _),
There are various options for <dispatch>. The most commonly used option
is to start a Prolog thread to handle the connection. Alternatively,
input from multiple clients can be handled in a single thread by
listening to these clients using wait_for_input/3. Finally, on Unix
systems, we can use fork/1 to handle the connection in a new process.
Note that fork/1 and threads do not cooperate well. Combinations can be
realised but require good understanding of POSIX thread and
Below is the typical example using a thread. Note the use of
setup_call_cleanup/3 to guarantee that all resources are reclaimed, also
in case of failure or exceptions.
tcp_accept(AcceptFd, Socket, Peer),
thread_create(process_client(Socket, Peer), _,
process_client(Socket, Peer) :-
Errors that are trapped by the low-level library are mapped to an
exception of the shape below. In this term, Code is a lower case atom
that corresponds to the C macro name, e.g.,
epipe for a broken pipe.
Message is the human readable string for the error code returned by
the OS or the same as Code if the OS does not provide this
functionality. Note that Code is derived from a static set of macros
that may or may not be defines for the target OS. If the macro name is
not known, Code is
ERROR_nnn, where nnn is an integer.
error(socket_error(Code, Message), _)
Note that on Windows Code is a
wsa* code which makes it hard to
write portable code that handles specific socket errors. Even on POSIX
systems the exact set of errors produced by the network stack is not
Socket addresses (families)
The library supports both IP4 and IP6 addresses. On Unix systems it also
supports Unix domain sockets (
AF_UNIX). The address of a Unix
domain sockets is a file name. Unix domain sockets are created using
socket_create/2 or unix_domain_socket/1.
IP4 or IP6 sockets can be created using socket_create/2 or tcp_connect/3
inet (default, IP3) or
inet6 domain option. Some of the
predicates produce or consume IP addresses as a Prolog term. The format
of this term is one of:
- ip(A, B, C, D)
- Represents an IP4 address. Each field is an integer in the range
0..255 (8 bit).
- ip(A, B, C, D, E, F, G, H)
- Represents an IP6 address. Each field is an integer in the range
0..65535 (16 bit).
The predicate ip_name/2 translates between the canonical textual
representation and the above defined address terms.
Socket predicate reference
- socket_create(-SocketId, +Options) is det
- Create a socket according to Options. Supported Options are:
- One of
- 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
- tcp_socket(-SocketId) is det
- Equivalent to
socket_create(SocketId, ) or, explicit,
socket_create(SocketId, [domain(inet), type(stream)]).
- unix_domain_socket(-SocketId) is det
- Equivalent to
socket_create(SocketId, [domain(unix)]) or,
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 (
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.
- tcp_close_socket(+SocketId) is det
- 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
- 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.
- tcp_open_socket(+SocketId, -StreamPair) is det
- 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.
- tcp_open_socket(+SocketId, -InStream, -OutStream) is det
- Similar to tcp_open_socket/2, but creates two separate sockets
where tcp_open_socket/2 would have created a stream pair.
- - New code should use tcp_open_socket/2 because
closing a stream pair is much easier to perform safely.
- tcp_bind(SocketId, ?Address) is det
- 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
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_listen(+SocketId, +BackLog) is det
- 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.
- tcp_accept(+Socket, -Slave, -Peer) is det
- 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
- tcp_connect(+SocketId, +Address) is det
- 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:
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.
tcp_connect(Host:Port, StreamPair, ),
If SocketId is an AF_UNIX socket (see unix_domain_socket/1), Address
is an atom or string denoting a file name.
- rewrite_host(+HostIn, -HostOut, +Socket) is nondet[multifile]
- 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
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.
- tcp_connect(+Socket, +Address, -Read, -Write) is det
- 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 tcp_connect_hook/4
with the same signature. Hooking can be used to deal with proxy
:- multifile socket:tcp_connect_hook/4.
socket:tcp_connect_hook(Socket, Address, Read, Write) :-
tcp_open_socket(Socket, Read, Write),
proxy_connect(Address, Read, Write).
- - New code should use tcp_connect/3 called as
tcp_connect(+Address, -StreamPair, +Options).
- tcp_connect(+Address, -StreamPair, +Options) is det
- tcp_connect(+Socket, +Address, -StreamPair) is det
- 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:
- Defaults to
true, do not attempt to use any
proxies to obtain the connection
- Defaults to
true, set nodelay on the
resulting socket using
- One of `inet' or
inet6. When omitted we use host_address/2
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
|Address||- is either a Host:Port term or a file name (atom or
string). The latter connects to an AF_UNIX socket and requires
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
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
- 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
underlying wait_for_input/3. As input multiplexing typically happens
in a background thread anyway we accept the loss of timeouts and
- - Use wait_for_input/3
- try_proxy(+Proxy, +TargetAddress, -Socket, -StreamPair) is semidet[multifile]
- 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 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
- Do not use any proxy
- socks(Host, Port)
- Use a SOCKS5 proxy
- proxy_for_url(+URL, +Hostname, -Proxy) is nondet[multifile]
- 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:
- 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
Additional methods can be returned if suitable clauses for
http:http_connection_over_proxy/6 or try_proxy/4 are defined.
- udp_socket(-SocketId) is det
- Equivalent to
socket_create(SocketId, [type(dgram)]) or, explicit,
socket_create(SocketId, [domain(inet), type(dgram)]).
- udp_receive(+Socket, -Data, -From, +Options) is det
- 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:
- Defines the type for Data. Possible values are
string (default) or
term (parse as Prolog term).
- Specify the encoding used to interpret the message. It is one of
- 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.
udp_receive(Socket, Data, From, [as(atom)]),
format('Got ~q from ~q~n', [Data, From]),
- udp_send(+Socket, +Data, +To, +Options) is det
- 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:
- Specifies the encoding to use for the string. See
udp_receive/4 for details
- 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.
term maps to CVT_WRITE_CANONICAL. This implies that
arbitrary Prolog terms can be sent reliably using the option
encoding(utf8)])`, using the same option list
send(Host, Port, Message) :-
udp_send(S, Message, Host:Port, ),
A broadcast is achieved by using
prior to sending the datagram and using the local network
broadcast address as a ip/4 term.
- tcp_setopt(+SocketId, +Option) is det
- Set options on the socket. Defined options are:
- Allow servers to reuse a port without the system being
completely sure the port is no longer in use.
- 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
setsockopt() and the socket interface (e.g.,
socket(7) on Linux) for details.
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
- 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, +LocalInterface)
- ip_add_membership(+MultiCastGroup, +LocalInterface, +InterfaceIndex)
- ip_drop_membership(+MultiCastGroup, +LocalInterface)
- ip_drop_membership(+MultiCastGroup, +LocalInterface, +InterfaceIndex)
- Join/leave a multicast group. Calls
setsockopt() with the
- In GUI environments (using XPCE or the Windows
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
- 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.
for Microsoft's discussion
- tcp_fcntl(+Stream, +Action, ?Argument) is det
- 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,
- tcp_getopt(+Socket, ?Option) is semidet
- Get information about Socket. Defined properties are below.
Requesting an unknown option results in a
- Get the OS file handle as an integer. This may be used for
debugging and integration.
- host_address(+HostName, -Address, +Options) is nondet
- host_address(-HostName, +Address, +Options) is det
- Translate between a machines host-name and it's (IP-)address.
- One of
inet6 to limit the results to the given
- One of
false), return the canonical host name
in the frist answer
In mode (+,-,+) Address is unified to a dict with the following keys:
- A Prolog terms describing the ip address.
- One of
inet6. The underlying
family. We use
domain for consistency with
- Currently one of
- Available if
canonname(true) is specified on the first
returned address. Holds the official canonical host name.
- tcp_host_to_address(?HostName, ?Address) is det
- 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
and the canonical hostname is unified with HostName.
- - 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.
- gethostname(-Hostname) is det
- Return the canonical fully qualified name of this host. This is
achieved by calling
gethostname() and return the canonical name
- ip_name(?IP, ?Name) is det
- 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 = '18.104.22.168'.
?- 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).
- negotiate_socks_connection(+DesiredEndpoint, +StreamPair) is det
- Negotiate a connection to DesiredEndpoint over StreamPair.
DesiredEndpoint should be in the form of either:
- hostname : port
ip(A,B,C,D) : port
socks_error(Details) if the SOCKS negotiation failed.