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 three
options:
socket which synchronises socket
events in the GUI event-loop.
Using this library to establish a TCP connection to a server is as simple as opening a file. See also http_open/3.
dump_swi_homepage :-
setup_call_cleanup(
tcp_connect(www.swi-prolog.org:http, Stream, []),
( format(Stream,
'GET / HTTP/1.1~n\c
Host: www.swi-prolog.org~n\c
Connection: close~n~n', []),
flush_output(Stream),
copy_stream_data(Stream, current_output)
),
close(S)).
To deal with timeouts and multiple connections, threads, wait_for_input/3 and/or non-blocking streams (see tcp_fcntl/3) can be used.
The typical sequence for generating a server application is given below. To close the server, use close/1 on AcceptFd.
create_server(Port) :-
tcp_socket(Socket),
tcp_bind(Socket, Port),
tcp_listen(Socket, 5),
tcp_open_socket(Socket, AcceptFd, _),
<dispatch>
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 fork-semantics.
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.
dispatch(AcceptFd) :-
tcp_accept(AcceptFd, Socket, Peer),
thread_create(process_client(Socket, Peer), _,
[ detached(true)
]),
dispatch(AcceptFd).
process_client(Socket, Peer) :-
setup_call_cleanup(
tcp_open_socket(Socket, StreamPair),
handle_service(StreamPair),
close(StreamPair)).
handle_service(StreamPair) :-
...
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 defined.
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.
af_unix if Socket
is an AF_UNIX socket (see
unix_domain_socket/1).
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.
:- 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).
tcp_connect(+Address, -StreamPair, +Options).false. If true, do not attempt to
use any proxies to obtain the connectionfalse. If true, set nodelay on the
resulting socket using tcp_setopt(Socket, nodelay)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. |
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.library(http/http_proxy) defines a hook that allows to
connect through HTTP proxies that support the CONNECT
method.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.
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.
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.
setsockopt()
and the socket interface (e.g.,
socket(7) on Linux) for details.
tcp_socket(Socket), tcp_setopt(Socket, bindtodevice(lo))
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.setsockopt() with the
corresponding arguments.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.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.
domain_error
exception.
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.
gethostname() and return the canonical name
returned by getaddrinfo().
ip(A,B,C,D) : portsocks_error(Details) if the SOCKS negotiation failed.
Unix domain sockets (sockets with address family AF_UNIX) are represented as a (socket) file in the file system. They can only be used to connect processes on the same host. The main advantage of AF_UNIX sockets is that name conflicts are much easier to manage than port conflicts and that access is determined by the file system permission rules.
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.
The socket library provides support for UDP sockets. The UDP protocol is a connection-less and unreliable datagram based protocol. That means that messages sent may or may not arrive at the client side, may arrive in a different order then they were sent or even may arrive multiple times. UDP messages are often used for data replication, streaming media, or service discovery.
The library library(udp_broadcast) provides a high-level
interface that builds on top of the library(broadcast)
facilitating an unreliable
publish-subscribe pattern based communication channel. The
library(udp_broadcast) library supports multiple network
architectures, e.g., classical LAN broadcasting, multicast and
unicast.
SOCK_DGRAM protocol, ready
for UDP connections.atom, codes, string (default) or
term. The value term causes the data to be
parsed to a Prolog string.octet. iso_latin_1, text
or
utf8.The typical sequence to receive UDP data is:
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.
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 terms1write_canonical/2
does not support blobs nor cyclic terms
can be sent reliably using the option list [as(term),encoding(utf8)]),
using the same option list for udp_receive/4.A simple example to send UDP data is:
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.
The normal mechanism to discover a service on the local network is for the client to send a broadcast message to an agreed port. The server receives this message and replies to the client with a message indicating further details to establish the communication.