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(In, 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.
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))
:- 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).
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.
The current library provides limited support for UDP packets. 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 and may arrive in a different order as they are sent. UDP messages are often used for streaming media or for service discovery using the broadcasting mechanism.
SOCK_DGRAM protocol, ready
for UDP connections.atom, codes or string (default).The typical sequence to receive UDP data is:
receive(Port) :-
udp_socket(S),
tcp_bind(S, Port),
repeat,
udp_receive(Socket, Data, From, [as(atom)]),
format('Got ~q from ~q~n', [Data, From]),
fail.
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.