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|library(socket): Network socket (TCP and UDP) library|
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
socketwhich 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)).
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.,
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
where nnn is an integer.
error(socket_error(Code, Message), _)
Note that on Windows Code is a
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.
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_unixif 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).
true, do not attempt to use any proxies to obtain the connection
true, set nodelay on the resulting socket using
|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
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
proxy(Host,Port) which allows for HTTP proxies using the
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.exeexecutable) 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
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),
at_end_of_stream/1 fails. On actual
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
socks_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
(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
representation_error(af_unix_name) exception is
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.
library(udp_broadcast) provides a high-level
interface that builds on top of the
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
SOCK_DGRAMprotocol, ready for UDP connections.
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
prior to sending the datagram and using the local network broadcast
address as a ip/4
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.