1socket(n) Tcl Built-In Commands socket(n)
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8 socket - Open a TCP network connection
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11 socket ?options? host port
12 socket -server command ?options? port
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18 This command opens a network socket and returns a channel identifier
19 that may be used in future invocations of commands like read, puts and
20 flush. At present only the TCP network protocol is supported; future
21 releases may include support for additional protocols. The socket com‐
22 mand may be used to open either the client or server side of a connec‐
23 tion, depending on whether the -server switch is specified.
24 Note that the default encoding for all sockets is the system encoding,
26 as returned by encoding system. Most of the time, you will need to use
27 fconfigure to alter this to something else, such as utf-8 (ideal for
28 communicating with other Tcl processes) or iso8859-1 (useful for many
29 network protocols, especially the older ones).
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32 If the -server option is not specified, then the client side of a con‐
33 nection is opened and the command returns a channel identifier that can
34 be used for both reading and writing. Port and host specify a port to
35 connect to; there must be a server accepting connections on this port.
36 Port is an integer port number (or service name, where supported and
37 understood by the host operating system) and host is either a domain-
38 style name such as www.tcl.tk or a numerical IP address such as
39 127.0.0.1. Use localhost to refer to the host on which the command is
40 invoked.
41 The following options may also be present before host to specify addi‐
43 tional information about the connection:
44 -myaddr addr
46 Addr gives the domain-style name or numerical IP address of the
47 client-side network interface to use for the connection. This
48 option may be useful if the client machine has multiple network
49 interfaces. If the option is omitted then the client-side
50 interface will be chosen by the system software.
51 -myport port
53 Port specifies an integer port number (or service name, where
54 supported and understood by the host operating system) to use
55 for the client's side of the connection. If this option is
56 omitted, the client's port number will be chosen at random by
57 the system software.
58 -async The -async option will cause the client socket to be connected
60 asynchronously. This means that the socket will be created imme‐
61 diately but may not yet be connected to the server, when the
62 call to socket returns. When a gets or flush is done on the
63 socket before the connection attempt succeeds or fails, if the
64 socket is in blocking mode, the operation will wait until the
65 connection is completed or fails. If the socket is in nonblock‐
66 ing mode and a gets or flush is done on the socket before the
67 connection attempt succeeds or fails, the operation returns
68 immediately and fblocked on the socket returns 1.
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71 If the -server option is specified then the new socket will be a server
72 for the port given by port (either an integer or a service name, where
73 supported and understood by the host operating system; if port is zero,
74 the operating system will allocate a free port to the server socket
75 which may be discovered by using fconfigure to read the -sockname
76 option). Tcl will automatically accept connections to the given port.
77 For each connection Tcl will create a new channel that may be used to
78 communicate with the client. Tcl then invokes command with three addi‐
79 tional arguments: the name of the new channel, the address, in network
80 address notation, of the client's host, and the client's port number.
81 The following additional option may also be specified before host:
83 -myaddr addr
85 Addr gives the domain-style name or numerical IP address of the
86 server-side network interface to use for the connection. This
87 option may be useful if the server machine has multiple network
88 interfaces. If the option is omitted then the server socket is
89 bound to the special address INADDR_ANY so that it can accept
90 connections from any interface.
91 Server channels cannot be used for input or output; their sole use is
93 to accept new client connections. The channels created for each incom‐
94 ing client connection are opened for input and output. Closing the
95 server channel shuts down the server so that no new connections will be
96 accepted; however, existing connections will be unaffected.
97 Server sockets depend on the Tcl event mechanism to find out when new
99 connections are opened. If the application doesn't enter the event
100 loop, for example by invoking the vwait command or calling the C proce‐
101 dure Tcl_DoOneEvent, then no connections will be accepted.
102 If port is specified as zero, the operating system will allocate an
104 unused port for use as a server socket. The port number actually allo‐
105 cated may be retrieved from the created server socket using the fcon‐
106 figure command to retrieve the -sockname option as described below.
107
109 The fconfigure command can be used to query several readonly configura‐
110 tion options for socket channels:
111 -error This option gets the current error status of the given socket.
113 This is useful when you need to determine if an asynchronous
114 connect operation succeeded. If there was an error, the error
115 message is returned. If there was no error, an empty string is
116 returned.
117 -sockname
119 This option returns a list of three elements, the address, the
120 host name and the port number for the socket. If the host name
121 cannot be computed, the second element is identical to the
122 address, the first element of the list.
123 -peername
125 This option is not supported by server sockets. For client and
126 accepted sockets, this option returns a list of three elements;
127 these are the address, the host name and the port to which the
128 peer socket is connected or bound. If the host name cannot be
129 computed, the second element of the list is identical to the
130 address, its first element.
131
133 Here is a very simple time server:
134 proc Server {channel clientaddr clientport} {
135 puts "Connection from $clientaddr registered"
136 puts $channel [clock format [clock seconds]]
137 close $channel
138 }
139 socket -server Server 9900
141 vwait forever
142 And here is the corresponding client to talk to the server:
144 set server localhost
145 set sockChan [socket $server 9900]
146 gets $sockChan line
147 close $sockChan
148 puts "The time on $server is $line"
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152 fconfigure(n), flush(n), open(n), read(n)
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156 bind, channel, connection, domain name, host, network address, socket,
157 tcp
158Tcl 8.0 socket(n)