'\" '\" Copyright (c) 1996 Sun Microsystems, Inc. '\" Copyright (c) 1998-1999 by Scriptics Corporation. '\" '\" See the file "license.terms" for information on usage and redistribution '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES. '\" '\" RCS: @(#) socket.n,v 1.5 2003/01/21 19:40:00 hunt Exp .so man.macros .TH socket n 8.0 Tcl "Tcl Built-In Commands" .BS '\" Note: do not modify the .SH NAME line immediately below! .SH NAME socket \- Open a TCP network connection .SH SYNOPSIS .sp \fBsocket \fR?\fIoptions\fR? \fIhost port\fR .sp \fBsocket\fR \fB\-server \fIcommand\fR ?\fIoptions\fR? \fIport\fR .BE .SH DESCRIPTION .PP This command opens a network socket and returns a channel identifier that may be used in future invocations of commands like \fBread\fR, \fBputs\fR and \fBflush\fR. At present only the TCP network protocol is supported; future releases may include support for additional protocols. The \fBsocket\fR command may be used to open either the client or server side of a connection, depending on whether the \fB\-server\fR switch is specified. .SH "CLIENT SOCKETS" .PP If the \fB\-server\fR option is not specified, then the client side of a connection is opened and the command returns a channel identifier that can be used for both reading and writing. \fIPort\fR and \fIhost\fR specify a port to connect to; there must be a server accepting connections on this port. \fIPort\fR is an integer port number (or service name, where supported and understood by the host operating system) and \fIhost\fR is either a domain-style name such as \fBwww.sunlabs.com\fR or a numerical IP address such as \fB127.0.0.1\fR. Use \fIlocalhost\fR to refer to the host on which the command is invoked. .PP The following options may also be present before \fIhost\fR to specify additional information about the connection: .TP \fB\-myaddr\fI addr\fR \fIAddr\fR gives the domain-style name or numerical IP address of the client-side network interface to use for the connection. This option may be useful if the client machine has multiple network interfaces. If the option is omitted then the client-side interface will be chosen by the system software. .TP \fB\-myport\fI port\fR \fIPort\fR specifies an integer port number (or service name, where supported and understood by the host operating system) to use for the client's side of the connection. If this option is omitted, the client's port number will be chosen at random by the system software. .TP \fB\-async\fR The \fB\-async\fR option will cause the client socket to be connected asynchronously. This means that the socket will be created immediately but may not yet be connected to the server, when the call to \fBsocket\fR returns. When a \fBgets\fR or \fBflush\fR is done on the socket before the connection attempt succeeds or fails, if the socket is in blocking mode, the operation will wait until the connection is completed or fails. If the socket is in nonblocking mode and a \fBgets\fR or \fBflush\fR is done on the socket before the connection attempt succeeds or fails, the operation returns immediately and \fBfblocked\fR on the socket returns 1. .SH "SERVER SOCKETS" .PP If the \fB\-server\fR option is specified then the new socket will be a server for the port given by \fIport\fR (either an integer or a service name, where supported and understood by the host operating system). Tcl will automatically accept connections to the given port. For each connection Tcl will create a new channel that may be used to communicate with the client. Tcl then invokes \fIcommand\fR with three additional arguments: the name of the new channel, the address, in network address notation, of the client's host, and the client's port number. .PP The following additional option may also be specified before \fIhost\fR: .TP \fB\-myaddr\fI addr\fR \fIAddr\fR gives the domain-style name or numerical IP address of the server-side network interface to use for the connection. This option may be useful if the server machine has multiple network interfaces. If the option is omitted then the server socket is bound to the special address INADDR_ANY so that it can accept connections from any interface. .PP Server channels cannot be used for input or output; their sole use is to accept new client connections. The channels created for each incoming client connection are opened for input and output. Closing the server channel shuts down the server so that no new connections will be accepted; however, existing connections will be unaffected. .PP Server sockets depend on the Tcl event mechanism to find out when new connections are opened. If the application doesn't enter the event loop, for example by invoking the \fBvwait\fR command or calling the C procedure \fBTcl_DoOneEvent\fR, then no connections will be accepted. .PP If \fIport\fR is specified as zero, the operating system will allocate an unused port for use as a server socket. The port number actually allocated my be retrieved from the created server socket using the \fBfconfigure\fR command to retrieve the \fB\-sockname\fR option as described below. .SH "CONFIGURATION OPTIONS" The \fBfconfigure\fR command can be used to query several readonly configuration options for socket channels: .TP \fB\-error\fR This option gets the current error status of the given socket. This is useful when you need to determine if an asynchronous connect operation succeeded. If there was an error, the error message is returned. If there was no error, an empty string is returned. .TP \fB\-sockname\fR This option returns a list of three elements, the address, the host name and the port number for the socket. If the host name cannot be computed, the second element is identical to the address, the first element of the list. .TP \fB\-peername\fR This option is not supported by server sockets. For client and accepted sockets, this option returns a list of three elements; these are the address, the host name and the port to which the peer socket is connected or bound. If the host name cannot be computed, the second element of the list is identical to the address, its first element. .PP .SH "SEE ALSO" flush(n), open(n), read(n) .SH KEYWORDS bind, channel, connection, domain name, host, network address, socket, tcp