Tcl7.6 C API Man Page -- OpenTcp (n)
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Tcl_OpenTcpClient, Tcl_OpenTcpServer - procedures to open channels using TCP sockets


#include <tcl.h>

Tcl_OpenTcpClient(interp, port, host, myaddr, myport, async)


Tcl_OpenTcpServer(interp, port, myaddr, proc, clientData)


Tcl_Interp *interp (in) Tcl interpreter to use for error reporting. If non-NULL and an error occurs, an error message is left in interp>result.

port (in) A port number to connect to as a client or to listen on as a server.

*host (in) A string specifying a host name or address for the remote end of the connection.

myport (in) A port number for the client's end of the socket. If 0, a port number is allocated at random.

*myaddr (in) A string specifying the host name or address for network interface to use for the local end of the connection. If NULL, a default interface is chosen.

async (in) If nonzero, the client socket is connected asynchronously to the server.

ClientData sock (in) Platformspecific handle for client TCP socket.

Tcl_TcpAcceptProc *proc
(in) Pointer to a procedure to invoke each time a new connection is accepted via the socket.

ClientData clientData (in) Arbitrary one-word value to pass to proc.


These functions are convenience procedures for creating channels that communicate over TCP sockets. The operations on a channel are described in the manual entry for Tcl_OpenFileChannel.


Tcl_OpenTcpClient opens a client TCP socket connected to a port on a specific host, and returns a channel that can be used to communicate with the server. The host to connect to can be specified either as a domain name style name (e.g., or as a string containing the alphanumeric representation of its four-byte address (e.g. Use the string localhost to connect to a TCP socket on the host on which the function is invoked.

The myaddr and myport arguments allow a client to specify an address for the local end of the connection. If myaddr is NULL, then an interface is chosen automatically by the operating system. If myport is 0, then a port number is chosen at random by the operating system.

If async is zero, the call to Tcl_OpenTcpClient returns only after the client socket has either successfully connected to the server, or the attempted connection has failed. If async is nonzero the socket is connected asynchronously and the returned channel may not yet be connected to the server when the call to Tcl_OpenTcpClient returns. If the channel is in blocking mode and an input or output operation is done on the channel before the connection is completed or fails, that operation will wait until the connection either completes successfully or fails. If the channel is in nonblocking mode, the input or output operation will return immediately and a subsequent call to Tcl_InputBlocked on the channel will return nonzero.

The returned channel is opened for reading and writing. If an error occurs in opening the socket, Tcl_OpenTcpClient returns NULL and records a POSIX error code that can be retrieved with Tcl_GetErrno. In addition, if interp is non-NULL, an error message is left in interp->result.


Tcl_MakeTcpClientChannel creates a Tcl_Channel around an existing, platform specific, handle for a client TCP socket.


Tcl_OpenTcpServer opens a TCP socket on the local host on a specified port and uses the Tcl event mechanism to accept requests from clients to connect to it. The myaddr argument specifies the network interface. If myaddr is NULL the special address INADDR_ANY should be used to allow connections from any network interface. Each time a client connects to this socket, Tcl creates a channel for the new connection and invokes proc with information about the channel. Proc must match the following prototype:
typedef void Tcl_TcpAcceptProc(
ClientData clientData,
Tcl_Channel channel,
char *hostName,
int port);

The clientData argument will be the same as the clientData argument to Tcl_OpenTcpServer, channel will be the handle for the new channel, hostName points to a string containing the name of the client host making the connection, and port will contain the client's port number. The new channel is opened for both input and output. If proc raises an error, the connection is closed automatically. Proc has no return value, but if it wishes to reject the connection it can close channel.

Tcl_OpenTcpServer normally returns a pointer to a channel representing the server socket. If an error occurs, Tcl_OpenTcpServer returns NULL and records a POSIX error code that can be retrieved with Tcl_GetErrno. In addition, if interp->result is non-NULL, an error message is left in interp->result.

The channel returned by Tcl_OpenTcpServer cannot be used for either input or output. It is simply a handle for the socket used to accept connections. The caller can close the channel to shut down the server and disallow further connections from new clients.

TCP server channels operate correctly only in applications that dispatch events through Tcl_DoOneEvent or through Tcl commands such as vwait; otherwise Tcl will never notice that a connection request from a remote client is pending.


Tcl_OpenFileChannel(3), vwait(n)


client, server, TCP

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