## Tcl8.6.15/Tk8.6.15 Documentation > Tcl C API, version 8.6.15 > OpenTcp

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  NAME
    Tcl_OpenTcpClient, Tcl_MakeTcpClientChannel, Tcl_OpenTcpServer — procedures to open channels using TCP sockets
  SYNOPSIS
    #include <tcl.h>
    Tcl_Channel
    Tcl_OpenTcpClient(interp, port, host, myaddr, myport, async)
    Tcl_Channel
    Tcl_MakeTcpClientChannel(sock)
    Tcl_Channel
    Tcl_OpenTcpServer(interp, port, myaddr, proc, clientData)
  ARGUMENTS
  DESCRIPTION
  TCL_OPENTCPCLIENT
  TCL_MAKETCPCLIENTCHANNEL
  TCL_OPENTCPSERVER
  PLATFORM ISSUES
  SEE ALSO
  KEYWORDS

### NAME

Tcl_OpenTcpClient, Tcl_MakeTcpClientChannel, Tcl_OpenTcpServer — procedures to open channels using TCP sockets

### SYNOPSIS

#include <tcl.h>
Tcl_Channel
Tcl_OpenTcpClient(interp, port, host, myaddr, myport, async)
Tcl_Channel
Tcl_MakeTcpClientChannel(sock)
Tcl_Channel
Tcl_OpenTcpServer(interp, port, myaddr, proc, clientData)

### ARGUMENTS

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

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

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

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

const char *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.

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

ClientData sock (in)
  Platform-specific 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.

### DESCRIPTION

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

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. www.sunlabs.com), or as a string containing the alphanumeric representation of its four-byte address (e.g. 127.0.0.1). 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 the interpreter's result.

The newly created channel is not registered in the supplied interpreter; to register it, use Tcl_RegisterChannel. If one of the standard channels, stdin, stdout or stderr was previously closed, the act of creating the new channel also assigns it as a replacement for the standard channel.

#### TCL_MAKETCPCLIENTCHANNEL

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

The newly created channel is not registered in the supplied interpreter; to register it, use Tcl_RegisterChannel. If one of the standard channels, stdin, stdout or stderr was previously closed, the act of creating the new channel also assigns it as a replacement for the standard channel.

#### TCL_OPENTCPSERVER

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 the interpreter is non-NULL, an error message is left in the interpreter's 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.

The newly created channel is not registered in the supplied interpreter; to register it, use Tcl_RegisterChannel. If one of the standard channels, stdin, stdout or stderr was previously closed, the act of creating the new channel also assigns it as a replacement for the standard channel.

### PLATFORM ISSUES

On Unix platforms, the socket handle is a Unix file descriptor as returned by the socket system call. On the Windows platform, the socket handle is a SOCKET as defined in the WinSock API.

### SEE ALSO

Tcl_OpenFileChannel, Tcl_RegisterChannel, vwait

### KEYWORDS

channel, client, server, socket, TCP
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