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Technical Reference: Communications, Volume 2

connect Subroutine


Connects two sockets.


Standard C Library (libc.a


#include <sys/socket.h> 

int connect ( Socket, Name, NameLength)
int Socket;
const struct sockaddr *Name;
socklen_t NameLength;


The connect subroutine requests a connection between two sockets. The kernel sets up the communication link between the sockets; both sockets must use the same address format and protocol.

If a connect subroutine is issued on an unbound socket, the system automatically binds the socket.

The connect subroutine performs a different action for each of the following two types of initiating sockets:


Socket Specifies the unique name of the socket.
Name Specifies the address of target socket that will form the other end of the communication line
NameLength Specifies the length of the address structure.

Return Values

Upon successful completion, the connect subroutine returns a value of 0.

If the connect subroutine is unsuccessful, the system handler performs the following functions:

Error Codes

The connect subroutine is unsuccessful if any of the following errors occurs:

EBADF The Socket parameter is not valid.
ENOTSOCK The Socket parameter refers to a file, not a socket.
EADDRNOTAVAIL The specified address is not available from the local machine.
EAFNOSUPPORT The addresses in the specified address family cannot be used with this socket.
EISCONN The socket is already connected.
ETIMEDOUT The establishment of a connection timed out before a connection was made.
ECONNREFUSED The attempt to connect was rejected.
ENETUNREACH No route to the network or host is present.
EADDRINUSE The specified address is already in use.
EFAULT The Address parameter is not in a writable part of the user address space.
EINPROGRESS The socket is marked as nonblocking. The connection cannot be immediately completed. The application program can select the socket for writing during the connection process.
EINVAL The specified path name contains a character with the high-order bit set.
ENETDOWN The specified physical network is down.
ENOSPC There is no space left on a device or system table.
ENOTCONN The socket could not be connected.


The following program fragment illustrates the use of the connect subroutine by a client to initiate a connection to a server's socket.

struct sockaddr_un server;
connect(s,(struct sockaddr*)&server, sun_len(&server));

Implementation Specifics

The connect subroutine is part of Base Operating System (BOS) Runtime.

The socket applications can be compiled with COMPAT_43 defined. This will make the sockaddr structure BSD 4.3 compatible. For more details refer to socket.h.

If the environment variable SOCKS5C_CONFIG is set, the connect subroutine may attempt to make the requested connection through a SOCKS server.

Related Information

The accept subroutine, bind subroutine, getsockname subroutine, send subroutine, socket, subroutine, socks5tcp_connect subroutine.

Initiating UNIX Stream Connections Example Program, Sockets Overview, and Understanding Socket Connections in AIX 5L Version 5.1 Communications Programming Concepts.

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