connect Subroutine
Purpose
Connects two sockets.
Library
Standard C Library (libc.a
Syntax
#include <sys/socket.h>
int Socket;
const struct sockaddr *Name;
socklen_t NameLength;
Description
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 or a partially bound socket (a socket that is assigned a port number but no IP address), the system automatically binds the socket. The connect subroutine can be used to connect a socket to itself. This can be done, for example, by binding a socket to a local port (using bind) and then connecting it to the same port with a local IP address (using connect).
The connect subroutine performs a different action for each of the following two types of initiating sockets:
- If the initiating socket is SOCK_DGRAM, the connect subroutine establishes the peer address. The peer address identifies the socket where all datagrams are sent on subsequent send subroutines. No connections are made by this connect subroutine. If the UDP socket is receiving datagrams when the connect subroutine is called, the subroutine will change the IP address, preventing the socket from receiving datagram packets based on the previous address.
- If the initiating socket is SOCK_STREAM or SOCK_CONN_DGRAM, the connect subroutine attempts to make a connection to the socket specified by the Name parameter. Each communication space interprets the Name parameter differently. For SOCK_CONN_DGRAM socket type and ATM protocol, some of the ATM parameters may have been modified by the remote station, applications may query new values of ATM parameters using the appropriate socket options.
- In the case of a UNIX domain socket, a connect call only succeeds if the process that calls connect has read and write permissions on the socket file created by the bind call. Permissions are determined by the umask< value of the process that created the file.
Implementation Specifics
Parameters
| Item | Description |
|---|---|
| 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:
- Returns a value of -1 to the calling program.
- Moves an error code, indicating the specific error, into the errno global variable.
Error Codes
The connect subroutine is unsuccessful if any of the following errors occurs:
| Value | Description |
|---|---|
| EADDRINUSE | The specified address is already in use. This error will also occur if the SO_REUSEADDR socket option was set and the local address (whether specified or selected by the system) is already in use. |
| 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. |
| EALREADY | The socket is specified with O_NONBLOCK or O_NDLAY, and a previous connecttion attempt has not yet completed. |
| EINTR | The attempt to establish a connection was interrupted by delivery of a signal that was caught; the connection will be established asynchronously. |
| EACCES | Search permission is denied on a component of the path prefix or write access to the named socket is denied. |
| ENOBUFS | The system ran out of memory for an internal data structure. |
| EOPNOTSUPP | The socket referenced by Socket parameter does not support connect. |
| EWOULDBLOCK | The range allocated for TCP/UDP ephemeral ports has been exhausted. |
| EBADF | The Socket parameter is not valid. |
| ECONNREFUSED | The attempt to connect was rejected. |
| 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. |
| EISCONN | The socket is already connected. |
| ENETDOWN | The specified physical network is down. |
| ENETUNREACH | No route to the network or host is present. |
| ENOSPC | There is no space left on a device or system table. |
| ENOTCONN | The socket could not be connected. |
| ENOTSOCK | The Socket parameter refers to a file, not a socket. |
| ETIMEDOUT | The establishment of a connection timed out before a connection was made. |
| EPROTOTYPE | The specified address has a different type from the socket that is bound to the specified peer address. |
| ELOOP | Too many symbolic links were encountered in translating the path name in address. |
| ENOENT | A component of the path name does not name an existing file or the path name is an empty string. |
| ENOTDIR | A component of the path prefix of the path name in address is not a directory. |
Examples
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));