connect

The Windows Sockets connect function establishes a connection to a peer.

int connect (

SOCKET s,
const struct sockaddr FAR* name,
int namelen
);

Parameters

s

[in] A descriptor identifying an unconnected socket.

name

[in] The name of the peer to which the socket is to be connected.

namelen

[in] The length of the name.

Remarks

This function is used to create a connection to the specified destination. If the socket, s, is unbound, unique values are assigned to the local association by the system, and the socket is marked as bound.

For connection-oriented sockets (for example, type SOCK_STREAM), an active connection is initiated to the foreign host using name (an address in the name space of the socket; for a detailed description, please see bind). When the socket call completes successfully, the socket is ready to send/receive data. If the address field of the name structure is all zeroes, connect will return the error WSAEADDRNOTAVAIL. Any attempt to re-connect an active connection will fail with the error code WSAEISCONN.

For a connectionless socket (for example, type SOCK_DGRAM), the operation performed by connect is merely to establish a default destination address which will be used on subsequent send/WSASend and recv/WSARecv calls. Any datagrams received from an address other than the destination address specified will be discarded. If the address field of the name structure is all zeroes, the socket will be "dis-connected." Then, the default remote address will be indeterminate, so send/WSASend and recv/WSARecv calls will return the error code WSAENOTCONN. However, sendto/WSASendTo and recvfrom/WSARecvFrom can still be used. The default destination can be changed by simply calling connect again, even if the socket is already "connected". Any datagrams queued for receipt are discarded if name is different from the previous connect.

For connectionless sockets, name can indicate any valid address, including a broadcast address. However, to connect to a broadcast address, a socket must have setsockopt SO_BROADCAST enabled. Otherwise, connect will fail with the error code WSAEACCES.

Comments

When connected sockets break (that is, become closed for whatever reason), they should be discarded and recreated. It is safest to assume that when things go awry for any reason on a connected socket, the application must discard and recreate the needed sockets in order to return to a stable point.

Return Values

If no error occurs, connect returns zero. Otherwise, it returns SOCKET_ERROR, and a specific error code can be retrieved by calling WSAGetLastError.

On a blocking socket, the return value indicates success or failure of the connection attempt.

With a nonblocking socket, the connection attempt cannot be completed immediately. In this case, connect will return SOCKET_ERROR, and WSAGetLastError will return WSAEWOULDBLOCK. In this case, the application can:

1. Use select to determine the completion of the connection request by checking if the socket is writeable, or

2. If your application is using WSAAsyncSelect to indicate interest in connection events, then your application will receive an FD_CONNECT notification when the connect operation is complete, or

3. If your application is using WSAEventSelect to indicate interest in connection events, then the associated event object will be signaled when the connect operation is complete.

For a nonblocking socket, until the connection attempt completes all subsequent calls to connect on the same socket will fail with the error code WSAEALREADY.

If the return error code indicates the connection attempt failed (that is, WSAECONNREFUSED, WSAENETUNREACH, WSAETIMEDOUT) the application can call connect again for the same socket.

Error Codes

WSANOTINITIALISED	A successful WSAStartup must occur before using this function.
WSAENETDOWN	The network subsystem has failed.
WSAEADDRINUSE	The specified address is already in use.
WSAEINTR	The (blocking) call was canceled through WSACancelBlockingCall.
WSAEINPROGRESS	A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
WSAEALREADY	A nonblocking connect call is in progress on the specified socket. Note In order to preserve backward compatibility, this error is reported as WSAEINVAL to Windows Sockets 1.1 applications that link to either WINSOCK.DLL or WSOCK32.DLL.
WSAEADDRNOTAVAIL	The specified address is not available from the local machine.
WSAEAFNOSUPPORT	Addresses in the specified family cannot be used with this socket.
WSAECONNREFUSED	The attempt to connect was forcefully rejected.
WSAEFAULT	The name or the namelen argument is not a valid part of the user address space, the namelen argument is too small, or the name argument contains incorrect address format for the associated address family.
WSAEINVAL	The parameter s is a listening socket, or the destination address specified is not consistent with that of the constrained group the socket belongs to.
WSAEISCONN	The socket is already connected (connection-oriented sockets only).
WSAENETUNREACH	The network cannot be reached from this host at this time.
WSAENOBUFS	No buffer space is available. The socket cannot be connected.
WSAENOTSOCK	The descriptor is not a socket.
WSAETIMEDOUT	Attempt to connect timed out without establishing a connection.
WSAEWOULDBLOCK	The socket is marked as nonblocking and the connection cannot be completed immediately. It is possible to select the socket while it is connecting by selecting it for writing.
WSAEACCES	Attempt to connect datagram socket to broadcast address failed because setsockopt SO_BROADCAST is not enabled.

See Also

accept, bind, getsockname, select, socket, WSAAsyncSelect, WSAConnect

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