connect function (winsock2.h)

The connect function establishes a connection to a specified socket.


int WSAAPI connect(
  [in] SOCKET         s,
  [in] const sockaddr *name,
  [in] int            namelen


[in] s

A descriptor identifying an unconnected socket.

[in] name

A pointer to the sockaddr structure to which the connection should be established.

[in] namelen

The length, in bytes, of the sockaddr structure pointed to by the name parameter.

Return value

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, there are three possible scenarios:

  • Use the select function to determine the completion of the connection request by checking to see if the socket is writable.
  • If the application is using WSAAsyncSelect to indicate interest in connection events, then the application will receive an FD_CONNECT notification indicating that the connect operation is complete (successfully or not).
  • If the application is using WSAEventSelect to indicate interest in connection events, then the associated event object will be signaled indicating that the connect operation is complete (successfully or not).

Until the connection attempt completes on a nonblocking socket, all subsequent calls to connect on the same socket will fail with the error code WSAEALREADY, and WSAEISCONN when the connection completes successfully. Due to ambiguities in version 1.1 of the Windows Sockets specification, error codes returned from connect while a connection is already pending may vary among implementations. As a result, it is not recommended that applications use multiple calls to connect to detect connection completion. If they do, they must be prepared to handle WSAEINVAL and WSAEWOULDBLOCK error values the same way that they handle WSAEALREADY, to assure robust operation.

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

Error code Meaning
A successful WSAStartup call must occur before using this function.
The network subsystem has failed.
The socket's local address is already in use and the socket was not marked to allow address reuse with SO_REUSEADDR. This error usually occurs when executing bind, but could be delayed until the connect function if the bind was to a wildcard address (INADDR_ANY or in6addr_any) for the local IP address. A specific address needs to be implicitly bound by the connect function.
The blocking Windows Socket 1.1 call was canceled through WSACancelBlockingCall.
A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
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.
The remote address is not a valid address (such as INADDR_ANY or in6addr_any) .
Addresses in the specified family cannot be used with this socket.
The attempt to connect was forcefully rejected.
The sockaddr structure pointed to by the name contains incorrect address format for the associated address family or the namelen parameter is too small. This error is also returned if the sockaddr structure pointed to by the name parameter with a length specified in the namelen parameter is not in a valid part of the user address space.
The parameter s is a listening socket.
The socket is already connected (connection-oriented sockets only).
The network cannot be reached from this host at this time.
A socket operation was attempted to an unreachable host.
Note  No buffer space is available. The socket cannot be connected.
The descriptor specified in the s parameter is not a socket.
An attempt to connect timed out without establishing a connection.
The socket is marked as nonblocking and the connection cannot be completed immediately.
An attempt to connect a datagram socket to broadcast address failed because setsockopt option SO_BROADCAST is not enabled.


The connect function is used to create a connection to the specified destination. If 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 namespace of the socket; for a detailed description, see bind and sockaddr).

Note  If a socket is opened, a setsockopt call is made, and then a sendto call is made, Windows Sockets performs an implicit bind function call.


When the socket call completes successfully, the socket is ready to send and receive data. If the address member of the structure specified by the name parameter is filled with zeros, connect will return the error WSAEADDRNOTAVAIL. Any attempt to reconnect an active connection will fail with the error code WSAEISCONN.

For connection-oriented, nonblocking sockets, it is often not possible to complete the connection immediately. In such a case, this function returns the error WSAEWOULDBLOCK. However, the operation proceeds.

When the success or failure outcome becomes known, it may be reported in one of two ways, depending on how the client registers for notification.

  • If the client uses the select function, success is reported in the writefds set and failure is reported in the exceptfds set.
  • If the client uses the functions WSAAsyncSelect or WSAEventSelect, the notification is announced with FD_CONNECT and the error code associated with the FD_CONNECT indicates either success or a specific reason for failure.

If the connection is not completed immediately, the client should wait for connection completion before attempting to set socket options using setsockopt. Calling setsockopt while a connection is in progress is not supported.

For a connectionless socket (for example, type SOCK_DGRAM), the operation performed by connect is merely to establish a default destination address that can 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 member of the structure specified by name is filled with zeros, the socket will be disconnected. 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 use setsockopt to enable the SO_BROADCAST option. Otherwise, connect will fail with the error code WSAEACCES.

When a connection between sockets is broken, the socket that was connected should be discarded and new socket should be created. When a problem develops on a connected socket, the application must discard the socket and create the socket again in order to return to a stable point.

Note  When issuing a blocking Winsock call such as connect, Winsock may need to wait for a network event before the call can complete. Winsock performs an alertable wait in this situation, which can be interrupted by an asynchronous procedure call (APC) scheduled on the same thread. Issuing another blocking Winsock call inside an APC that interrupted an ongoing blocking Winsock call on the same thread will lead to undefined behavior, and must never be attempted by Winsock clients.

Example Code

The following example demonstrates the use of the connect function.
#ifndef UNICODE
#define UNICODE


#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdio.h>

// Need to link with Ws2_32.lib
#pragma comment(lib, "ws2_32.lib")

int wmain()
    // Initialize Winsock
    WSADATA wsaData;
    int iResult = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (iResult != NO_ERROR) {
        wprintf(L"WSAStartup function failed with error: %d\n", iResult);
        return 1;
    // Create a SOCKET for connecting to server
    SOCKET ConnectSocket;
    ConnectSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (ConnectSocket == INVALID_SOCKET) {
        wprintf(L"socket function failed with error: %ld\n", WSAGetLastError());
        return 1;
    // The sockaddr_in structure specifies the address family,
    // IP address, and port of the server to be connected to.
    sockaddr_in clientService;
    clientService.sin_family = AF_INET;
    clientService.sin_addr.s_addr = inet_addr("");
    clientService.sin_port = htons(27015);

    // Connect to server.
    iResult = connect(ConnectSocket, (SOCKADDR *) & clientService, sizeof (clientService));
    if (iResult == SOCKET_ERROR) {
        wprintf(L"connect function failed with error: %ld\n", WSAGetLastError());
        iResult = closesocket(ConnectSocket);
        if (iResult == SOCKET_ERROR)
            wprintf(L"closesocket function failed with error: %ld\n", WSAGetLastError());
        return 1;

    wprintf(L"Connected to server.\n");

    iResult = closesocket(ConnectSocket);
    if (iResult == SOCKET_ERROR) {
        wprintf(L"closesocket function failed with error: %ld\n", WSAGetLastError());
        return 1;

    return 0;

For another example that uses the connect function, see Getting Started With Winsock.

Notes for IrDA Sockets

  • The Af_irda.h header file must be explicitly included.
  • If an existing IrDA connection is detected at the media-access level, WSAENETDOWN is returned.
  • If active connections to a device with a different address exist, WSAEADDRINUSE is returned.
  • If the socket is already connected or an exclusive/multiplexed mode change failed, WSAEISCONN is returned.
  • If the socket was previously bound to a local service name to accept incoming connections using bind, WSAEINVAL is returned. Note that once a socket is bound, it cannot be used for establishing an outbound connection.

IrDA implements the connect function with addresses of the form sockaddr_irda. Typically, a client application will create a socket with the socket function, scan the immediate vicinity for IrDA devices with the IRLMP_ENUMDEVICES socket option, choose a device from the returned list, form an address, and then call connect. There is no difference between blocking and nonblocking semantics.


Requirement Value
Minimum supported client Windows Vista [desktop apps | UWP apps]
Minimum supported server Windows Server 2003 [desktop apps | UWP apps]
Target Platform Windows
Header winsock2.h
Library Ws2_32.lib
DLL Ws2_32.dll

See also




Winsock Functions

Winsock Reference