WSAAsyncSelect function (winsock.h)

[The WSAAsyncSelect function is available for use in the operating systems specified in the Requirements section. It may be altered or unavailable in subsequent versions. Rather than use Select-style I/O, use Overlapped I/O and Event Objects with WinSock2.]

The WSAAsyncSelect function requests Windows message-based notification of network events for a socket.


int WSAAsyncSelect(
  [in] SOCKET s,
  [in] HWND   hWnd,
  [in] u_int  wMsg,
  [in] long   lEvent


[in] s

A descriptor that identifies the socket for which event notification is required.

[in] hWnd

A handle that identifies the window that will receive a message when a network event occurs.

[in] wMsg

A message to be received when a network event occurs.

[in] lEvent

A bitmask that specifies a combination of network events in which the application is interested.

Return value

If the WSAAsyncSelect function succeeds, the return value is zero, provided that the application's declaration of interest in the network event set was successful. Otherwise, the value SOCKET_ERROR is returned, and a specific error number can be retrieved by calling WSAGetLastError.

Error code Meaning
A successful WSAStartup call must occur before using this function.
The network subsystem failed.
One of the specified parameters was invalid, such as the window handle not referring to an existing window, or the specified socket is in an invalid state.
A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
The descriptor is not a socket.

Additional error codes can be set when an application window receives a message. This error code is extracted from the lParam in the reply message using the WSAGETSELECTERROR macro. Possible error codes for each network event are listed in the following table.


Error code Meaning
WSAEAFNOSUPPORT Addresses in the specified family cannot be used with this socket.
WSAECONNREFUSED The attempt to connect was rejected.
WSAENETUNREACH The network cannot be reached from this host at this time.
WSAEFAULT The namelen parameter is invalid.
WSAEINVAL The socket is already bound to an address.
WSAEISCONN The socket is already connected.
WSAEMFILE No more file descriptors are available.
WSAENOBUFS No buffer space is available. The socket cannot be connected.
WSAENOTCONN The socket is not connected.
WSAETIMEDOUT Attempt to connect timed out without establishing a connection.


Error code Meaning
WSAENETDOWN The network subsystem failed.
WSAECONNRESET The connection was reset by the remote side.
WSAECONNABORTED The connection was terminated due to a time-out or other failure.
Error code Meaning
WSAENETDOWN The network subsystem failed.


Error code Meaning
WSAENETUNREACH The specified destination is no longer reachable.
WSAENETDOWN The network subsystem failed.


The WSAAsyncSelect function is used to request that WS2_32.DLL should send a message to the window hWnd when it detects any network event specified by the lEvent parameter. The message that should be sent is specified by the wMsg parameter. The socket for which notification is required is identified by the s parameter.

The WSAAsyncSelect function automatically sets socket s to nonblocking mode, regardless of the value of lEvent. To set socket s back to blocking mode, it is first necessary to clear the event record associated with socket s via a call to WSAAsyncSelect with lEvent set to zero. You can then call ioctlsocket or WSAIoctl to set the socket back to blocking mode. For more information about how to set the nonblocking socket back to blocking mode, see the ioctlsocket and WSAIoctl functions.

The lEvent parameter is constructed by using the bitwise OR operator with any value listed in the following table.

Value Meaning
FD_READ Set to receive notification of readiness for reading.
FD_WRITE Wants to receive notification of readiness for writing.
FD_OOB Wants to receive notification of the arrival of OOB data.
FD_ACCEPT Wants to receive notification of incoming connections.
FD_CONNECT Wants to receive notification of completed connection or multipoint join operation.
FD_CLOSE Wants to receive notification of socket closure.
FD_QOS Wants to receive notification of socket Quality of Service (QoS) changes.
FD_GROUP_QOS Wants to receive notification of socket group Quality of Service (QoS) changes (reserved for future use with socket groups). Reserved.
FD_ROUTING_INTERFACE_CHANGE Wants to receive notification of routing interface changes for the specified destination(s).
FD_ADDRESS_LIST_CHANGE Wants to receive notification of local address list changes for the socket protocol family.

Issuing a WSAAsyncSelect for a socket cancels any previous WSAAsyncSelect or WSAEventSelect for the same socket. For example, to receive notification for both reading and writing, the application must call WSAAsyncSelect with both FD_READ and FD_WRITE, as follows:

rc = WSAAsyncSelect(s, hWnd, wMsg, FD_READ|FD_WRITE);

It is not possible to specify different messages for different events. The following code will not work; the second call will cancel the effects of the first, and only FD_WRITE events will be reported with message wMsg2:

rc = WSAAsyncSelect(s, hWnd, wMsg1, FD_READ);
rc = WSAAsyncSelect(s, hWnd, wMsg2, FD_WRITE);

To cancel all notification indicating that Windows Sockets should send no further messages related to network events on the socket, lEvent is set to zero.

rc = WSAAsyncSelect(s, hWnd, 0, 0);

Although WSAAsyncSelect immediately disables event message posting for the socket in this instance, it is possible that messages could be waiting in the application message queue. Therefore, the application must be prepared to receive network event messages even after cancellation. Closing a socket with closesocket also cancels WSAAsyncSelect message sending, but the same caveat about messages in the queue still applies.

The socket created by the accept function has the same properties as the listening socket used to accept it. Consequently, WSAAsyncSelect events set for the listening socket also apply to the accepted socket. For example, if a listening socket has WSAAsyncSelect events FD_ACCEPT, FD_READ, and FD_WRITE, then any socket accepted on that listening socket will also have FD_ACCEPT, FD_READ, and FD_WRITE events with the same wMsg value used for messages. If a different wMsg or events are desired, the application should call WSAAsyncSelect, passing the accepted socket and the desired new data.

When one of the nominated network events occurs on the specified socket s, the application window hWnd receives message wMsg. The wParam parameter identifies the socket on which a network event has occurred. The low word of lParam specifies the network event that has occurred. The high word of lParam contains any error code. The error code be any error as defined in Winsock2.h.

Note  Upon receipt of an event notification message, the WSAGetLastError function cannot be used to check the error value because the error value returned can differ from the value in the high word of lParam.
The error and event codes can be extracted from the lParam using the macros WSAGETSELECTERROR and WSAGETSELECTEVENT, defined in Winsock2.h as:
#include <windows.h>

#define WSAGETSELECTEVENT(lParam)       LOWORD(lParam)
#define WSAGETSELECTERROR(lParam)       HIWORD(lParam)

The use of these macros will maximize the portability of the source code for the application.

The possible network event codes that can be returned are listed in the following table.

Value Meaning
FD_READ Socket s ready for reading.
FD_WRITE Socket s ready for writing.
FD_OOB OOB data ready for reading on socket s
FD_ACCEPT Socket s ready for accepting a new incoming connection.
FD_CONNECT Connection or multipoint join operation initiated on socket s completed.
FD_CLOSE Connection identified by socket s has been closed.
FD_QOS Quality of Service associated with socket s has changed.
FD_GROUP_QOS Reserved. Quality of Service associated with the socket group to which s belongs has changed (reserved for future use with socket groups).
FD_ROUTING_INTERFACE_CHANGE Local interface that should be used to send to the specified destination has changed.
FD_ADDRESS_LIST_CHANGE The list of addresses of the socket protocol family to which the application client can bind has changed.

Although WSAAsyncSelect can be called with interest in multiple events, the application window will receive a single message for each network event.

As in the case of the select function, WSAAsyncSelect will frequently be used to determine when a data transfer operation (send or recv) can be issued with the expectation of immediate success. Nevertheless, a robust application must be prepared for the possibility that it can receive a message and issue a Windows Sockets 2 call that returns WSAEWOULDBLOCK immediately. For example, the following sequence of events is possible:

  1. Data arrives on socket s; Windows Sockets 2 posts WSAAsyncSelect message
  2. Application processes some other message
  3. While processing, application issues an ioctlsocket(s, FIONREAD...) and notices that there is data ready to be read
  4. Application issues a recv(s,...) to read the data
  5. Application loops to process next message, eventually reaching the WSAAsyncSelect message indicating that data is ready to read
  6. Application issues recv(s,...), which fails with the error WSAEWOULDBLOCK.
Other sequences are also possible.

The WS2_32.DLL will not continually flood an application with messages for a particular network event. Having successfully posted notification of a particular event to an application window, no further message(s) for that network event will be posted to the application window until the application makes the function call that implicitly reenables notification of that network event.

Event Reenabling function
FD_READ recv, recvfrom, WSARecv, or WSARecvFrom.
FD_WRITE send, sendto, WSASend, or WSASendTo.
FD_OOB recv, recvfrom, WSARecv, or WSARecvFrom.
FD_ACCEPT accept or WSAAccept unless the error code is WSATRY_AGAIN indicating that the condition function returned CF_DEFER.
FD_QOS WSAIoctl with command SIO_GET_QOS.
FD_GROUP_QOS Reserved. WSAIoctl with command SIO_GET_GROUP_QOS (reserved for future use with socket groups).

Any call to the reenabling routine, even one that fails, results in reenabling of message posting for the relevant event.

For FD_READ, FD_OOB, and FD_ACCEPT events, message posting is level-triggered. This means that if the reenabling routine is called and the relevant condition is still met after the call, a WSAAsyncSelect message is posted to the application. This allows an application to be event-driven and not be concerned with the amount of data that arrives at any one time. Consider the following sequence:

  1. Network transport stack receives 100 bytes of data on socket s and causes Windows Sockets 2 to post an FD_READ message.
  2. The application issues recv( s, buffptr, 50, 0) to read 50 bytes.
  3. Another FD_READ message is posted because there is still data to be read.
With these semantics, an application need not read all available data in response to an FD_READ message—a single recv in response to each FD_READ message is appropriate. If an application issues multiple recv calls in response to a single FD_READ, it can receive multiple FD_READ messages. Such an application can require disabling FD_READ messages before starting the recv calls by calling WSAAsyncSelect with the FD_READ event not set.

The FD_QOS and FD_GROUP_QOS events are considered edge triggered. A message will be posted exactly once when a quality of service change occurs. Further messages will not be forthcoming until either the provider detects a further change in quality of service or the application renegotiates the quality of service for the socket.

The FD_ROUTING_INTERFACE_CHANGE message is posted when the local interface that should be used to reach the destination specified in WSAIoctl with SIO_ROUTING_INTERFACE_CHANGE changes after such IOCTL has been issued.

The FD_ADDRESS_LIST_CHANGE message is posted when the list of addresses to which the application can bind changes after WSAIoctl with SIO_ADDRESS_LIST_CHANGE has been issued.

If any event has occurred when the application calls WSAAsyncSelect or when the reenabling function is called, then a message is posted as appropriate. For example, consider the following sequence:

  1. An application calls listen.
  2. A connect request is received, but not yet accepted.
  3. The application calls WSAAsyncSelect specifying that it requires receiving FD_ACCEPT messages for the socket. Due to the persistence of events, Windows Sockets 2 posts an FD_ACCEPT message immediately.
The FD_WRITE event is handled slightly differently. An FD_WRITE message is posted when a socket is first connected with connect or WSAConnect (after FD_CONNECT, if also registered) or accepted with accept or WSAAccept, and then after a send operation fails with WSAEWOULDBLOCK and buffer space becomes available. Therefore, an application can assume that sends are possible starting from the first FD_WRITE message and lasting until a send returns WSAEWOULDBLOCK. After such a failure the application will be notified that sends are again possible with an FD_WRITE message.

The FD_OOB event is used only when a socket is configured to receive OOB data separately. If the socket is configured to receive OOB data inline, the OOB (expedited) data is treated as normal data and the application should register an interest in, and will receive, FD_READ events, not FD_OOB events. An application can set or inspect the way in which OOB data is to be handled by using setsockopt or getsockopt for the SO_OOBINLINE option.

The error code in an FD_CLOSE message indicates whether the socket close was graceful or abortive. If the error code is zero, then the close was graceful; if the error code is WSAECONNRESET, then the socket's virtual circuit was reset. This only applies to connection-oriented sockets such as SOCK_STREAM.

The FD_CLOSE message is posted when a close indication is received for the virtual circuit corresponding to the socket. In TCP terms, this means that the FD_CLOSE is posted when the connection goes into the TIME WAIT or CLOSE WAIT states. This results from the remote end performing a shutdown on the send side or a closesocket. FD_CLOSE should only be posted after all data is read from a socket, but an application should check for remaining data upon receipt of FD_CLOSE to avoid any possibility of losing data.

Be aware that the application will only receive an FD_CLOSE message to indicate closure of a virtual circuit, and only when all the received data has been read if this is a graceful close. It will not receive an FD_READ message to indicate this condition.

The FD_QOS or FD_GROUP_QOS message is posted when any parameter in the flow specification associated with socket s or the socket group that s belongs to has changed, respectively. Applications should use WSAIoctl with command SIO_GET_QOS or SIO_GET_GROUP_QOS to get the current quality of service for socket s or for the socket group s belongs to, respectively.

The FD_ROUTING_INTERFACE_CHANGE and FD_ADDRESS_LIST_CHANGE events are considered edge triggered as well. A message will be posted exactly once when a change occurs after the application has requested the notification by issuing WSAIoctl with SIO_ROUTING_INTERFACE_CHANGE or SIO_ADDRESS_LIST_CHANGE correspondingly. Further messages will not be forthcoming until the application reissues the IOCTL and another change is detected because the IOCTL has been issued.

Here is a summary of events and conditions for each asynchronous notification message.

  • FD_READ:
    1. When WSAAsyncSelect is called, if there is data currently available to receive.
    2. When data arrives, if FD_READ is not already posted.
    3. After recv or recvfrom is called, with or without MSG_PEEK), if data is still available to receive.
      Note  When setsockopt SO_OOBINLINE is enabled, data includes both normal data and OOB data in the instances noted above.
    1. When WSAAsyncSelect called, if a send or sendto is possible.
    2. After connect or accept called, when connection established.
    3. After send or sendto fail with WSAEWOULDBLOCK, when send or sendto are likely to succeed.
    4. After bind on a connectionless socket. FD_WRITE may or may not occur at this time (implementation-dependent). In any case, a connectionless socket is always writable immediately after a bind operation.
  • FD_OOB: Only valid when setsockopt SO_OOBINLINE is disabled (default).
    1. When WSAAsyncSelect called, if there is OOB data currently available to receive with the MSG_OOB flag.
    2. When OOB data arrives, if FD_OOB not already posted.
    3. After recv or recvfrom called with or without MSG_OOB flag, if OOB data is still available to receive.
    1. When WSAAsyncSelect called, if there is currently a connection request available to accept.
    2. When a connection request arrives, if FD_ACCEPT not already posted.
    3. After accept called, if there is another connection request available to accept.
    1. When WSAAsyncSelect called, if there is currently a connection established.
    2. After connect called, when connection is established, even when connect succeeds immediately, as is typical with a datagram socket.
    3. After calling WSAJoinLeaf, when join operation completes.
    4. After connect, WSAConnect, or WSAJoinLeaf was called with a nonblocking, connection-oriented socket. The initial operation returned with a specific error of WSAEWOULDBLOCK, but the network operation went ahead. Whether the operation eventually succeeds or not, when the outcome has been determined, FD_CONNECT happens. The client should check the error code to determine whether the outcome was successful or failed.
  • FD_CLOSE: Only valid on connection-oriented sockets (for example, SOCK_STREAM)
    1. When WSAAsyncSelect called, if socket connection has been closed.
    2. After remote system initiated graceful close, when no data currently available to receive (Be aware that, if data has been received and is waiting to be read when the remote system initiates a graceful close, the FD_CLOSE is not delivered until all pending data has been read).
    3. After local system initiates graceful close with shutdown and remote system has responded with "End of Data" notification (for example, TCP FIN), when no data currently available to receive.
    4. When remote system terminates connection (for example, sent TCP RST), and lParam will contain WSAECONNRESET error value.
      Note  FD_CLOSE is not posted after closesocket is called.
  • FD_QOS:
    1. When WSAAsyncSelect called, if the quality of service associated with the socket has been changed.
    2. After WSAIoctl with SIO_GET_QOS called, when the quality of service is changed.
  • FD_GROUP_QOS: Reserved.
    • After WSAIoctl with SIO_ROUTING_INTERFACE_CHANGE called, when the local interface that should be used to reach the destination specified in the IOCTL changes.
    • After WSAIoctl with SIO_ADDRESS_LIST_CHANGE called, when the list of local addresses to which the application can bind changes.


Minimum supported client Windows 2000 Professional [desktop apps only]
Minimum supported server Windows 2000 Server [desktop apps only]
Target Platform Windows
Header winsock.h (include Winsock2.h)
Library Ws2_32.lib
DLL Ws2_32.dll

See also


Winsock Functions

Winsock Reference