The Windows Sockets WSAEventSelect function specifies an event object to be associated with the supplied set of FD_XXX network events.

int WSAEventSelect (


WSAEVENT hEventObject,

long lNetworkEvents




[in] A descriptor identifying the socket.


[in] A handle identifying the event object to be associated with the supplied set of FD_XXX network events.


[in] A bitmask which specifies the combination of FD_XXX network events in which the application has interest.


This function is used to specify an event object, hEventObject, to be associated with the selected FD_XXX network events, lNetworkEvents. The socket for which an event object is specified is identified by s. The event object is set when any of the nominated network events occur.

WSAEventSelect operates very similarly to WSAAsyncSelect, the difference being in the actions taken when a nominated network event occurs. Whereas WSAAsyncSelect causes an application-specified Windows message to be posted, WSAEventSelect sets the associated event object and records the occurrence of this event in an internal network event record. An application can use WSAWaitForMultipleEvents to wait or poll on the event object, and use WSAEnumNetworkEvents to retrieve the contents of the internal network event record and thus determine which of the nominated network events have occurred.

This function automatically sets socket s to nonblocking mode, regardless of the value of lNetworkEvents. See ioctlsocket/WSAIoctl about how to set the socket back to blocking mode.

The lNetworkEvents parameter is constructed by or'ing any of the values specified in the following list.

Want to receive notification of readiness for reading
Want to receive notification of readiness for writing
Want to receive notification of the arrival of out-of-band data
Want to receive notification of incoming connections
Want to receive notification of completed connection
Want to receive notification of socket closure
Want to receive notification of socket Quality of Service (QOS) changes
Want to receive notification of socket group Quality of Service (QOS) changes

Issuing a WSAEventSelect for a socket cancels any previous WSAAsyncSelect or WSAEventSelect for the same socket and clears the internal network event record. For example, to associate an event object with both reading and writing network events, the application must call WSAEventSelect with both FD_READ and FD_WRITE, as follows:

rc = WSAEventSelect(s, hEventObject, FD_READ|FD_WRITE);

It is not possible to specify different event objects for different network events. The following code will not work; the second call will cancel the effects of the first, and only FD_WRITE network event will be associated with hEventObject2:

rc = WSAEventSelect(s, hEventObject1, FD_READ);

rc = WSAEventSelect(s, hEventObject2, FD_WRITE); //bad

To cancel the association and selection of network events on a socket, lNetworkEvents should be set to zero, in which case the hEventObject parameter will be ignored.

rc = WSAEventSelect(s, hEventObject, 0);

Closing a socket with closesocket also cancels the association and selection of network events specified in WSAEventSelect for the socket. The application, however, still must call WSACloseEvent to explicitly close the event object and free any resources.

Since an accept'ed socket has the same properties as the listening socket used to accept it, any WSAEventSelect association and network events selection set for the listening socket apply to the accepted socket. For example, if a listening socket has WSAEventSelect association of hEventOject with 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 network events associated with the same hEventObject. If a different hEventObject or network events are desired, the application should call WSAEventSelect, passing the accepted socket and the desired new information.

Return Values

The return value is zero if the application's specification of the network events and the associated event object was successful. Otherwise, the value SOCKET_ERROR is returned, and a specific error number may be retrieved by calling WSAGetLastError.

As in the case of the select and WSAAsyncSelect functions, WSAEventSelect 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 the event object is set and it issues a Windows Sockets call which returns WSAEWOULDBLOCK immediately. For example, the following sequence of operations is possible:

  1. data arrives on socket s; Windows Sockets sets the WSAEventSelect event object

  2. application does some other processing

  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 eventually waits on event object specified in WSAEventSelect, which returns immediately indicating that data is ready to read

  6. application issues recv(s,...), which fails with the error WSAEWOULDBLOCK.

Other sequences are possible.

Having successfully recorded the occurrence of the network event (by setting the corresponding bit in the internal network event record) and signaled the associated event object, no further actions are taken for that network event until the application makes the function call which implicitly re-enables the setting of that network event and signaling of the associated event object.

Network Event
Re-enabling function
recv, recvfrom, WSARecv, or WSARecvFrom
send, sendto, WSASend, or WSASendTo
recv, recvfrom, WSARecv, or WSARecvFrom
accept or WSAAccept unless the error code returned is WSATRY_AGAIN indicating that the condition function returned CF_DEFER
WSAIoctl with command SIO_GET_QOS
WSAIoctl with command SIO_GET_GROUP_QOS

Any call to the re-enabling routine, even one which fails, results in re-enabling of recording and signaling for the relevant network event and event object, respectively.

For FD_READ, FD_OOB, and FD_ACCEPT network events, network event recording and event object signaling are "level-triggered." This means that if the re-enabling routine is called and the relevant network condition is still valid after the call, the network event is recorded and the associated event object is set. 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. Transport provider receives 100 bytes of data on socket s and causes Windows Sockets DLL to record the FD_READ network event and set the associated event object.

  2. The application issues recv( s, buffptr, 50, 0) to read 50 bytes.

  3. The transport provider causes Windows Sockets DLL to record the FD_READ network event and sets the associated event object again since there is still data to be read.

With these semantics, an application need not read all available data in response to an FD_READ network eventpics/SOCK200090000.gifa single recv in response to each FD_READ network event is appropriate.

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

If a network event has already happened when the application calls WSAEventSelect or when the re-enabling function is called, then a network event is recorded and the associated event object is set 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 WSAEventSelect specifying that it is interested in the FD_ACCEPT network event for the socket. Due to the persistence of network events, Windows Sockets records the FD_ACCEPT network event and sets the associated event object immediately.

The FD_WRITE network event is handled slightly differently. An FD_WRITE network event is recorded when a socket is first connected with connect/WSAConnect or accepted with accept/WSAAccept, and then after a send fails with WSAEWOULDBLOCK and buffer space becomes available. Therefore, an application can assume that sends are possible starting from the first FD_WRITE network event setting and lasting until a send returns WSAEWOULDBLOCK. After such a failure the application will find out that sends are again possible when an FD_WRITE network event is recorded and the associated event object is set.

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

The error code in an FD_CLOSE network event 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 network event is recorded when a close indication is received for the virtual circuit corresponding to the socket. In TCP terms, this means that the FD_CLOSE is recorded when the connection goes into the FIN WAIT or CLOSE WAIT states. This results from the remote end performing a shutdown on the send side or a closesocket.

Please note Windows Sockets will record ONLY an FD_CLOSE network event to indicate closure of a virtual circuit. It will not record an FD_READ network event to indicate this condition.

The FD_QOS or FD_GROUP_QOS network event is recorded when any field 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 QOS for socket s or for the socket group s belongs to, respectively.

Error Codes

A successful WSAStartup must occur before using this function.
The network subsystem has failed.
Indicates that one of the specified parameters was invalid, 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.

See Also

, WSACreateEvent, WSAEnumNetworkEvents, WSAWaitForMultipleEvents

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