WSPSelect function

The WSPSelect function determines the status of one or more sockets.


int WSPSelect(
  _In_    int                  nfds,
  _Inout_ fd_set               *readfds,
  _Inout_ fd_set               *writefds,
  _Inout_ fd_set               *exceptfds,
  _In_    const struct timeval *timeout,
  _Out_   LPINT                lpErrno


nfds [in]

Ignored and included only for the sake of compatibility.

readfds [in, out]

Optional pointer to a set of sockets to be checked for readability.

writefds [in, out]

Optional pointer to a set of sockets to be checked for writability.

exceptfds [in, out]

Optional pointer to a set of sockets to be checked for errors.

timeout [in]

Maximum time for WSPSelect to wait, or null for a blocking operation, in the form of a timeval structure.

lpErrno [out]

Pointer to the error code.

Return value

The WSPSelect function returns the total number of descriptors that are ready and contained in the fd_set structures, or SOCKET_ERROR if an error occurred. If the return value is SOCKET_ERROR, a specific error code is available in lpErrno.

Error codeMeaning

Windows Sockets service provider was unable to allocated needed resources for its internal operations, or the readfds, writefds, exceptfds or timeval parameters are not part of the user address space.


The network subsystem has failed.


The timeout value is not valid, or all three descriptor parameters were NULL.


(Blocking) call was canceled through WSPCancelBlockingCall.


Blocking Windows Sockets call is in progress, or the service provider is still processing a callback function.


One of the descriptor sets contains an entry that is not a socket.




This function is used to determine the status of one or more sockets. For each socket, the caller can request information on read, write, or error status. The set of sockets for which a given status is requested is indicated by an fd_set structure. All entries in an fd_set correspond to sockets created by the service provider (that is, the WSAPROTOCOL_INFO structures describing their protocols have the same providerId value). Upon return, the structures are updated to reflect the subset of these sockets that meet the specified condition, and WSPSelect returns the total number of sockets meeting the conditions. A set of macros is provided for manipulating an fd_set. These macros are compatible with those used in the Berkeley software, but the underlying representation is completely different.

The parameter readfds identifies those sockets that are to be checked for readability. If the socket is currently listening through WSPListen, it will be marked as readable if an incoming connection request has been received, so that a WSPAccept is guaranteed to complete without blocking. For other sockets, readability means that queued data is available for reading so that a WSPRecv or WSPRecvfrom is guaranteed not to block.

For connection-oriented sockets, readability can also indicate that a close request has been received from the peer. If the virtual circuit was closed gracefully, then a WSPRecv will return immediately with zero bytes read. If the virtual circuit was reset, then a WSPRecv will complete immediately with an error code, such as WSAECONNRESET. The presence of OOB data will be checked if the socket option SO_OOBINLINE has been enabled (see WSPSetSockOpt).

The parameter writefds identifies those sockets that are to be checked for writability:

  • If a socket is connecting through WSPConnect, writability means that the connection establishment successfully completed.
  • If the socket is not in the process of listening through WSPConnect, writability means that a WSPSend or WSPSendTo are guaranteed to succeed.

However, they can block on a blocking socket if the len exceeds the amount of outgoing system buffer space available. It is not specified how long these guarantees can be assumed to be valid, particularly in a multithreaded environment.

The parameter exceptfds identifies those sockets that are to be checked for the presence of OOB data or any exceptional error conditions. Note that OOB data will only be reported in this way if the option SO_OOBINLINE is FALSE. If a socket is making a WSPConnect (nonblocking) connection, failure of the connect attempt is indicated in exceptfds. This specification does not define which other errors will be included.

Any two of readfds, writefds, or exceptfds can be given as null if no descriptors are to be checked for the condition of interest. At least one must be non-null, and any non-null descriptor set must contain at least one socket descriptor.

Summary: A socket will be identified in a particular set when WSPSelect returns according to the following.

readfds:If WSPListen is called, a connection is pending, WSPAccept will succeed.
Data is available for reading (includes OOB data if SO_OOBINLINE is enabled).
The connection has been closed/reset/terminated.
writefds:If WSPConnect (nonblocking), connection has succeeded.
Data can be sent.
exceptfds:If WSPConnect (nonblocking), connection attempt failed.
OOB data is available for reading (only if SO_OOBINLINE is disabled).



Three macros and one upcall function are defined in the header file Ws2spi.h for manipulating and checking the descriptor sets. The variable FD_SETSIZE determines the maximum number of descriptors in a set. (The default value of FD_SETSIZE is 64, which can be modified by #defining FD_SETSIZE to another value before #including Ws2spi.h.) Internally, socket handles in a fd_set are not represented as bit flags as in Berkeley UNIX. Their data representation is opaque. Use of these macros will maintain software portability between different socket environments.

The macros to manipulate and check fd_set contents are:

FD_CLR(s, *set)

Removes the descriptor s from set.

FD_SET(s, *set)

Adds descriptor s to set.


Initializes the set to the null set.

The upcall function used to check the membership is:


which will return nonzero if s is a member of the set or otherwise zero.

The parameter timeout controls how long the WSPSelect can take to complete. If timeout is a null pointer, WSPSelect will block indefinitely until at least one descriptor meets the specified criteria. Otherwise, timeout points to a timeval structure that specifies the maximum time that WSPSelect should wait before returning. When WSPSelect returns, the contents of the timeval structure are not altered. If timeval is initialized to {0, 0}, WSPSelect will return immediately; this is used to poll the state of the selected sockets. If this is the case, then the WSPSelect call is considered nonblocking and the standard assumptions for nonblocking calls apply. For example, the blocking hook will not be called, and the Windows Sockets provider will not yield.

Note  The WSPSelect function has no effect on the persistence of socket events registered with WSPAsyncSelect or WSPEventSelect.


Minimum supported client

Windows 2000 Professional [desktop apps only]

Minimum supported server

Windows 2000 Server [desktop apps only]



See also