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

The following table shows the possible error codes.

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_setstructure. All entries in an fd_setcorrespond to sockets created by the service provider. On return, the structures are updated to reflect the subset of these sockets that meet the specified condition, and WSPSelectreturns 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 readfdsidentifies 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 WSPAcceptis guaranteed to complete without blocking. For other sockets, readability means that queued data is available for reading so that a WSPRecvor WSPRecvFromis 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 WSPRecvwill return immediately with zero bytes read. If the virtual circuit was reset, then a WSPRecvwill 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 following list shows ways in which the parameter writefdsidentifies 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 WSPSendor WSPSendToare guaranteed to succeed.
  
  

However, they can block on a blocking socket if the lenexceeds 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 exceptfdsidentifies 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 exceptfdscan 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.

A socket will be identified in a particular set when WSPSelectreturns specific parameters. The following table shows these parameters.

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_setare 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.

You can manipulate and check fd_setcontents for the following macros:

FD_CLR( s ,* set )

Removes the descriptor sfrom set.

fd_set( s ,* set )

Adds descriptor sto set.

FD_ZERO(* set )

Initializes the setto the NULL set.

The upcall function used to check the membership is int  WPUFDIsSet (  SOCKET  s, fd_set FAR *  set  ), which will return nonzero if sis a member of the setor otherwise zero.

The parameter timeoutcontrols how long the WSPSelectcan take to complete. If timeoutis a NULL pointer, WSPSelectwill block indefinitely until at least one descriptor meets the specified criteria. Otherwise, timeoutpoints to a timevalstructure that specifies the maximum time that WSPSelectshould wait before returning. When WSPSelectreturns, the contents of the timevalstructure are not altered. If timevalis initialized to {0, 0}, WSPSelectwill return immediately; this is used to poll the state of the selected sockets. If this is the case, then the WSPSelectcall is considered nonblocking and the standard assumptions for nonblocking calls apply. For example, the blocking hook will notbe called, and the Windows Sockets provider will not yield.

Note:
WSPSelecthas no effect on the persistence of socket events registered with WSPEventSelect.