int WSAAPI select(
[in] int nfds,
[in, out] fd_set *readfds,
[in, out] fd_set *writefds,
[in, out] fd_set *exceptfds,
[in] const timeval *timeout
Parameters
[in] nfds
Ignored. The nfds parameter is included only for compatibility with Berkeley sockets.
[in, out] readfds
An optional pointer to a set of sockets to be checked for readability.
[in, out] writefds
An optional pointer to a set of sockets to be checked for writability.
[in, out] exceptfds
An optional pointer to a set of sockets to be checked for errors.
[in] timeout
The maximum time for
select to wait, provided in the form of a
TIMEVAL structure. Set the timeout parameter to null for blocking operations.
Return value
select function returns the total number of socket handles that are ready and contained in the
fd_set structures, zero if the time limit expired, or SOCKET_ERROR if an error occurred. If the return value is SOCKET_ERROR,
WSAGetLastError can be used to retrieve a specific error code.
Error code
Meaning
select 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. The sockets contained within the
fd_set structures must be associated with a single service provider. For the purpose of this restriction, sockets are considered to be from the same service provider if 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. The
select function returns the number of sockets meeting the conditions. A set of macros is provided for manipulating an
fd_set structure. These macros are compatible with those used in the Berkeley software, but the underlying representation is completely different.
The parameter readfds identifies the sockets that are to be checked for readability. If the socket is currently in the
listen state, it will be marked as readable if an incoming connection request has been received such that an
accept is guaranteed to complete without blocking. For other sockets, readability means that queued data is available for reading such that a call to
recv,
WSARecv,
WSARecvFrom, or
recvfrom is guaranteed not to block.
For connection-oriented sockets, readability can also indicate that a request to close the socket has been received from the peer. If the virtual circuit was closed gracefully, and all data was received, then a
recv will return immediately with zero bytes read. If the virtual circuit was reset, then a
recv 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
setsockopt).
The parameter writefds identifies the sockets that are to be checked for writability. If a socket is processing a
connect call (nonblocking), a socket is writable if the connection establishment successfully completes. If the socket is not processing a
connect call, writability means a
send,
sendto, or
WSASendto are guaranteed to succeed. However, they can block on a blocking socket if the len parameter 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 the sockets that are to be checked for the presence of OOB data or any exceptional error conditions.
Note Out-of-band data will only be reported in this way if the option SO_OOBINLINE is
FALSE. If a socket is processing a
connect call (nonblocking), failure of the connect attempt is indicated in
exceptfds (application must then call
getsockopt SO_ERROR to determine the error value to describe why the failure occurred). This document does not define which other errors will be included.
Any two of the parameters, readfds, writefds, or exceptfds, can be given as null. At least one must be non-null, and any non-null descriptor set must contain at least one handle to a socket.
In summary, a socket will be identified in a particular set when
select returns if:
readfds:
listen has been called and a connection is pending,
accept will succeed.
Data is available for reading (includes OOB data if SO_OOBINLINE is enabled).
Connection has been closed/reset/terminated.
writefds:
If processing a
connect call (nonblocking), connection has succeeded.
Data can be sent.
exceptfds:
If processing a
connect call (nonblocking), connection attempt failed.
OOB data is available for reading (only if SO_OOBINLINE is disabled).
Four macros are defined in the header file Winsock2.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 Winsock2.h.) Internally, socket handles in an
fd_set structure 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_ZERO(*set) - Initializes set to the empty set. A set should always be cleared before using.
FD_CLR(s, *set) - Removes socket s from set.
FD_ISSET(s, *set) - Checks to see if s is a member of set and returns TRUE if so.
FD_SET(s, *set) - Adds socket s to set.
The parameter time-out controls how long the
select can take to complete. If time-out is a null pointer,
select will block indefinitely until at least one descriptor meets the specified criteria. Otherwise, time-out points to a
TIMEVAL structure that specifies the maximum time that
select should wait before returning. When
select returns, the contents of the TIMEVAL structure are not altered. If TIMEVAL is initialized to {0, 0},
select will return immediately; this is used to poll the state of the selected sockets. If
select returns immediately, then the
select call is considered nonblocking and the standard assumptions for nonblocking calls apply. For example, the blocking hook will not be called, and Windows Sockets will not yield.
Note When issuing a blocking Winsock call such as select with the timeout parameter set to NULL, 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.
Windows Phone 8: This function is supported for Windows Phone Store apps on Windows Phone 8 and later.
Windows 8.1 and Windows Server 2012 R2: This function is supported for Windows Store apps on Windows 8.1, Windows Server 2012 R2, and later.
Requirements
Requirement
Value
