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select function

Applies to: desktop apps only

The select function determines the status of one or more sockets, waiting if necessary, to perform synchronous I/O.

Syntax

Copy

int select(
  __in     int nfds,
  __inout  fd_set *readfds,
  __inout  fd_set *writefds,
  __inout  fd_set *exceptfds,
  __in     const struct timeval *timeout
);

Parameters

nfds [in]: Ignored. The nfds parameter is included only for compatibility with Berkeley sockets.
readfds [in, out]: An optional pointer to a set of sockets to be checked for readability.
writefds [in, out]: An optional pointer to a set of sockets to be checked for writability.
exceptfds [in, out]: An optional pointer to a set of sockets to be checked for errors.
timeout [in]: 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

The 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
WSANOTINITIALISED	A successful WSAStartup call must occur before using this function.
WSAEFAULT	The Windows Sockets implementation was unable to allocate needed resources for its internal operations, or the readfds, writefds, exceptfds, or timeval parameters are not part of the user address space.
WSAENETDOWN	The network subsystem has failed.
WSAEINVAL	The time-out value is not valid, or all three descriptor parameters were null.
WSAEINTR	A blocking Windows Socket 1.1 call was canceled through WSACancelBlockingCall.
WSAEINPROGRESS	A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
WSAENOTSOCK	One of the descriptor sets contains an entry that is not a socket.

Remarks

The 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 writeable 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:

If 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_CLR(s, *set): Removes the descriptor s from set.
FD_ISSET(s, *set): Nonzero if s is a member of the set. Otherwise, zero.
FD_SET(s, *set): Adds descriptor s to set.
FD_ZERO(*set): Initializes the set to the null 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 The select function has no effect on the persistence of socket events registered with WSAAsyncSelect or WSAEventSelect.

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.

Requirements

Minimum supported client	Windows 2000 Professional
Minimum supported server	Windows 2000 Server
Header	Winsock2.h
Library	Ws2_32.lib
DLL	Ws2_32.dll

The use of the FD_ macros seems rather confusing. The documentation doesn't tell us that select() removes fds from the set. Looking at the implementation of the macros to get further insight is misleading because it can easily lead to code that won't port to other platforms - for example it would be tempting after select() to iterate the set instead of iterating all your fds and checking if they are in the set; or to optimize and only add fds back into the set if they weren't signaled last time. Presumably that won't necessarily port. So I think the advice has to be:
- on first use initialise the set with FD_ZERO
- add all fds to the set prior to each call to select (don't make assumptions about any being in there already)
- check all fds with FD_ISSET after the call to select
- no need to use FD_CLR with each iteration, only when an fd is not of interest anymore

History

2/23/2012
Kevin Robertson

Thanks MR.Huang

Thank you for your sharing, MR.Huang. Your project is exactly what I am looking for.

History

10/22/2011
canterburyman

Implement OO multiplex by winsock select/epoll/kqueue

Pls go to: http://sourceforge.net/projects/speed/

History

2/10/2010
Daemon Huang

Performance of FD_SET/FD_ISSET functions

Unlike the Linux versions of these macros which use a single calculation to set/check the fd, the Winsock versions use a loop which goes through the entire set of fds each time you call FD_SET or FD_ISSET (check out winsock2.h and you'll see). So you might want to consider an alternative if you have thousands of sockets!

History

8/12/2009
David Jameson - groupboard.com

Don't forget to reset the fd_set:s between select calls

Don't forget to reset the fd_set:s between select calls. Otherwise select() will return SOCKET_ERROR and raise WSAEINVAL through WSAGetLastError(). This error was also mentioned here:
http://www.codeguru.com/forum/archive/index.php/t-294323.html

History

10/8/2008
Ztyx

select function

Syntax

Parameters

Return value

Remarks

Requirements

See also