This function determines the status of one or more sockets, waiting if necessary.

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


  • nfds
    [in] Ignored; included only for compatibility with Berkeley sockets.
  • 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 select to wait. NULL blocks indefinitely.

Return Values

Zero indicates that the time limit expired. SOCKET_ERROR indicates failure. To get a specific error value, call WSAGetLastError.


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. 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 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 value such as WSAECONNRESET.

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 or sendto 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 any exceptional error conditions.

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.

Summary: A socket will be identified in a particular set when select returns if:


  • If listen has been called and a connection is pending, accept will succeed
  • Connection has been closed/reset/terminated


  • If processing a connect call (nonblocking), connection has succeeded
  • Data can be sent


  • If processing a connect call (nonblocking), connection attempt failed

Four macros are defined in the header file Winsock.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 Winsock.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.


Runs on Versions Defined in Include Link to
Windows CE OS 1.0 and later Winsock.h    

Note   This API is part of the complete Windows CE OS package as provided by Microsoft. The functionality of a particular platform is determined by the original equipment manufacturer (OEM) and some devices may not support this API.

See Also

accept, connect, recv, recvfrom, send, sendto, setsockopt, WSAStartup

 Last updated on Tuesday, July 13, 2004

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