The Windows Sockets select function determines the status of one or more sockets, waiting if necessary.

int select (

int nfds,

fd_set FAR * readfds,

fd_set FAR * writefds,

fd_set FAR * exceptfds,

const struct timeval FAR * timeout




[in] This argument is ignored and included only for the sake of compatibility.


[in/out] An optional pointer to a set of sockets to be checked for readability.


[in/out] An optional pointer to a set of sockets to be checked for writability


[in/out] An optional pointer to a set of sockets to be checked for errors.


[in] The maximum time for select to wait, or NULL for blocking operation.


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. The sockets contained within the fd_set structures must be associated with a single service provider. Upon return, the structures are updated to reflect the subset of these sockets which meet the specified condition, and select returns the 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 which are to be checked for readability. If the socket is currently listening, it will be marked as readable if an incoming connection request has been received, so that an accept is guaranteed to complete without blocking. For other sockets, readability means that queued data is available for reading so that a recv or recvfrom 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 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 out-of-band data will be checked if the socket option SO_OOBINLINE has been enabled (see setsockopt).

The parameter writefds identifies those sockets which are to be checked for writability. If a socket is connecting (nonblocking), writability means that the connection establishment successfully completed. If the socket is not in the process of connecting, writability means that a send or sendto 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 which are to be checked for the presence of out-of-band data (see section Out-Of-Band data for a discussion of this topic) or any exceptional error conditions. Note that out-of-band data will only be reported in this way if the option SO_OOBINLINE is FALSE. If a socket is connecting (nonblocking), 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 select returns if:


  • If listening, 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


  • If connecting (nonblocking), connection has succeeded

  • Data can be sent


  • If connecting (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 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_ISSET(s, *set)

Nonzero if s is a member of the set. Otherwise, zero.

FD_SET(s, *set)

Adds descriptor s to set.


Initializes the set to the NULL set.

The parameter timeout controls how long the select can take to complete. If timeout is a null pointer, select will block indefinitely until at least one descriptor meets the specified criteria. Otherwise, timeout points to a struct timeval which specifies the maximum time that select should wait before returning. When select returns, the contents of the struct timeval are not altered. If the timeval is initialized to {0, 0}, select will return immediately; this is used to "poll" the state of the selected sockets. If this is the case, 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.

Return Values

returns the total number of descriptors which 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.


has no effect on the persistence of socket events registered with WSAAsyncSelect or WSAEventSelect.

Error Codes

A successful WSAStartup must occur before using this function.
The Windows Sockets implementation 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.
The (blocking) call was canceled through WSACancelBlockingCall.
A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
One of the descriptor sets contains an entry which is not a socket.

See Also

, connect, recv, recvfrom, send, WSAAsyncSelect, WSAEventSelect

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