select

select is used by an application to receive notifications of events that occur on one or more sockets of interest.

Syntax

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

Parameters

nfds

Ignored. The nfds parameter is included only for compatibility with Berkeley sockets.

readfds

The address of the descriptor set that contains a set of sockets for which the application wishes to be notified when any of them becomes readable.

writefds

The address of the descriptor set that contains a set of sockets for which the application wishes to be notified when any of them becomes writeable.

exceptfds

The address of the descriptor set that contains a set of sockets for which the application wishes to be notified when any of them has a pending exception condition. Unlike in Windows, the only exception condition that can create a notification is when out-of-band data is available on a socket.

timeout

The address of a struct timeval which specifies how long the application is willing to block on the select call while waiting for a notification.

Return Value

0 if a timeout occurs and none of the selected events has occurred on any of the selected sockets, or

A positive integer which is the sum of the number of sockets in each fd_set for which events have occurred, or

SOCKET_ERROR if the function fails

Use WSAGetLastError to retrieve a specific error code.

Remarks

The following macros are provided for manipulating socket descriptor sets:

Macro	Description
FD_ZERO(&fdset);	Clears a socket descriptor set.
FD_SET(fd, &fdset);	Adds a socket descriptor fd to fdset.
FD_CLR(fd, &fdset);	Removes a socket descriptor fd from fdset.
FD_ISSET(fd, &fdset);	Is non-zero if fd is a member of fdset, zero otherwise.

FD_SETSIZE is a maximum number of socket descriptors which could be used in a descriptor set. The default is 64.

The RTX64 RT-TCP/IP Stack requires FD_SETSIZE to be greater than or equal to the number of sockets configured by the Stack. Setting it to a smaller value may result in a system crash.

Resolution of timeout depends on the HAL timer period. Note that timeout can sometimes occur within one HAL timer tick earlier than the requested time.