select

Allows the user to poll or check a group of sockets for I/O activity. It can check what sockets are ready to be read or written, or what sockets have a pending exception.

As of OpenVMS Version 7.0, this routine can operate on any number of sockets up to the limit of open files supported by the DEC C RTL (65535 on OpenVMS Alpha; 2047 on OpenVMS VAX). However, for select to use more than 32 sockets, the DEC TCP/IP Services for OpenVMS used with the DEC C RTL must offer the same support.

Format

#include  <socket.h>

int select  (int nfds, int *readfds, int
            *writefds, int *execptfds, struct
            timeval *timeout);

Arguments

nfds

The highest numbered socket descriptor to search for. That is, the highest numbered bit +1 in readfds, writefds, and exceptfds that should be examined. Descriptor s is represented by 1<< s (1 shifted to the left s number of times).

readfds

A pointer to an array of bits, organized as integers (each integer describing 32 descriptors), that should be examined for read readiness. If bit n of the longword is set, socket descriptor n will be checked to see if it is ready to be read. All bits set in the bit mask must correspond to the file descriptors of sockets. The select routine cannot be used on normal files.

On return, the array of bits to which readfds points contains a bit mask of the sockets that are ready for reading. Only bits that are set on entry to select will be set on exit.

writefds

A pointer to a longword bit mask of the socket descriptors that should be examined for write readiness. If bit n of the longword is set, socket descriptor n will be checked to see if it is ready to be written to. All bits set in the bit mask must correspond to socket descriptors.

On return, the array of bits that writefds points to contains a bit mask of the sockets that are ready for writing. Only bits that are set on entry to select will be set on exit.

exceptfds

A pointer to a longword bit mask of the socket descriptors that should be examined for exceptions. If bit n of the longword is set, socket descriptor n will be checked to see if it has any pending exceptions. All bits set in the bit mask must correspond to the file descriptors of sockets.

On return, the array of bits pointer exceptfds contains a bit mask of the sockets that have exceptions pending. Only bits that are set on entry to select will be set on exit.

timeout

The length of time that select should examine the sockets before returning. If one of the sockets specified in the readfds, writefds, and exceptfds bit masks is ready for I/O, select will return before the timeout period has expired.

The timeout structure points to a timeval structure. See <socket.h> for a description of the timeval structure.

Description

If the timeout argument is NULL, select will block indefinitely. In order to effect a poll, timeout should be non-NULL, and should point to a zero-valued structure.

If a process is blocked on a select while waiting for input from a socket and the sending process closes the socket, the select notes this as an event and will unblock the process. The descriptors are always modified on return if select returns because of the timeout.

Note

When the socket option SO_ OOBINLINE is set on the device_socket, a select on both read and exception events returns the socket mask set on both the read and exception mask. Otherwise, only the exception mask is set.

See also accept, connect, read, recv, recvfrom, recvmsg, send, sendmsg, sendto, and write in this section.

Return Values

n	The number of sockets that were ready for I/O or that had pending exceptions. This value matches the number of returned bits that are set in all output masks.
0	Indicates that select timed out before any socket became ready for I/O.
- 1	Indicates an error; errno is set to one of the following values: EBADF - One of the bit masks specified an invalid descriptor. EINVAL - The specified time limit is unacceptable. One of its components is negative or too large.

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