VMS Help TCPIP Services, Programming Interfaces, Sockets API, select()  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Allows you to poll or check a group of sockets for I/O activity.
    This function indicates which sockets are ready to be read or
    written, or which sockets have an exception pending.
    Format
      #include  <time.h>
      int select  ( int nfds, int *readfds, int *writefds, int
                  *execptfds, struct timeval *timeout );

1 - Arguments

 nfds
    Specifies the number of open objects that may be ready for
    reading or writing or that have exceptions pending. The nfds
    argument is normally limited to FD_SETSIZE. Note that a single
    process can have a maximum of 65535 simultaneous channels
    (including sockets) on OpenVMS Alpha systems, and a maximum of
    2047 on OpenVMS VAX systems.
 readfds
    A pointer to an array of bits, organized as integers, that should
    be examined for read readiness. If bit n of the longword is set,
    socket descriptor n is checked to see whether it is ready to be
    read. All bits set in the bit mask must correspond to the file
    descriptors of sockets. The select() function cannot be used on
    normal files.
    On return, the array to which readfds points contains a bit mask
    of the sockets that are ready for reading. Only bits that were
    set on entry to the select() function can be set on exit.
 writefds
    A pointer to an array of bits, organized as integers, that should
    be examined for write readiness. If bit n of the longword is
    set, socket descriptor n is checked to see whether it is ready
    to be written to. All bits set in the bit mask must correspond to
    socket descriptors.
    On return, the array to which writefds points contains a bit mask
    of the sockets that are ready for writing. Only bits that were
    set on entry to the select() function are set on exit.
 exceptfds
    A pointer to an array of bits, organized as integers, that
    is examined for exceptions. If bit n of the longword is set,
    socket descriptor n is checked to see whether it has any pending
    exceptions. All bits set in the bit mask must correspond to the
    file descriptors of sockets.
    On return, the array exceptfds pointer contains a bit mask of the
    sockets that have exceptions pending. Only bits that were set on
    entry to the select() function can be set on exit.
 timeout
    The length of time that the select() function 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, the
    select() function returns before the timeout period expires.
    The timeout argument points to a timeval structure.

2 - Description

    This function determines the I/O status of the sockets specified
    in the various mask arguments. It returns when a socket is ready
    to be read or written, when the timeout period expires, or when
    exceptions occur. If timeout is a non-null pointer, it specifies
    a maximum interval to wait for the selection to complete.
    If the timeout argument is null, the select() function blocks
    indefinitely until a selected event occurs. To effect a poll,
    the value for timeout should be non-null, and should point to a
    zero-value structure.
    If a process is blocked on a select() function while waiting for
    input for a socket and the sending process closes the socket,
    then the select() function notes this as an event and unblocks
    the process. The descriptors are always modified on return if the
    select() function returns because of the timeout.
                                   NOTE
       When the socket option SO_OOBINLINE is set on the device
       socket, the select() function on both read and exception
       events returns the socket mask that is set on both the read
       and the exception mask. Otherwise, only the exception mask
       is set.
    Related Functions
    See also accept(), connect(),  read(), recv(), recvfrom(),
    recvmsg(), send(),  sendmsg(), sendto(), and write().

3 - Return Values

    n                  The number of sockets ready for I/O or pending
                       exceptions. This value matches the number
                       of returned bits that are set in all output
                       masks.
    0                  The select() function timed out before any
                       socket became ready for I/O.
    -1                 Error; errno is set to indicate the error.

4 - Errors

    EBADF              One or more of the I/O descriptor sets
                       specified an invalid file descriptor.
    EINTR              A signal was delivered before the time limit
                       specified by the timeout argument expired and
                       before any of the selected events occurred.
    EINVAL             The time limit specified by the timeout
                       argument is invalid.
                       The nfds argument is less than zero, or
                       greater than or equal to FD_SETSIZE.
    EAGAIN             Allocation of internal data structures failed.
                       A later call to the select() function may
                       complete successfully.
    ENETDOWN           TCP/IP Services was not started.
    ENOTSOCK           The socket descriptor is invalid.