select(2)                                                     select(2)

  select()

  NAME

    select() - synchronous I/O multiplexing

  SYNOPSIS

    #include 
    #include 
    #include 

    int select (int nfds, fd_set *readfds, fd_set *writefds,
                fd_set *exceptfds, struct timeval *timeout)
    FD_SET (fd, &fdset)
    FD_CLR (fd, &fdset)
    FD_ISSET (fd, &fdset)
    FD_ZERO (&fdset)

  DESCRIPTION

    The select(2) function examines the I/O descriptor sets whose addresses
    are passed in readfds, writefds, and exceptfds to see if some of their
    descriptors are ready for reading, are ready for writing, or have an
    exceptional condition pending, respectively. The first nfds descriptors
    are checked in each set; that is, the descriptors from 0 through nfds-1 in
    the descriptor sets are examined. On return, select(2) replaces the given
    descriptor sets with subsets consisting of those descriptors that are
    ready for the requested operation. The select(2) function returns the
    total number of ready descriptors in all the sets.

    The descriptor sets are stored as bit fields in arrays of integers. The
    following macros are provided for manipulating such descriptor sets:
    FD_ZERO (&fdsetx)
        initializes a descriptor set fdset to the null set.
    FD_SET(fd, &fdset)
        includes a particular descriptor fd in fdset.
    FD_CLR(fd, &fdset)
        removes fd from fdset.
    FD_ISSET(fd, &fdset)
        is non-zero if fd is a member of fdset, zero otherwise.

    The behavior of these macros is undefined if a descriptor value is less
    than zero or greater than or equal to FD_SETSIZE, which is normally the
    maximum number of descriptors supported by the system or greater. In this
    implementation, FD_SETSIZE is the maximum number of descriptors supported
    by the system (the value of {OPEN_MAX}).

    If timeout is a non-nil pointer, it specifies a maximum interval to wait
    for the selection to complete. If timeout is a nil pointer, the select
    blocks indefinitely. To effect a poll, the timeout argument should be non-
    nil, pointing to a zero-valued timeval structure.

    Any of readfds, writefds, and exceptfds may be given as nil pointers if no
    descriptors are of interest.

  RETURN VALUES

    The select(2) function returns the number of ready descriptors that are
    contained in the descriptor sets, or -1 if an error occurred. If the time
    limit expires, select(2) returns 0. If select(2) returns with an error,
    including one due to an interrupted call, the descriptor sets will be
    unmodified.

  ERRORS

    An error return from select(2) indicates:

    [EBADF]
        One of the descriptor sets specified an invalid descriptor.

    [EINTR]
        A signal was delivered before the time limit expired and before any of
        the selected events occurred.

    [EINVAL]
        The specified time limit is invalid. One of its components is negative
        or too large.

  SEE ALSO

    accept(2)

    connect(2)

    getdtablesize(3)

    gettimeofday(3)

    read(2)

    recv(2)

    send(2)

    write(2)

  BUGS

    User programs cannot be written independent of the limit on the number of
    open files, and the dimension of a sufficiently large bit field for select
    remains a problem. The default size FD_SETSIZE is the current maximum
    number of open files ({OPEN_MAX}). In this implementation, you cannot open
    more files than {OPEN_MAX} with select(2) so there is no reason to change
    the value of FD_SETSIZE.

    The select(2) function should probably return the time remaining from the
    original time-out, if any, by modifying the time value in place. This may
    be implemented in future versions of the system. Thus, it is unwise to
    assume that the time-out value will be unmodified by the select(2) call.

  USAGE NOTES

    The select function is thread safe.

    The select function is async-signal safe.