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     poll — synchronous I/O multiplexing


     Standard C Library (libc, -lc)


     #include <poll.h>

     poll(struct pollfd fds[], nfds_t nfds, int timeout);


     The poll() system call examines a set of file descriptors to see if some
     of them are ready for I/O.  The fds argument is a pointer to an array of
     pollfd structures as defined in <poll.h> (shown below).  The nfds
     argument determines the size of the fds array.

     struct pollfd {
         int    fd;       /* file descriptor */
         short  events;   /* events to look for */
         short  revents;  /* events returned */

     The fields of struct pollfd are as follows:

     fd          File descriptor to poll.  If fd is equal to -1 then revents
                 is cleared (set to zero), and that pollfd is not checked.

     events      Events to poll for.  (See below.)

     revents     Events which may occur.  (See below.)

     The event bitmasks in events and revents have the following bits:

     POLLIN         Data other than high priority data may be read without

     POLLRDNORM     Normal data may be read without blocking.

     POLLRDBAND     Data with a non-zero priority may be read without

     POLLPRI        High priority data may be read without blocking.


     POLLWRNORM     Normal data may be written without blocking.

     POLLWRBAND     Data with a non-zero priority may be written without

     POLLERR        An exceptional condition has occurred on the device or
                    socket.  This flag is always checked, even if not present
                    in the events bitmask.

     POLLHUP        The device or socket has been disconnected.  This flag is
                    always checked, even if not present in the events bitmask.
                    Note that POLLHUP and POLLOUT should never be present in
                    the revents bitmask at the same time.

     POLLNVAL       The file descriptor is not open.  This flag is always
                    checked, even if not present in the events bitmask.

     If timeout is neither zero nor INFTIM (-1), it specifies a maximum
     interval to wait for any file descriptor to become ready, in
     milliseconds.  If timeout is INFTIM (-1), the poll blocks indefinitely.
     If timeout is zero, then poll() will return without blocking.


     The poll() system call returns the number of descriptors that are ready
     for I/O, or -1 if an error occurred.  If the time limit expires, poll()
     returns 0.  If poll() returns with an error, including one due to an
     interrupted system call, the fds array will be unmodified.


     This implementation differs from the historical one in that a given file
     descriptor may not cause poll() to return with an error.  In cases where
     this would have happened in the historical implementation (e.g. trying to
     poll a revoke(2)ed descriptor), this implementation instead copies the
     events bitmask to the revents bitmask.  Attempting to perform I/O on this
     descriptor will then return an error.  This behaviour is believed to be
     more useful.


     An error return from poll() indicates:

     [EFAULT]           The fds argument points outside the process's
                        allocated address space.

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

     [EINVAL]           The specified time limit is negative.


     accept(2), connect(2), kqueue(2), read(2), recv(2), select(2), send(2),


     The poll() function appeared in AT&T System V UNIX.  This manual page and
     the core of the implementation was taken from NetBSD.


     The distinction between some of the fields in the events and revents
     bitmasks is really not useful without STREAMS.  The fields are defined
     for compatibility with existing software.