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POLL(2)			    BSD	System Calls Manual		       POLL(2)

     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 argu-
     ment 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 block-

     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	inter-
     val 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	occured.  If the time limit expires, poll()
     returns 0.	 If poll() returns with	an error, including one	due to an in-
     terrupted 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 allo-
			cated 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), read(2), recv(2), select(2), send(2), write(2)

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

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

BSD				 July 8, 2002				   BSD


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