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POLL(2)			  FreeBSD System Calls Manual		       POLL(2)

NAME
     poll -- synchronous I/O multiplexing

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <sys/types.h>
     #include <poll.h>

     int
     poll(struct pollfd	*fds, unsigned int nfds, int timeout);

DESCRIPTION
     Poll() 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
		    blocking.

     POLLRDNORM	    Normal data	may be read without blocking.

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

     POLLPRI	    High priority data may be read without blocking.

     POLLOUT

     POLLWRNORM	    Normal data	may be written without blocking.

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

     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.

RETURN VALUES
     Poll() 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 interrupted call,
     the fds array will	be unmodified.

COMPATIBILITY
     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.

ERRORS
     An	error return from poll() indicates:

     [EFAULT]		Fds 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.

SEE ALSO
     accept(2),	connect(2), read(2), recv(2), select(2), send(2), write(2)

BUGS
     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.

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

FreeBSD	10.1		       September 7, 1996		  FreeBSD 10.1

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | COMPATIBILITY | ERRORS | SEE ALSO | BUGS | HISTORY

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