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POLL(2)			  FreeBSD 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);

     ppoll(struct pollfd fds[],	nfds_t nfds,
	 const struct timespec * restrict timeout,
	 const sigset_t	* restrict newsigmask);

     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, or	in capability mode the
		    file descriptor has	insufficient rights.  This flag	is al-
		    ways 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 ppoll() system	call, unlike poll(), is	used to	safely wait until ei-
     ther a set	of file	descriptors becomes ready or until a signal is caught.
     The fds and nfds arguments	are identical to the analogous arguments of
     poll().  The timeout argument in ppoll() points to	a const	struct
     timespec which is defined in <sys/timespec.h> (shown below) rather	than
     the int timeout used by poll().  A	null pointer may be passed to indicate
     that ppoll() should wait indefinitely.  Finally, newsigmask specifies a
     signal mask which is set while waiting for	input.	When ppoll() returns,
     the original signal mask is restored.

     struct timespec {
	     time_t  tv_sec;	     /*	seconds	*/
	     long    tv_nsec;	     /*	and nanoseconds	*/

     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 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 invalid.  One of its com-
			ponents	is negative or too large.

     [EINVAL]		The number of pollfd structures	specified by nfds ex-
			ceeds the system tunable kern.maxfilesperproc and

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

     The poll()	function conforms to IEEE Std 1003.1-2001 ("POSIX.1").	The
     ppoll() is	not specified by POSIX.

     The poll()	function appeared in AT&T System V UNIX.  This manual page and
     the core of the implementation was	taken from NetBSD.  The	ppoll()	func-
     tion first	appeared in FreeBSD 11.0

     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.

FreeBSD	13.0		       February	27, 2019		  FreeBSD 13.0


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