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

NAME
     poll, pollts -- synchronous I/O multiplexing

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <poll.h>

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

     #include <poll.h>
     #include <signal.h>
     #include <time.h>

     int
     pollts(struct pollfd * restrict fds, nfds_t nfds,
	 const struct timespec * restrict ts,
	 const sigset_t	* restrict sigmask);

DESCRIPTION
     poll() and	pollts() examine 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 the value	in fd is negative, the
		 file descriptor is ignored and	revents	is set to 0.

     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	    Normal data	may be written without blocking.

     POLLWRNORM	    Equivalent to POLLOUT.

     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.  If the remote end
		    of a socket	is closed, poll() returns a POLLIN event,
		    rather than	a POLLHUP.

     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.

     If	ts is a	non-null pointer, it references	a timespec structure which
     specifies a maximum interval to wait for any file descriptor to become
     ready.  If	ts is a	null pointer, pollts() blocks indefinitely.  If	ts is
     a non-null	pointer, referencing a zero-valued timespec structure, then
     pollts() will return without blocking.

     If	sigmask	is a non-null pointer, then the	pollts() function shall	re-
     place the signal mask of the caller by the	set of signals pointed to by
     sigmask before examining the descriptors, and shall restore the signal
     mask of the caller	before returning.

RETURN VALUES
     poll() 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 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)d 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 or	the number of
			pollfd structures specified is larger than the current
			file descriptor	resource limit.

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

HISTORY
     The poll()	function appeared in AT&T System V Release 3 UNIX.  The
     pollts() function first appeared in NetBSD	3.0.

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.

FreeBSD	13.0			  May 5, 2019			  FreeBSD 13.0

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

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