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FreeBSD 11.0 - man page for poll (freebsd section 2)

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);

     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 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 blocking.

     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 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 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 ppoll() system call, unlike poll(), is used to safely wait until either a set of file
     descriptors becomes ready or until a signal is caught.  The fds and nfds arguments are iden-
     tical 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 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 his-
     torical 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 invalid. One of its components is negative or
			too large.

     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() function first appeared in FreeBSD 11.0

     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.

BSD					November 13, 2014				      BSD

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