poll man page on FreeBSD

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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 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 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), kqueue(2), read(2), recv(2), select(2), send(2),

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

     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.

BSD				 July 8, 2002				   BSD

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