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POLL(2) 		       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 blocking.

     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 block-
		    ing.

     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.

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 4.5			September 7, 1996			 POLL(2)
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