poll man page on NetBSD

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POLL(2)			    BSD 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
     replace 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.

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.

BSD			       September 8, 2006			   BSD
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