POLL(2) 						      BSD System Calls Manual							   POLL(2)

NAME

     poll -- synchronous I/O multiplexing

SYNOPSIS

     #include <poll.h>

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

DESCRIPTION

     Poll() examines a set of file descriptors to see if some of them are ready for I/O or if certain events have occurred on them.  The fds argu-
     ment is a pointer to an array of pollfd structures, as defined in <poll.h> (shown below).	The nfds argument specifies 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.

     events	    Events to poll for.  (See below.)

     revents	    Events which may occur or have occurred.  (See below.)

     The event bitmasks in events and revents have the following bits:

     POLLERR	    An exceptional condition has occurred on the device or socket.  This flag is output only, and ignored if present in the input
		    events bitmask.

     POLLHUP	    The device or socket has been disconnected.  This flag is output only, and ignored if present in the input events bitmask.
		    Note that POLLHUP and POLLOUT are mutually exclusive and should never be present in the revents bitmask at the same time.

     POLLIN	    Data other than high priority data may be read without blocking.  This is equivalent to ( POLLRDNORM | POLLRDBAND ).

     POLLNVAL	    The file descriptor is not open.  This flag is output only, and ignored if present in the input events bitmask.

     POLLOUT	    Normal data may be written without blocking.  This is equivalent to POLLWRNORM.

     POLLPRI	    High priority data may be read without blocking.

     POLLRDBAND     Priority data may be read without blocking.

     POLLRDNORM     Normal data may be read without blocking.

     POLLWRBAND     Priority data may be written without blocking.

     POLLWRNORM     Normal data may be written without blocking.

     The distinction between normal, priority, and high-priority data is specific to particular file types or devices.

     If timeout is greater than zero, it specifies a maximum interval (in milliseconds) to wait for any file descriptor to become ready.  If
     timeout is zero, then poll() will return without blocking. If the value of timeout is -1, the poll blocks indefinitely.

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 and the global variable errno
     will be set to indicate the error.

ERRORS

     Poll() will fail if:

     [EAGAIN]		Allocation of internal data structures fails.  A subsequent request may succeed.

     [EFAULT]		Fds points outside the process's allocated address space.

     [EINTR]		A signal is delivered before the time limit expires and before any of the selected events occurs.

     [EINVAL]		The nfds argument is greater than OPEN_MAX or the timeout argument is less than -1.

BUGS

     The poll() system call currently does not support devices.

SEE ALSO

     accept(2), connect(2), kevent(2), read(2), recv(2), select(2), send(2), write(2)

HISTORY

     The poll() function call appeared in AT&T System V UNIX.

BSD
								 February 27, 2005							       BSD