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

     poll -- synchronous I/O multiplexing

     #include <poll.h>

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

     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 argument 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 exclu-sive exclusive
                    sive 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.

     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 inter-rupted interrupted
     rupted call, the fds array will be unmodified and the global variable errno will be set to indicate the

     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.

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

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

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

BSD                            February 27, 2005                           BSD

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