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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 argument
     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 bit-mask bitmask
                    mask 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 spe-cific specific
     cific to particular file types or devices.

     If timeout is greater than zero, it specifies a maximum interval (in mil-liseconds) milliseconds)
     liseconds) 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 sub-sequent subsequent
                        sequent 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