poll(2) - NetBSD Manual Pages

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POLL(2)                   NetBSD Programmer's 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, nfds_t 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 de-
     termines 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

     POLLWRNORM     Normal data may be written without blocking.

     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 ze-
     ro, 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 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)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)


HISTORY

     The poll() function call appeared in AT&T System V.3 UNIX.


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.

NetBSD 1.6                     September 7, 1996                             2