NAME

       poll — synchronous I/O multiplexing

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

       #include <poll.h>

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

DESCRIPTION

       The  poll() system call 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 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 fd is equal to -1 then revents is cleared (set to zero), and
                   that pollfd is not checked.

       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 blocking.

       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.

       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  interval  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.

RETURN VALUES

       The poll() system call 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 system 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]           The fds argument 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), kqueue(2), read(2), recv(2), select(2), send(2), write(2)

HISTORY

       The poll() function appeared in AT&T System V UNIX.  This manual page and the core of the  implementation
       was taken from NetBSD.

BUGS

       The  distinction  between  some  of  the  fields  in the events and revents bitmasks is really not useful
       without STREAMS.  The fields are defined for compatibility with existing software.

Debian                                            July 8, 2002                                           POLL(2)