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);

       int
       ppoll(struct pollfd fds[],             nfds_t nfds,             const struct timespec * restrict timeout,
           const sigset_t * restrict newsigmask);

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.

       The  ppoll()  system  call,  unlike poll(), is used to safely wait until either a set of file descriptors
       becomes ready or until a signal is caught.  The fds and nfds arguments are  identical  to  the  analogous
       arguments  of poll().  The timeout argument in ppoll() points to a const struct timespec which is defined
       in <sys/timespec.h> (shown below) rather than the int timeout used by poll().   A  null  pointer  may  be
       passed  to  indicate  that ppoll() should wait indefinitely.  Finally, newsigmask specifies a signal mask
       which is set while waiting for input.  When ppoll() returns, the original signal mask is restored.

       struct timespec {
               time_t  tv_sec;         /* seconds */
               long    tv_nsec;        /* and nanoseconds */
       };

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 invalid. One of its components is negative or too large.

SEE ALSO

       accept(2), connect(2), kqueue(2), pselect(2), read(2), recv(2), select(2), send(2), write(2)

STANDARDS

       The poll() function conforms to IEEE Std 1003.1-2001 (“POSIX.1”).  The ppoll() is not specified by POSIX.

HISTORY

       The poll() function appeared in AT&T System V UNIX.  This manual page and the core of the  implementation
       was taken from NetBSD.  The ppoll() function first appeared in FreeBSD 11.0

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                                          November 13, 2014                                        POLL(2)