POLL(2)                   FreeBSD System Calls Manual                  POLL(2)

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, or in capability mode the
                    file descriptor has insufficient rights.  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.

     [EINVAL]           The number of pollfd structures specified by nfds
                        exceeds the system tunable kern.maxfilesperproc and
                        FD_SETSIZE.

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.

FreeBSD 13.1-RELEASE-p6        February 27, 2019       FreeBSD 13.1-RELEASE-p6