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.