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.

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                                          February 27, 2019                                        POLL(2)