POLL(3)                    POSIX Programmer's Manual                   POLL(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       poll -- input/output multiplexing

SYNOPSIS
       #include <poll.h>

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

DESCRIPTION
       The poll() function provides applications with a mechanism for
       multiplexing input/output over a set of file descriptors. For each
       member of the array pointed to by fds, poll() shall examine the given
       file descriptor for the event(s) specified in events.  The number of
       pollfd structures in the fds array is specified by nfds.  The poll()
       function shall identify those file descriptors on which an application
       can read or write data, or on which certain events have occurred.

       The fds argument specifies the file descriptors to be examined and the
       events of interest for each file descriptor. It is a pointer to an
       array with one member for each open file descriptor of interest. The
       array's members are pollfd structures within which fd specifies an open
       file descriptor and events and revents are bitmasks constructed by
       OR'ing a combination of the following event flags:

       POLLIN      Data other than high-priority data may be read without
                   blocking.

                   For STREAMS, this flag is set in revents even if the
                   message is of zero length. This flag shall be equivalent to
                   POLLRDNORM | POLLRDBAND.

       POLLRDNORM  Normal data may be read without blocking.

                   For STREAMS, data on priority band 0 may be read without
                   blocking. This flag is set in revents even if the message
                   is of zero length.

       POLLRDBAND  Priority data may be read without blocking.

                   For STREAMS, data on priority bands greater than 0 may be
                   read without blocking. This flag is set in revents even if
                   the message is of zero length.

       POLLPRI     High-priority data may be read without blocking.

                   For STREAMS, this flag is set in revents even if the
                   message is of zero length.

       POLLOUT     Normal data may be written without blocking.

                   For STREAMS, data on priority band 0 may be written without
                   blocking.

       POLLWRNORM  Equivalent to POLLOUT.

       POLLWRBAND  Priority data may be written.

                   For STREAMS, data on priority bands greater than 0 may be
                   written without blocking. If any priority band has been
                   written to on this STREAM, this event only examines bands
                   that have been written to at least once.

       POLLERR     An error has occurred on the device or stream. This flag is
                   only valid in the revents bitmask; it shall be ignored in
                   the events member.

       POLLHUP     A device has been disconnected, or a pipe or FIFO has been
                   closed by the last process that had it open for writing.
                   Once set, the hangup state of a FIFO shall persist until
                   some process opens the FIFO for writing or until all read-
                   only file descriptors for the FIFO are closed. This event
                   and POLLOUT are mutually-exclusive; a stream can never be
                   writable if a hangup has occurred. However, this event and
                   POLLIN, POLLRDNORM, POLLRDBAND, or POLLPRI are not
                   mutually-exclusive. This flag is only valid in the revents
                   bitmask; it shall be ignored in the events member.

       POLLNVAL    The specified fd value is invalid. This flag is only valid
                   in the revents member; it shall ignored in the events
                   member.

       The significance and semantics of normal, priority, and high-priority
       data are file and device-specific.

       If the value of fd is less than 0, events shall be ignored, and revents
       shall be set to 0 in that entry on return from poll().

       In each pollfd structure, poll() shall clear the revents member, except
       that where the application requested a report on a condition by setting
       one of the bits of events listed above, poll() shall set the
       corresponding bit in revents if the requested condition is true. In
       addition, poll() shall set the POLLHUP, POLLERR, and POLLNVAL flag in
       revents if the condition is true, even if the application did not set
       the corresponding bit in events.

       If none of the defined events have occurred on any selected file
       descriptor, poll() shall wait at least timeout milliseconds for an
       event to occur on any of the selected file descriptors. If the value of
       timeout is 0, poll() shall return immediately. If the value of timeout
       is -1, poll() shall block until a requested event occurs or until the
       call is interrupted.

       Implementations may place limitations on the granularity of timeout
       intervals. If the requested timeout interval requires a finer
       granularity than the implementation supports, the actual timeout
       interval shall be rounded up to the next supported value.

       The poll() function shall not be affected by the O_NONBLOCK flag.

       The poll() function shall support regular files, terminal and pseudo-
       terminal devices, FIFOs, pipes, sockets and STREAMS-based files.  The
       behavior of poll() on elements of fds that refer to other types of file
       is unspecified.

       Regular files shall always poll TRUE for reading and writing.

       A file descriptor for a socket that is listening for connections shall
       indicate that it is ready for reading, once connections are available.
       A file descriptor for a socket that is connecting asynchronously shall
       indicate that it is ready for writing, once a connection has been
       established.

       Provided the application does not perform any action that results in
       unspecified or undefined behavior, the value of the fd and events
       members of each element of fds shall not be modified by poll().

RETURN VALUE
       Upon successful completion, poll() shall return a non-negative value. A
       positive value indicates the total number of pollfd structures that
       have selected events (that is, those for which the revents member is
       non-zero). A value of 0 indicates that the call timed out and no file
       descriptors have been selected. Upon failure, poll() shall return -1
       and set errno to indicate the error.

ERRORS
       The poll() function shall fail if:

       EAGAIN The allocation of internal data structures failed but a
              subsequent request may succeed.

       EINTR  A signal was caught during poll().

       EINVAL The nfds argument is greater than {OPEN_MAX}, or one of the fd
              members refers to a STREAM or multiplexer that is linked
              (directly or indirectly) downstream from a multiplexer.

       The following sections are informative.

EXAMPLES
   Checking for Events on aa Stream
       The following example opens a pair of STREAMS devices and then waits
       for either one to become writable. This example proceeds as follows:

        1. Sets the timeout parameter to 500 milliseconds.

        2. Opens the STREAMS devices /dev/dev0 and /dev/dev1, and then polls
           them, specifying POLLOUT and POLLWRBAND as the events of interest.

           The STREAMS device names /dev/dev0 and /dev/dev1 are only examples
           of how STREAMS devices can be named; STREAMS naming conventions may
           vary among systems conforming to the POSIX.1-2008.

        3. Uses the ret variable to determine whether an event has occurred on
           either of the two STREAMS. The poll() function is given 500
           milliseconds to wait for an event to occur (if it has not occurred
           prior to the poll() call).

        4. Checks the returned value of ret.  If a positive value is returned,
           one of the following can be done:

            a. Priority data can be written to the open STREAM on priority
               bands greater than 0, because the POLLWRBAND event occurred on
               the open STREAM (fds[0] or fds[1]).

            b. Data can be written to the open STREAM on priority-band 0,
               because the POLLOUT event occurred on the open STREAM (fds[0]
               or fds[1]).

        5. If the returned value is not a positive value, permission to write
           data to the open STREAM (on any priority band) is denied.

        6. If the POLLHUP event occurs on the open STREAM (fds[0] or fds[1]),
           the device on the open STREAM has disconnected.


           #include <stropts.h>
           #include <poll.h>
           ...
           struct pollfd fds[2];
           int timeout_msecs = 500;
           int ret;
               int i;

           /* Open STREAMS device. */
           fds[0].fd = open("/dev/dev0", ...);
           fds[1].fd = open("/dev/dev1", ...);
           fds[0].events = POLLOUT | POLLWRBAND;
           fds[1].events = POLLOUT | POLLWRBAND;

           ret = poll(fds, 2, timeout_msecs);

           if (ret > 0) {
               /* An event on one of the fds has occurred. */
               for (i=0; i<2; i++) {
                   if (fds[i].revents & POLLWRBAND) {
                   /* Priority data may be written on device number i. */
           ...
                   }
                   if (fds[i].revents & POLLOUT) {
                   /* Data may be written on device number i. */
           ...
                   }
                   if (fds[i].revents & POLLHUP) {
                   /* A hangup has occurred on device number i. */
           ...
                   }
               }
           }

APPLICATION USAGE
       None.

RATIONALE
       The POLLHUP event does not occur for FIFOs just because the FIFO is not
       open for writing. It only occurs when the FIFO is closed by the last
       writer and persists until some process opens the FIFO for writing or
       until all read-only file descriptors for the FIFO are closed.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section 2.6, STREAMS, getmsg(), pselect(), putmsg(), read(), write()

       The Base Definitions volume of POSIX.1-2017, <poll.h>, <stropts.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                              POLL(3)
