xref: /openbmc/linux/Documentation/userspace-api/media/mediactl/request-func-poll.rst (revision cbecf716ca618fd44feda6bd9a64a8179d031fc5)

.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.1-no-invariants-or-later
.. c:namespace:: MC

.. _request-func-poll:

**************
request poll()
**************

Name
====

request-poll - Wait for some event on a file descriptor

Synopsis
========

.. code-block:: c

    #include <sys/poll.h>

.. c:function:: int poll( struct pollfd *ufds, unsigned int nfds, int timeout )

Arguments
=========

``ufds``
   List of file descriptor events to be watched

``nfds``
   Number of file descriptor events at the \*ufds array

``timeout``
   Timeout to wait for events

Description
===========

With the :c:func:`poll()` function applications can wait
for a request to complete.

On success :c:func:`poll()` returns the number of file
descriptors that have been selected (that is, file descriptors for which the
``revents`` field of the respective struct :c:type:`pollfd`
is non-zero). Request file descriptor set the ``POLLPRI`` flag in ``revents``
when the request was completed.  When the function times out it returns
a value of zero, on failure it returns -1 and the ``errno`` variable is
set appropriately.

Attempting to poll for a request that is not yet queued will
set the ``POLLERR`` flag in ``revents``.

Return Value
============

On success, :c:func:`poll()` returns the number of
structures which have non-zero ``revents`` fields, or zero if the call
timed out. On error -1 is returned, and the ``errno`` variable is set
appropriately:

``EBADF``
    One or more of the ``ufds`` members specify an invalid file
    descriptor.

``EFAULT``
    ``ufds`` references an inaccessible memory area.

``EINTR``
    The call was interrupted by a signal.

``EINVAL``
    The ``nfds`` value exceeds the ``RLIMIT_NOFILE`` value. Use
    ``getrlimit()`` to obtain this value.