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Diffstat (limited to 'Documentation/userspace-api/media/v4l/func-poll.rst')
| -rw-r--r-- | Documentation/userspace-api/media/v4l/func-poll.rst | 40 |
1 files changed, 18 insertions, 22 deletions
diff --git a/Documentation/userspace-api/media/v4l/func-poll.rst b/Documentation/userspace-api/media/v4l/func-poll.rst index 95cf9c6fedcd..cbf4a0a10ae2 100644 --- a/Documentation/userspace-api/media/v4l/func-poll.rst +++ b/Documentation/userspace-api/media/v4l/func-poll.rst @@ -1,4 +1,5 @@ .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later +.. c:namespace:: V4L .. _func-poll: @@ -11,7 +12,6 @@ Name v4l2-poll - Wait for some event on a file descriptor - Synopsis ======== @@ -19,19 +19,16 @@ Synopsis #include <sys/poll.h> - .. c:function:: int poll( struct pollfd *ufds, unsigned int nfds, int timeout ) - :name: v4l2-poll Arguments ========= - Description =========== -With the :ref:`poll() <func-poll>` function applications can suspend execution +With the :c:func:`poll()` function applications can suspend execution until the driver has captured data or is ready to accept data for output. @@ -44,57 +41,56 @@ display. When buffers are already in the outgoing queue of the driver (capture) or the incoming queue isn't full (display) the function returns immediately. -On success :ref:`poll() <func-poll>` returns the number of file descriptors +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 :c:func:`struct pollfd` structure +``revents`` field of the respective ``struct pollfd`` structure is non-zero). Capture devices set the ``POLLIN`` and ``POLLRDNORM`` flags in the ``revents`` field, output devices the ``POLLOUT`` and ``POLLWRNORM`` flags. When the function timed out it returns a value of zero, on failure it returns -1 and the ``errno`` variable is set appropriately. When the application did not call -:ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` the :ref:`poll() <func-poll>` +:ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` the :c:func:`poll()` function succeeds, but sets the ``POLLERR`` flag in the ``revents`` field. When the application has called :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` for a capture device but hasn't yet called :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`, the -:ref:`poll() <func-poll>` function succeeds and sets the ``POLLERR`` flag in +:c:func:`poll()` function succeeds and sets the ``POLLERR`` flag in the ``revents`` field. For output devices this same situation will cause -:ref:`poll() <func-poll>` to succeed as well, but it sets the ``POLLOUT`` and +:c:func:`poll()` to succeed as well, but it sets the ``POLLOUT`` and ``POLLWRNORM`` flags in the ``revents`` field. If an event occurred (see :ref:`VIDIOC_DQEVENT`) then ``POLLPRI`` will be set in the ``revents`` field and -:ref:`poll() <func-poll>` will return. +:c:func:`poll()` will return. -When use of the :ref:`read() <func-read>` function has been negotiated and the -driver does not capture yet, the :ref:`poll() <func-poll>` function starts +When use of the :c:func:`read()` function has been negotiated and the +driver does not capture yet, the :c:func:`poll()` function starts capturing. When that fails it returns a ``POLLERR`` as above. Otherwise it waits until data has been captured and can be read. When the driver captures continuously (as opposed to, for example, still images) the function may return immediately. -When use of the :ref:`write() <func-write>` function has been negotiated and the -driver does not stream yet, the :ref:`poll() <func-poll>` function starts +When use of the :c:func:`write()` function has been negotiated and the +driver does not stream yet, the :c:func:`poll()` function starts streaming. When that fails it returns a ``POLLERR`` as above. Otherwise it waits until the driver is ready for a non-blocking -:ref:`write() <func-write>` call. +:c:func:`write()` call. If the caller is only interested in events (just ``POLLPRI`` is set in -the ``events`` field), then :ref:`poll() <func-poll>` will *not* start +the ``events`` field), then :c:func:`poll()` will *not* start streaming if the driver does not stream yet. This makes it possible to just poll for events and not for buffers. -All drivers implementing the :ref:`read() <func-read>` or :ref:`write() <func-write>` -function or streaming I/O must also support the :ref:`poll() <func-poll>` +All drivers implementing the :c:func:`read()` or :c:func:`write()` +function or streaming I/O must also support the :c:func:`poll()` function. -For more details see the :ref:`poll() <func-poll>` manual page. - +For more details see the :c:func:`poll()` manual page. Return Value ============ -On success, :ref:`poll() <func-poll>` returns the number structures which have +On success, :c:func:`poll()` returns the number 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: |