^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 1) .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 2) .. c:namespace:: V4L
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 3)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 4) .. _func-select:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 5)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 6) *************
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 7) V4L2 select()
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 8) *************
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 9)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 10) Name
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 11) ====
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 12)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 13) v4l2-select - Synchronous I/O multiplexing
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 14)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 15) Synopsis
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 16) ========
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 17)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 18) .. code-block:: c
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 19)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 20) #include <sys/time.h>
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 21) #include <sys/types.h>
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 22) #include <unistd.h>
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 23)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 24) .. c:function:: int select( int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout )
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 25)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 26) Arguments
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 27) =========
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 28)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 29) ``nfds``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 30) The highest-numbered file descriptor in any of the three sets, plus 1.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 31)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 32) ``readfds``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 33) File descriptions to be watched if a read() call won't block.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 34)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 35) ``writefds``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 36) File descriptions to be watched if a write() won't block.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 37)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 38) ``exceptfds``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 39) File descriptions to be watched for V4L2 events.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 40)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 41) ``timeout``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 42) Maximum time to wait.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 43)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 44) Description
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 45) ===========
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 46)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 47) With the :c:func:`select()` function applications can suspend
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 48) execution until the driver has captured data or is ready to accept data
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 49) for output.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 50)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 51) When streaming I/O has been negotiated this function waits until a
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 52) buffer has been filled or displayed and can be dequeued with the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 53) :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. When buffers are already in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 54) the outgoing queue of the driver the function returns immediately.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 55)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 56) On success :c:func:`select()` returns the total number of bits set in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 57) ``fd_set``. When the function timed out it returns
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 58) a value of zero. On failure it returns -1 and the ``errno`` variable is
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 59) set appropriately. When the application did not call
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 60) :ref:`VIDIOC_QBUF` or
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 61) :ref:`VIDIOC_STREAMON` yet the :c:func:`select()`
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 62) function succeeds, setting the bit of the file descriptor in ``readfds``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 63) or ``writefds``, but subsequent :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 64) calls will fail. [#f1]_
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 65)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 66) When use of the :c:func:`read()` function has been negotiated and the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 67) driver does not capture yet, the :c:func:`select()` function starts
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 68) capturing. When that fails, :c:func:`select()` returns successful and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 69) a subsequent :c:func:`read()` call, which also attempts to start
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 70) capturing, will return an appropriate error code. When the driver
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 71) captures continuously (as opposed to, for example, still images) and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 72) data is already available the :c:func:`select()` function returns
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 73) immediately.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 74)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 75) When use of the :c:func:`write()` function has been negotiated the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 76) :c:func:`select()` function just waits until the driver is ready for a
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 77) non-blocking :c:func:`write()` call.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 78)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 79) All drivers implementing the :c:func:`read()` or :c:func:`write()`
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 80) function or streaming I/O must also support the :c:func:`select()`
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 81) function.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 82)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 83) For more details see the :c:func:`select()` manual page.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 84)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 85) Return Value
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 86) ============
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 87)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 88) On success, :c:func:`select()` returns the number of descriptors
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 89) contained in the three returned descriptor sets, which will be zero if
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 90) the timeout expired. On error -1 is returned, and the ``errno`` variable
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 91) is set appropriately; the sets and ``timeout`` are undefined. Possible
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 92) error codes are:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 93)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 94) EBADF
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 95) One or more of the file descriptor sets specified a file descriptor
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 96) that is not open.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 97)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 98) EBUSY
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 99) The driver does not support multiple read or write streams and the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 100) device is already in use.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 101)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 102) EFAULT
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 103) The ``readfds``, ``writefds``, ``exceptfds`` or ``timeout`` pointer
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 104) references an inaccessible memory area.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 105)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 106) EINTR
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 107) The call was interrupted by a signal.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 108)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 109) EINVAL
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 110) The ``nfds`` argument is less than zero or greater than
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 111) ``FD_SETSIZE``.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 112)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 113) .. [#f1]
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 114) The Linux kernel implements :c:func:`select()` like the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 115) :c:func:`poll()` function, but :c:func:`select()` cannot
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 116) return a ``POLLERR``.
Orange Pi5 kernel
Deprecated Linux kernel 5.10.110 for OrangePi 5/5B/5+ boards
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