1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2.. c:namespace:: V4L 3 4.. _func-read: 5 6*********** 7V4L2 read() 8*********** 9 10Name 11==== 12 13v4l2-read - Read from a V4L2 device 14 15Synopsis 16======== 17 18.. code-block:: c 19 20 #include <unistd.h> 21 22.. c:function:: ssize_t read( int fd, void *buf, size_t count ) 23 24Arguments 25========= 26 27``fd`` 28 File descriptor returned by :c:func:`open()`. 29 30``buf`` 31 Buffer to be filled 32 33``count`` 34 Max number of bytes to read 35 36Description 37=========== 38 39:c:func:`read()` attempts to read up to ``count`` bytes from file 40descriptor ``fd`` into the buffer starting at ``buf``. The layout of the 41data in the buffer is discussed in the respective device interface 42section, see ##. If ``count`` is zero, :c:func:`read()` returns zero 43and has no other results. If ``count`` is greater than ``SSIZE_MAX``, 44the result is unspecified. Regardless of the ``count`` value each 45:c:func:`read()` call will provide at most one frame (two fields) 46worth of data. 47 48By default :c:func:`read()` blocks until data becomes available. When 49the ``O_NONBLOCK`` flag was given to the :c:func:`open()` 50function it returns immediately with an ``EAGAIN`` error code when no data 51is available. The :c:func:`select()` or 52:c:func:`poll()` functions can always be used to suspend 53execution until data becomes available. All drivers supporting the 54:c:func:`read()` function must also support :c:func:`select()` and 55:c:func:`poll()`. 56 57Drivers can implement read functionality in different ways, using a 58single or multiple buffers and discarding the oldest or newest frames 59once the internal buffers are filled. 60 61:c:func:`read()` never returns a "snapshot" of a buffer being filled. 62Using a single buffer the driver will stop capturing when the 63application starts reading the buffer until the read is finished. Thus 64only the period of the vertical blanking interval is available for 65reading, or the capture rate must fall below the nominal frame rate of 66the video standard. 67 68The behavior of :c:func:`read()` when called during the active picture 69period or the vertical blanking separating the top and bottom field 70depends on the discarding policy. A driver discarding the oldest frames 71keeps capturing into an internal buffer, continuously overwriting the 72previously, not read frame, and returns the frame being received at the 73time of the :c:func:`read()` call as soon as it is complete. 74 75A driver discarding the newest frames stops capturing until the next 76:c:func:`read()` call. The frame being received at :c:func:`read()` 77time is discarded, returning the following frame instead. Again this 78implies a reduction of the capture rate to one half or less of the 79nominal frame rate. An example of this model is the video read mode of 80the bttv driver, initiating a DMA to user memory when :c:func:`read()` 81is called and returning when the DMA finished. 82 83In the multiple buffer model drivers maintain a ring of internal 84buffers, automatically advancing to the next free buffer. This allows 85continuous capturing when the application can empty the buffers fast 86enough. Again, the behavior when the driver runs out of free buffers 87depends on the discarding policy. 88 89Applications can get and set the number of buffers used internally by 90the driver with the :ref:`VIDIOC_G_PARM <VIDIOC_G_PARM>` and 91:ref:`VIDIOC_S_PARM <VIDIOC_G_PARM>` ioctls. They are optional, 92however. The discarding policy is not reported and cannot be changed. 93For minimum requirements see :ref:`devices`. 94 95Return Value 96============ 97 98On success, the number of bytes read is returned. It is not an error if 99this number is smaller than the number of bytes requested, or the amount 100of data required for one frame. This may happen for example because 101:c:func:`read()` was interrupted by a signal. On error, -1 is 102returned, and the ``errno`` variable is set appropriately. In this case 103the next read will start at the beginning of a new frame. Possible error 104codes are: 105 106EAGAIN 107 Non-blocking I/O has been selected using O_NONBLOCK and no data was 108 immediately available for reading. 109 110EBADF 111 ``fd`` is not a valid file descriptor or is not open for reading, or 112 the process already has the maximum number of files open. 113 114EBUSY 115 The driver does not support multiple read streams and the device is 116 already in use. 117 118EFAULT 119 ``buf`` references an inaccessible memory area. 120 121EINTR 122 The call was interrupted by a signal before any data was read. 123 124EIO 125 I/O error. This indicates some hardware problem or a failure to 126 communicate with a remote device (USB camera etc.). 127 128EINVAL 129 The :c:func:`read()` function is not supported by this driver, not 130 on this device, or generally not on this type of device. 131