func-select.rst (3529B)
1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2.. c:namespace:: V4L 3 4.. _func-select: 5 6************* 7V4L2 select() 8************* 9 10Name 11==== 12 13v4l2-select - Synchronous I/O multiplexing 14 15Synopsis 16======== 17 18.. code-block:: c 19 20 #include <sys/time.h> 21 #include <sys/types.h> 22 #include <unistd.h> 23 24.. c:function:: int select( int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout ) 25 26Arguments 27========= 28 29``nfds`` 30 The highest-numbered file descriptor in any of the three sets, plus 1. 31 32``readfds`` 33 File descriptions to be watched if a read() call won't block. 34 35``writefds`` 36 File descriptions to be watched if a write() won't block. 37 38``exceptfds`` 39 File descriptions to be watched for V4L2 events. 40 41``timeout`` 42 Maximum time to wait. 43 44Description 45=========== 46 47With the :c:func:`select()` function applications can suspend 48execution until the driver has captured data or is ready to accept data 49for output. 50 51When streaming I/O has been negotiated this function waits until a 52buffer has been filled or displayed and can be dequeued with the 53:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. When buffers are already in 54the outgoing queue of the driver the function returns immediately. 55 56On success :c:func:`select()` returns the total number of bits set in 57``fd_set``. When the function timed out it returns 58a value of zero. On failure it returns -1 and the ``errno`` variable is 59set appropriately. When the application did not call 60:ref:`VIDIOC_QBUF` or 61:ref:`VIDIOC_STREAMON` yet the :c:func:`select()` 62function succeeds, setting the bit of the file descriptor in ``readfds`` 63or ``writefds``, but subsequent :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` 64calls will fail. [#f1]_ 65 66When use of the :c:func:`read()` function has been negotiated and the 67driver does not capture yet, the :c:func:`select()` function starts 68capturing. When that fails, :c:func:`select()` returns successful and 69a subsequent :c:func:`read()` call, which also attempts to start 70capturing, will return an appropriate error code. When the driver 71captures continuously (as opposed to, for example, still images) and 72data is already available the :c:func:`select()` function returns 73immediately. 74 75When use of the :c:func:`write()` function has been negotiated the 76:c:func:`select()` function just waits until the driver is ready for a 77non-blocking :c:func:`write()` call. 78 79All drivers implementing the :c:func:`read()` or :c:func:`write()` 80function or streaming I/O must also support the :c:func:`select()` 81function. 82 83For more details see the :c:func:`select()` manual page. 84 85Return Value 86============ 87 88On success, :c:func:`select()` returns the number of descriptors 89contained in the three returned descriptor sets, which will be zero if 90the timeout expired. On error -1 is returned, and the ``errno`` variable 91is set appropriately; the sets and ``timeout`` are undefined. Possible 92error codes are: 93 94EBADF 95 One or more of the file descriptor sets specified a file descriptor 96 that is not open. 97 98EBUSY 99 The driver does not support multiple read or write streams and the 100 device is already in use. 101 102EFAULT 103 The ``readfds``, ``writefds``, ``exceptfds`` or ``timeout`` pointer 104 references an inaccessible memory area. 105 106EINTR 107 The call was interrupted by a signal. 108 109EINVAL 110 The ``nfds`` argument is less than zero or greater than 111 ``FD_SETSIZE``. 112 113.. [#f1] 114 The Linux kernel implements :c:func:`select()` like the 115 :c:func:`poll()` function, but :c:func:`select()` cannot 116 return a ``POLLERR``.