.. -*- coding: utf-8; mode: rst -*- .. _func-select: ************* V4L2 select() ************* Name ==== v4l2-select - Synchronous I/O multiplexing Synopsis ======== .. code-block:: c #include #include #include .. c:function:: int select( int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout ) :name: v4l2-select Arguments ========= ``nfds`` The highest-numbered file descriptor in any of the three sets, plus 1. ``readfds`` File descriptions to be watched if a read() call won't block. ``writefds`` File descriptions to be watched if a write() won't block. ``exceptfds`` File descriptions to be watched for V4L2 events. ``timeout`` Maximum time to wait. Description =========== With the :ref:`select() ` function applications can suspend execution until the driver has captured data or is ready to accept data for output. When streaming I/O has been negotiated this function waits until a buffer has been filled or displayed and can be dequeued with the :ref:`VIDIOC_DQBUF ` ioctl. When buffers are already in the outgoing queue of the driver the function returns immediately. On success :ref:`select() ` returns the total number of bits set in :c:func:`struct fd_set`. 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_QBUF` or :ref:`VIDIOC_STREAMON` yet the :ref:`select() ` function succeeds, setting the bit of the file descriptor in ``readfds`` or ``writefds``, but subsequent :ref:`VIDIOC_DQBUF ` calls will fail. [#f1]_ When use of the :ref:`read() ` function has been negotiated and the driver does not capture yet, the :ref:`select() ` function starts capturing. When that fails, :ref:`select() ` returns successful and a subsequent :ref:`read() ` call, which also attempts to start capturing, will return an appropriate error code. When the driver captures continuously (as opposed to, for example, still images) and data is already available the :ref:`select() ` function returns immediately. When use of the :ref:`write() ` function has been negotiated the :ref:`select() ` function just waits until the driver is ready for a non-blocking :ref:`write() ` call. All drivers implementing the :ref:`read() ` or :ref:`write() ` function or streaming I/O must also support the :ref:`select() ` function. For more details see the :ref:`select() ` manual page. Return Value ============ On success, :ref:`select() ` returns the number of descriptors contained in the three returned descriptor sets, which will be zero if the timeout expired. On error -1 is returned, and the ``errno`` variable is set appropriately; the sets and ``timeout`` are undefined. Possible error codes are: EBADF One or more of the file descriptor sets specified a file descriptor that is not open. EBUSY The driver does not support multiple read or write streams and the device is already in use. EFAULT The ``readfds``, ``writefds``, ``exceptfds`` or ``timeout`` pointer references an inaccessible memory area. EINTR The call was interrupted by a signal. EINVAL The ``nfds`` argument is less than zero or greater than ``FD_SETSIZE``. .. [#f1] The Linux kernel implements :ref:`select() ` like the :ref:`poll() ` function, but :ref:`select() ` cannot return a ``POLLERR``.