Name

v4l2-select - Synchronous I/O multiplexing

Synopsis

.. code-block:: c

#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

.. c:function:: int select( int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout )

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 :c:func: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 <VIDIOC_QBUF> ioctl. When buffers are already in the outgoing queue of the driver the function returns immediately.

On success :c:func:select() returns the total number of bits set in 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 :c:func:select() function succeeds, setting the bit of the file descriptor in readfds or writefds, but subsequent :ref:VIDIOC_DQBUF <VIDIOC_QBUF> calls will fail. [#f1]_

When use of the :c:func:read() function has been negotiated and the driver does not capture yet, the :c:func:select() function starts capturing. When that fails, :c:func:select() returns successful and a subsequent :c:func: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 :c:func:select() function returns immediately.

When use of the :c:func:write() function has been negotiated the :c:func:select() function just waits until the driver is ready for a non-blocking :c:func:write() call.

All drivers implementing the :c:func:read() or :c:func:write() function or streaming I/O must also support the :c:func:select() function.

For more details see the :c:func:select() manual page.

Return Value

On success, :c:func: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 :c:func:select() like the :c:func:poll() function, but :c:func:select() cannot return a POLLERR.