V4L2 poll()

Description

With the poll() 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 VIDIOC_DQBUF ioctl. When buffers are already in the outgoing queue of the driver the function returns immediately.

On success poll() returns the number of file descriptors that have been selected (that is, file descriptors for which the revents field of the respective pollfd structure is non-zero). Capture devices set the POLLIN and POLLRDNORM flags in the revents field, output devices the POLLOUT and POLLWRNORM flags. 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 VIDIOC_QBUF or VIDIOC_STREAMON yet the poll() function succeeds, but sets the POLLERR flag in the revents field.

When use of the read() function has been negotiated and the driver does not capture yet, the poll function starts capturing. When that fails it returns a POLLERR as above. Otherwise it waits until data has been captured and can be read. When the driver captures continuously (as opposed to, for example, still images) the function may return immediately.

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

All drivers implementing the read() or write() function or streaming I/O must also support the poll() function.

For more details see the poll() manual page.

Return Value

On success, poll() returns the number structures which have non-zero revents fields, or zero if the call timed out. On error -1 is returned, and the errno variable is set appropriately: