7.66. V4L2 poll()

7.66.1. Name

v4l2-poll - Wait for some event on a file descriptor

7.66.2. Synopsis

#include <sys/poll.h>
int poll(struct pollfd *ufds, unsigned int nfds, int timeout)

7.66.3. Arguments

7.66.4. 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 by the capture device and can be dequeued with the VIDIOC_DQBUF ioctl. For output devices this function waits until the device is ready to accept a new buffer to be queued up with the VIDIOC_QBUF ioctl for display. When buffers are already in the outgoing queue of the driver (capture) or the incoming queue isn’t full (display) 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 struct 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_STREAMON the poll() function succeeds, but sets the POLLERR flag in the revents field. When the application has called VIDIOC_STREAMON for a capture device but hasn’t yet called VIDIOC_QBUF, the poll() function succeeds and sets the POLLERR flag in the revents field. For output devices this same situation will cause poll() to succeed as well, but it sets the POLLOUT and POLLWRNORM flags in the revents field.

If an event occurred (see ioctl VIDIOC_DQEVENT) then POLLPRI will be set in the revents field and poll() will return.

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 and the driver does not stream yet, the poll() function starts streaming. When that fails it returns a POLLERR as above. Otherwise it waits until the driver is ready for a non-blocking write() call.

If the caller is only interested in events (just POLLPRI is set in the events field), then poll() will not start streaming if the driver does not stream yet. This makes it possible to just poll for events and not for buffers.

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.

7.66.5. 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:

One or more of the ufds members specify an invalid file descriptor.
The driver does not support multiple read or write streams and the device is already in use.
ufds references an inaccessible memory area.
The call was interrupted by a signal.
The nfds argument is greater than OPEN_MAX.