V4L2 poll()
&manvol;

# v4l2-poll

Wait for some event on a file descriptor

          #include <sys/poll.h>

    	int poll
    	struct pollfd *ufds
    	unsigned int nfds
    	int timeout

## 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 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 &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.

## 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:

EBADF

  One or more of the `ufds` members specify an invalid file descriptor.

EBUSY

  The driver does not support multiple read or write streams and the device is already in use.

EFAULT

  `ufds` references an inaccessible memory area.

EINTR

  The call was interrupted by a signal.

EINVAL

  The `nfds` argument is greater than `OPEN_MAX`.