Video for Linux Two API Specification: Revision 0.24
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# V4L2 poll()

## Name

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

## Synopsis

    #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 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:

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`.

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