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.
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
.