With the select() 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 select() returns the total
number of bits set in the fd_sets. 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
select() function succeeds, setting the bit of
the file descriptor in readfds or
writefds, but subsequent VIDIOC_DQBUF calls
will fail.[1]
When use of the read() function has
been negotiated and the driver does not capture yet, the
select() function starts capturing. When that
fails, select() returns successful and a
subsequent read() call, which also attempts to
start capturing, will return an appropriate error code. When the
driver captures continuously (as opposed to, for example, still
images) and data is already available the
select() function returns immediately.
When use of the write() function has
been negotiated the select() 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 select() function.
For more details see the select()
manual page.
On success, select() returns the number
of descriptors contained in the three returned descriptor sets, which
will be zero if the timeout expired. On error
-1 is returned, and the
errno variable is set appropriately; the sets and
timeout are undefined. Possible error codes
are:
One or more of the file descriptor sets specified a file descriptor that is not open.
The driver does not support multiple read or write streams and the device is already in use.
The readfds,
writefds, exceptfds or
timeout pointer references an inaccessible memory
area.
The call was interrupted by a signal.
The nfds argument is less than
zero or greater than FD_SETSIZE.
| [1] | The Linux kernel implements
|