7.69. V4L2 select()¶
v4l2-select - Synchronous I/O multiplexing
#include <sys/time.h> #include <sys/types.h> #include <unistd.h>
select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout)¶
- The highest-numbered file descriptor in any of the three sets, plus 1.
- File descriptions to be watched if a read() call won’t block.
- File descriptions to be watched if a write() won’t block.
- File descriptions to be watched for V4L2 events.
- Maximum time to wait.
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
struct fd_set(). 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
ioctl VIDIOC_QBUF, VIDIOC_DQBUF or
ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF yet the select()
function succeeds, setting the bit of the file descriptor in
writefds, but subsequent VIDIOC_DQBUF
calls will fail. 
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.
7.69.5. Return Value¶
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
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.
timeoutpointer references an inaccessible memory area.
- The call was interrupted by a signal.
nfdsargument is less than zero or greater than
|||The Linux kernel implements select() like the
poll() function, but select() cannot
return a |