7.68. V4L2 select()¶

7.68.1. Name¶

v4l2-select - Synchronous I/O multiplexing

7.68.2. Synopsis¶

#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout)¶

7.68.3. Arguments¶

nfds: The highest-numbered file descriptor in any of the three sets, plus 1.
readfds: File descriptions to be watched if a read() call won’t block.
writefds: File descriptions to be watched if a write() won’t block.
exceptfds: File descriptions to be watched for V4L2 events.
timeout: Maximum time to wait.

7.68.4. Description¶

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 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.

7.68.5. 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 errno variable is set appropriately; the sets and timeout are undefined. Possible error codes are:

EBADF: One or more of the file descriptor sets specified a file descriptor that is not open.
EBUSY: The driver does not support multiple read or write streams and the device is already in use.
EFAULT: The readfds, writefds, exceptfds or timeout pointer references an inaccessible memory area.
EINTR: The call was interrupted by a signal.
EINVAL: The nfds argument is less than zero or greater than FD_SETSIZE.

[1]	The Linux kernel implements select() like the poll() function, but select() cannot return a `POLLERR`.