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

## Name

v4l2-select -- Synchronous I/O multiplexing

## 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);`

## 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 the `fd_set`s. 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.

## 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`.

## Notes

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

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