7.69. V4L2 select()¶
7.69.1. Name¶
v4l2-select - Synchronous I/O multiplexing
7.69.2. 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)¶
7.69.3. Arguments¶
7.69.4. 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
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.
7.69.5. 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 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
ortimeout
pointer references an inaccessible memory area.- EINTR
The call was interrupted by a signal.
- EINVAL
The
nfds
argument is less than zero or greater thanFD_SETSIZE
.
- 1
The Linux kernel implements
select()
like thepoll()
function, butselect()
cannot return aPOLLERR
.