Hello !

I would like to open two file descriptors of IIO device node - /dev/iio:device0
int fd1 = open("/dev/iio:device0",O_RDONLY);
int fd2 = open("/dev/iio:device0",O_RDONLY);

But, only first open call succeeds.
The second open calls fails with error "Device or Resource busy".

In normal input device, we can open as many event node file descriptors as we like.
int fd1 = open("/dev/input/event1");
int fd2 = open("/dev/input/event1");

Both calls succeeds for input subsystem case.

So, why restriction is present in IIO subsystem ?
Why cannot open same iio device node as many times as desired ?

Thank you!

Hello Smallpond,

I checked the above suggested link - http://linux.die.net/man/2/dup
But it does not solve the problem.

It is mentioned on the Linux man page,
"They refer to the same open file descriptors and thus share file offset and file status flags;
for example, if the file offset is modified by using lseek(2) on one of the descriptors, the offset is also changed for the other."

So, "dup" only duplicates the the fd but I need two different file descriptors so that I can read the data independently.

Thanks!

Hello,

After discussing with IIO community,
below is the reason why same iio device node cannot be opened twice:

Basically IIO is a different design for the userspace interfaces.
IIO mainly deals with much faster devices so overhead of description of data
has been removed to avoid overhead in kernel.

static int iio_chrdev_open(struct inode *inode, struct file *filp)
{
...
if (test_and_set_bit(IIO_BUSY_BIT_POS, &indio_dev->flags))
return -EBUSY;

...
}

So, we get ebusy error upon opening same iio device node again.
If we comment this line, we will be able to get multiple fds successfully.
But currently, there is no support of multiple fds in iio for read
operation i.e. if we read data using one fd, the offset for other fd will also change
because they actually act as only one fd.
So two open fds will not be able to work independently.

Thanks !