connecting application with device driver

This is an interview question.

I had written device driver for a char device so I know that code structure looks like this

struct file_operations something {
 .owner=my_device_open;
 .read=my_device_read;
 .close=my_device_close;
 .write=my_device_write;

 }

When the device driver is active then in

/dev/mydevice
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you can actually read and write into it. But what I was not clear is how an application will read or write to this device. I know insmod will insert the module to kernel,and register_chrdev(); will register the driver in kernel but how will application program communicate with this driver. Let me know what will be correct answer for it.

Well Martin Beckett summed it up. It is not really more complex, although you could say the same with slightly more detail. Here is my try at it:

The program performs an open("/dev/mydevice", flags) syscall, then the kernel reads /dev/mydevice from disk. It is just an inode, with no associated data blocks, but it holds two important pieces of information: the major number and the minor number. From these numbers, the kernel finds the struct file_operations that you provided through register_chrdev(), and it calls it's .open field. It returns to the program a file descriptor that it associated with this particular struct file_operations. Next, when the kernel receives a syscall like write(fd, buf, count), it will call the .write field and so on.

In unix it simply opens the device node as a file and sends/receives data and commands from it.
The beauty of Unix is that from an app's point of view there is nothing special about devices - they are just files (except for ioctls to set some modes). There is work to do in the kernel to accomodate this but that's the kernel modules problem.

Or were you asking something more complex?