How Linux knows which ioctl function to call?

Question

Here is the ioctl call in user space:

int ioctl(int fd, int cmd, ...);

As far as I know, when we want to perfrom IO operations, we define our own ioctl function with a set of requests (commands), assign our ioctl to a file_operations structure like this:

struct file_operations fops = {
 .read = device_read,
 .write = device_write,
 .ioctl = device_ioctl, // device_ioctl is our function
 .open = device_open,
 .release = device_release,
};

And the device_ioctl function is defined differently compared to the user space interface:

static long device_ioctl(struct file *f, unsigned int cmd, unsigned long arg)

I think based on the file descriptor, the kernel can get the appropriate file structure and calls the device's ioctl.

This is just a guess because I cannot find it the generic function definition where the kernel selects appropriate ioctl function based on the file descriptor fd passed into the generic ioctl interface? There are only 3 ioctl definitions I can find, but apparently those are just the devices' definitions, not the kernel: ioctl

Answer 1

Look in the Linux source code, fs/ioctl.c (http://lxr.free-electrons.com/source/fs/ioctl.c)
There you will see the syscall for ioctl:

SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd, unsigned long, arg)

This in turns calls do_vfs_ioctl(), which calls vfs_ioctl() which then calls the unlocked_ioctl function defined for that filesystem in the file_operations structure.
This will be your device_ioctl function that you registered.