Interactivity with IOCTL

Linux contains a syscall called ioctl, which is often used to communicate with a driver. ioctl() takes three parameters:

• File Descriptor fd

• an unsigned int

• an unsigned long

The driver can be adapted to make the latter two virtually anything - perhaps a pointer to a struct or a string. In the driver source, the code looks along the lines of:

static ssize_t ioctl_handler(struct file *file, unsigned int cmd, unsigned long arg) {
    printk("Command: %d; Argument: %d", cmd, arg);

    return 0;
}

But if you want, you can interpret cmd and arg as pointers if that is how you wish your driver to work.

To communicate with the driver in this case, you would use the ioctl() function, which you can import in C:

#include <sys/ioctl.h>

// [...]

ioctl(fd, 0x100, 0x12345678);    // data is a string

And you would have to update the file_operations struct:

static struct file_operations fops = {
    .ioctl = ioctl_handler
};

On modern Linux kernel versions, .ioctl has been removed and replaced by .unlocked_ioctl and .compat_ioctl. The former is the replacement for .ioctl, with the latter allowing 32-bit processes to perform ioctl calls on 64-bit systems. As a result, the new file_operations is likely to look more like this:

static struct file_operations fops = {
    .compat_ioctl = ioctl_handler,
    .unlocked_ioctl = ioctl_handler
};