About IOCTL system call

Question

The prototype of IOCTL system call in linux is

int ioctl(struct inode *, struct file *, unsigned int, unsigned long);

All other file operations like read(),write(),llseek(),mmap() etc.. have only struct file * as argument. But, why IOCTL call needs struct inode * to be passed. Is there any specific use of it?

Answer 1

Which kernel version you are taking about, now ioctl doesn't has the inode pointer as its parameter. Previously there used to be, but I think from 2.6.36 kernel onwards it has been removed.

Answer 2

The prototype of ioctl is, at least according to the manpage, int ioctl(int d, int request, ...);. The ... bit is important - variadic arguments, meaning the remaining arguments depend on the first ones, much like printf. Any use for a struct inode * would stem from the specific ioctl request you're making.