Modern Unix derivatives have this really bad idea called ioctl. It’s a function which performs arbitrary operations on a file descriptor. It is essentially the kitchen sink of modern Unix derivatives, particularly Linux, in which they act almost like a second set of extra syscalls. For example, to get the size of the terminal window, you use an ioctl specific to TTY file descriptors:
This code performs the ioctl syscall against the provided file descriptor “fd”, using the “TIOCGWINSZ” operation, and setting the parameter to a pointer to a winsize structure. There are thousands of ioctls provided by Linux, and each of them is assigned a constant like TIOCGWINSZ (0x5413). Some constants, including this one, are assigned somewhat arbitrarily. However, some are assigned with some degree of structure.
Consider for instance the ioctl TUNSETOWNER, which is used for tun/tap network devices. This ioctl is assigned the number 0x400454cc, but this is not selected arbitrarily. It’s assigned with a macro, which we can find in /usr/include/linux/if_tun.h: