linux-kernel-module-cheat/kernel_module/ioctl.h

#ifndef IOCTL_H
#define IOCTL_H

#include <linux/ioctl.h>

/* Structs are the way to pass multiple arguments. */
typedef struct {
	int i;
	int j;
} lkmc_ioctl_struct;

/* Some random number I can't understand how to choose. */
#define LKMC_IOCTL_MAGIC 0x33

/*
 * I think those number does not *need* to be unique across, that is just to help debugging:
 * https://stackoverflow.com/questions/22496123/what-is-the-meaning-of-this-macro-iormy-macig-0-int
 *
 * However, the ioctl syscall highjacks several low values at do_vfs_ioctl, e.g.
 * This "forces" use to use the _IOx macros...
 * https://stackoverflow.com/questions/10071296/ioctl-is-not-called-if-cmd-2
 *
 * Some of those magic low values are used for fnctl, which can also be used on regular files:
 * e.g. FIOCLEX for close-on-exec:
 * https://stackoverflow.com/questions/6125068/what-does-the-fd-cloexec-fcntl-flag-do
 *
 * TODO are the W or R of _IOx and type functional, or only to help with uniqueness?
 * */
#define LKMC_IOCTL_INC     _IOWR(LKMC_IOCTL_MAGIC, 0, int)
#define LKMC_IOCTL_INC_DEC _IOWR(LKMC_IOCTL_MAGIC, 1, lkmc_ioctl_struct)

#endif