ioctl example complete!

kernel_module/ioctl.h

@@ -3,6 +3,30 @@
 
 #include <linux/ioctl.h>
 
-#define IOCTL0 _IOW(123, 0, char *)
+/* 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