ioctl begins to work

kernel_module/ioctl.c

@@ -1,5 +1,20 @@
 /*
 Give integer and a pointer to the kernel, and get one positive integer out.
+
+Feels like an archaic API... so many weird restrictions for something that could be so simple!
+
+TODO: must the ioctl numbers be globally unique? How to ensure that?
+
+Ioctl is super picky about the ioctl numbers, it is very annoying: https://stackoverflow.com/questions/10071296/ioctl-is-not-called-if-cmd-2
+
+See how do_vfs_ioctl highjacks several values. This "forces" use to use the _IOx macros...
+
+Documentation/ioctl/ioctl-number.txt has some info:
+
+    _IO    an ioctl with no parameters
+    _IOW   an ioctl with write parameters (copy_from_user)
+    _IOR   an ioctl with read parameters  (copy_to_user)
+    _IOWR  an ioctl with both write and read parameters.
 */
 
 #include <linux/debugfs.h>

kernel_module/ioctl.h

@@ -0,0 +1,8 @@
+#ifndef IOCTL_H
+#define IOCTL_H
+
+#include <linux/ioctl.h>
+
+#define IOCTL0 _IOW(123, 0, char *)
+
+#endif

kernel_module/test/init_hello.c

@@ -0,0 +1,11 @@
+/* Replacement init example for when we feel like running
+ * a single non-interactive single executable Linux distro. */
+
+#include <stdio.h>
+#include <unistd.h>
+
+int main(void) {
+	puts("hello world");
+	while (1)
+		sleep(0xFFFFFFFF);
+}

kernel_module/test/ioctl.c

@@ -1,4 +1,5 @@
 #define _GNU_SOURCE
+#include <errno.h>
 #include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -6,7 +7,8 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
-#include <errno.h>
+
+#include "../ioctl.h"
 
 int main(int argc, char **argv)
 {
@@ -21,7 +23,7 @@ int main(int argc, char **argv)
 		perror("open");
 		return EXIT_FAILURE;
 	}
-	ret = ioctl(fd, strtoul(argv[2], NULL, 10), argv[3]);
+	ret = ioctl(fd, IOCTL0, argv[3]);
 	if (ret == -1) {
 		perror("ioctl");
 		return EXIT_FAILURE;