ioctl done

ioctl/ioctl_const.h

@@ -0,0 +1,48 @@
+#ifndef _IOCTL_CHAR_DEVICE_H_
+#define _IOCTL_CHAR_DEVICE_H_
+
+#include <linux/ioctl.h>
+
+/**
+ *  ioctl constants are defined in this header file for our
+ *  char_driver_ioctl driver.
+ * 
+ *  This file explains how the constants are defined with
+ *  the use of inline comments
+ */
+
+/**
+ *  Constants have to be globally unique. Though this is not
+ *  compulsory, it is necessary. This is because of the simple
+ *  reason: we do not want our commands to clash with the commands
+ *  in other drivers. we don't want the right command doing to
+ *  the wrong driver or vice-versa
+ */
+
+/**
+ *  We have to base our constants based on a magic number. 
+ */
+
+#define DEMO_IOCTL_MAGIC 'a'
+
+/**
+ * Defining constants requires us to decide following values
+ * 
+ * 1. type or magic number (type)
+ * 2. sequence number which is eight bits wide. This means
+ *    we can have up to 256 ioctl commands (nr)
+ * 3. direction if the arg is involed, whether we are reading
+ *    or writing
+ * 4. size(composition) of the argument.
+ * 
+ *  To arrive at unique numbers easily we use the following macros
+ *  _IO(type, nr); -> no data is passed from kernel to user space, just like void function
+ *  _IOW(type, nr, dataitem) the fourth item calculated using sizeof -> to read from user to kernel
+ *  _IOR(type, nr, dataitem) -> to write to user from kernel
+ *  _IOWR(type, nr, dataitem) -> to do both read and write
+ */
+
+#define WR_VALUE _IOW(DEMO_IOCTL_MAGIC, 1, int32_t *)
+#define RD_VALUE _IOR(DEMO_IOCTL_MAGIC, 2, int32_t *)
+ 
+#endif