Character device

README.md

@@ -342,3 +342,4 @@ But TODO I don't think you can see where you are in the kernel source code and l
         1.  Module dependencies
             1. [dep.c](kernel_module/dep.c)
             1. [dep2.c](kernel_module/dep2.c)
+        1.  [character_device](kernel_module/character_device.c)

kernel_module/character_device.c

@@ -0,0 +1,67 @@
+/*
+Allows us to create device files with given file operations with mknod c X.
+
+Usage:
+
+	/character_device.sh
+
+The major number determines which module owns the device file.
+
+minor is to differentiate between multiple instances of the device,
+e.g. two NVIDIA GPUs using the same kernel module.
+
+We ask the kernel to automatically allocate a major number for us to avoid
+conlicts with other devices.
+
+Then we need to check /proc/devices to find out the assigned number,
+and use that for the mknod.
+*/
+
+#include <asm/uaccess.h> /* copy_from_user, copy_to_user */
+#include <linux/errno.h> /* EFAULT */
+#include <linux/fs.h> /* register_chrdev, unregister_chrdev */
+#include <linux/jiffies.h>
+#include <linux/module.h>
+#include <linux/printk.h> /* printk */
+#include <uapi/linux/stat.h> /* S_IRUSR */
+
+#define NAME "lkmc_character_device"
+
+MODULE_LICENSE("GPL");
+
+static int major;
+
+static ssize_t read(struct file *filp, char __user *buf, size_t len, loff_t *off)
+{
+	size_t ret;
+	char kbuf[] = {'a', 'b', 'c', 'd'};
+
+	ret = 0;
+	if (*off == 0) {
+		if (copy_to_user(buf, kbuf, sizeof(kbuf))) {
+			ret = -EFAULT;
+		} else {
+			ret = sizeof(kbuf);
+			*off = 1;
+		}
+	}
+	return ret;
+}
+
+static const struct file_operations fops = {
+	.read = read,
+};
+
+static int myinit(void)
+{
+	major = register_chrdev(0, NAME, &fops);
+	return 0;
+}
+
+static void myexit(void)
+{
+	unregister_chrdev(major, NAME);
+}
+
+module_init(myinit)
+module_exit(myexit)

rootfs_overlay/character_device.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+insmod /character_device.ko
+major="$(grep lkmc_character_device /proc/devices | cut -d ' ' -f 1)"
+mknod /character_device.dev c $major 0
+cat /character_device.dev
+# => abcd
+rmmod character_device