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A simple Linux loadable Kernel Module
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-105
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2 files changed

+115
-105
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‎Makefile

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obj-m += lkm_example.o
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all:
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make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
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clean:
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make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
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test:
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sudo dmesg -C
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sudo insmod lkm_example.ko
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sudo rmmod lkm_example.ko
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dmesg

‎chrdevic.c renamed to ‎lkm_example.c

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#include <linux/init.h>
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#include <linux/module.h>
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#include <linux/kernel.h>
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#include <linux/fs.h>
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#include <asm/uaccess.h>
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MODULE_LICENSE("GPL");
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MODULE_AUTHOR("P Srinivas Rao");
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MODULE_DESCRIPTION("A simple Linux loadable Kernel Module.");
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MODULE_VERSION("0.01");
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#define DEVICE_NAME "lkm_example"
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#define EXAMPLE_MSG "Hello, World!\n"
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#define MSG_BUFFER_LEN 15
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/* Prototypes for device functions */
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static int device_open(struct inode *, struct file *);
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static int device_release(struct inode *, struct file *);
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static ssize_t device_read(struct file *, char *, size_t, loff_t *);
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static ssize_t device_write(struct file *, const char *, size_t, loff_t *);
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static int major_num;
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static int device_open_count = 0;
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static char msg_buffer[MSG_BUFFER_LEN];
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static char *msg_ptr;
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/* This structure points to all of the device functions */
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static struct file_operations file_ops = {
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.owner = THIS_MODULE,
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.read = device_read,
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.write = device_write,
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.open = device_open,
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.release = device_release
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};
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/* When a process reads from our device, this gets called. */
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static ssize_t device_read(struct file *filep, char *buffer, size_t len, loff_t *offset) {
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int bytes_read = 0;
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/* If we're at the end, loop back to the beginning */
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if (*msg_ptr == 0) {
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msg_ptr = msg_buffer;
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}
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/* Put data in the buffer */
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while (len && *msg_ptr) {
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/* Buffer is in user data, not kernel, so you can't just reference
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* with a pointer. The function put_user handles this for us */
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put_user(*(msg_ptr++), buffer++);
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len--;
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bytes_read++;
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}
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return bytes_read;
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}
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/* Called when a process tries to write to our device */
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static ssize_t device_write(struct file *filep, const char *buffer, size_t len, loff_t *offset) {
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/* This is a read-only device */
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printk(KERN_ALERT "This operation is not supported.\n");
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return -EINVAL;
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}
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/* Called when a process opens our device */
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static int device_open(struct inode *inode, struct file *file) {
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/* If device is open, return busy */
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if (device_open_count) {
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return -EBUSY;
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}
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device_open_count++;
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try_module_get(THIS_MODULE);
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return 0;
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}
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/* Called when a process closes our device */
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static int device_release(struct inode *inode, struct file *file) {
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/* Decrement the open counter and usage count. Without this, the module would not unload. */
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device_open_count--;
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module_put(THIS_MODULE);
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return 0;
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}
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static int __init lkm_example_init(void) {
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/* Fill buffer with our message */
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strncpy(msg_buffer, EXAMPLE_MSG, MSG_BUFFER_LEN);
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/* Set the msg_ptr to the buffer */
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msg_ptr = msg_buffer;
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/* Try to register character device */
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major_num = register_chrdev(0, "lkm_example", &file_ops);
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if (major_num < 0) {
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printk(KERN_ALERT "Could not register device: %d\n", major_num);
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return major_num;
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} else {
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printk(KERN_INFO "lkm_example module loaded with device major number %d\n", major_num);
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return 0;
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}
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}
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static void __exit lkm_example_exit(void) {
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/* Remember: we have to clean up after ourselves. Unregister the character device. */
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unregister_chrdev(major_num, DEVICE_NAME);
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printk(KERN_INFO "Goodbye, World!\n");
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}
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/* Register module functions */
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module_init(lkm_example_init);
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module_exit(lkm_example_exit);
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#include <linux/init.h>
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#include <linux/module.h>
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#include <linux/kernel.h>
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#include <linux/fs.h>
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#include <asm/uaccess.h>
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MODULE_LICENSE("GPL");
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MODULE_AUTHOR("P Srinivas Rao");
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MODULE_DESCRIPTION("A simple Linux loadable Kernel Module.");
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MODULE_VERSION("0.01");
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#define DEVICE_NAME "lkm_example"
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#define EXAMPLE_MSG "Hello, World!\n"
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#define MSG_BUFFER_LEN 15
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/* Prototypes for device functions */
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static int device_open(struct inode *, struct file *);
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static int device_release(struct inode *, struct file *);
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static ssize_t device_read(struct file *, char *, size_t, loff_t *);
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static ssize_t device_write(struct file *, const char *, size_t, loff_t *);
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static int major_num;
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static int device_open_count = 0;
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static char msg_buffer[MSG_BUFFER_LEN];
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static char *msg_ptr;
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/* This structure points to all of the device functions */
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static struct file_operations file_ops = {
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.owner = THIS_MODULE,
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.read = device_read,
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.write = device_write,
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.open = device_open,
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.release = device_release
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};
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/* When a process reads from our device, this gets called. */
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static ssize_t device_read(struct file *filep, char *buffer, size_t len, loff_t *offset) {
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int bytes_read = 0;
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/* If we're at the end, loop back to the beginning */
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if (*msg_ptr == 0) {
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msg_ptr = msg_buffer;
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}
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/* Put data in the buffer */
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while (len && *msg_ptr) {
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/* Buffer is in user data, not kernel, so you can't just reference
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* with a pointer. The function put_user handles this for us */
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put_user(*(msg_ptr++), buffer++);
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len--;
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bytes_read++;
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}
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return bytes_read;
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}
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/* Called when a process tries to write to our device */
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static ssize_t device_write(struct file *filep, const char *buffer, size_t len, loff_t *offset) {
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/* This is a read-only device */
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printk(KERN_ALERT "This operation is not supported.\n");
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return -EINVAL;
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}
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/* Called when a process opens our device */
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static int device_open(struct inode *inode, struct file *file) {
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/* If device is open, return busy */
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if (device_open_count) {
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return -EBUSY;
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}
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device_open_count++;
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try_module_get(THIS_MODULE);
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return 0;
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}
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/* Called when a process closes our device */
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static int device_release(struct inode *inode, struct file *file) {
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/* Decrement the open counter and usage count. Without this, the module would not unload. */
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device_open_count--;
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module_put(THIS_MODULE);
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return 0;
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}
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static int __init lkm_example_init(void) {
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/* Fill buffer with our message */
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strncpy(msg_buffer, EXAMPLE_MSG, MSG_BUFFER_LEN);
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/* Set the msg_ptr to the buffer */
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msg_ptr = msg_buffer;
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/* Try to register character device */
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major_num = register_chrdev(0, "lkm_example", &file_ops);
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if (major_num < 0) {
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printk(KERN_ALERT "Could not register device: %d\n", major_num);
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return major_num;
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} else {
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printk(KERN_INFO "lkm_example module loaded with device major number %d\n", major_num);
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return 0;
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}
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}
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static void __exit lkm_example_exit(void) {
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/* Remember: we have to clean up after ourselves. Unregister the character device. */
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unregister_chrdev(major_num, DEVICE_NAME);
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printk(KERN_INFO "Goodbye, World!\n");
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}
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/* Register module functions */
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module_init(lkm_example_init);
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module_exit(lkm_example_exit);

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