#include
#include
#include
#include
#include
MODULE_LICENSE(“GPL”);
MODULE_AUTHOR(“Robert W. Oliver II”);
MODULE_DESCRIPTION(“A simple example Linux module.”);
MODULE_VERSION(“0.01”);
#define DEVICE_NAME “lkm_example”
#define EXAMPLE_MSG “Hello, World!\n”
#define MSG_BUFFER_LEN 15
/* Prototypes for device functions */
static int device_open(struct inode *, struct file *);
static int device_release(struct inode *, struct file *);
static ssize_t device_read(struct file *, char *, size_t, loff_t *);
static ssize_t device_write(struct file *, const char *, size_t, loff_t *);
static int major_num;
static int device_open_count = 0;
static char msg_buffer[MSG_BUFFER_LEN];
static char *msg_ptr;
/* This structure points to all of the device functions */
static struct file_operations file_ops = {
.read = device_read,
.write = device_write,
.open = device_open,
.release = device_release
};
/* When a process reads from our device, this gets called. */
static ssize_t device_read(struct file *flip, char *buffer, size_t len, loff_t *offset) {
int bytes_read = 0;
/* If we’re at the end, loop back to the beginning */
if (*msg_ptr == 0) {
msg_ptr = msg_buffer;
}
/* Put data in the buffer */
while (len && *msg_ptr) {
/* Buffer is in user data, not kernel, so you can’t just reference
* with a pointer. The function put_user handles this for us */
put_user(*(msg_ptr++), buffer++);
len--;
bytes_read++;
}
return bytes_read;
}
/* Called when a process tries to write to our device */
static ssize_t device_write(struct file *flip, const char *buffer, size_t len, loff_t *offset) {
/* This is a read-only device */
printk(KERN_ALERT “This operation is not supported.\n”);
return -EINVAL;
}
/* Called when a process opens our device */
static int device_open(struct inode *inode, struct file *file) {
/* If device is open, return busy */
if (device_open_count) {
return -EBUSY;
}
device_open_count++;
try_module_get(THIS_MODULE);
return 0;
}
/* Called when a process closes our device */
static int device_release(struct inode *inode, struct file *file) {
/* Decrement the open counter and usage count. Without this, the module would not unload. */
device_open_count--;
module_put(THIS_MODULE);
return 0;
}
static int __init lkm_example_init(void) {
/* Fill buffer with our message */
strncpy(msg_buffer, EXAMPLE_MSG, MSG_BUFFER_LEN);
/* Set the msg_ptr to the buffer */
msg_ptr = msg_buffer;
/* Try to register character device */
major_num = register_chrdev(0, “lkm_example”, &file_ops);
if (major_num < 0) {
