II我們將介紹如何向Linux註冊char device，並且read/write該device。
#include <linux/init.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h> // chrdev
#include <linux/cdev.h> // cdev_add()/cdev_del()
#include <asm/uaccess.h> // copy_*_user()
MODULE_LICENSE("GPL");
#define DEV_BUFSIZE 1024
int dev_major;
int dev_minor;
struct cdev *dev_cdevp = NULL;
int dev_open(struct inode*, struct file*);
int dev_release(struct inode*, struct file*);
ssize_t dev_read(struct file*, char __user*, size_t, loff_t*);
ssize_t dev_write(struct file*, const char __user *, size_t, loff_t*);
static void __exit exit_modules(void);
struct file_operations dev_fops = {
.owner = THIS_MODULE,
.open = dev_open,
.release = dev_release,
.read = dev_read,
.write = dev_write,
};
int dev_open(struct inode *inode, struct file *filp)
{
printk("%s():\n", __FUNCTION__);
return 0;
}
int dev_release(struct inode *inode, struct file *filp)
{
printk("%s():\n", __FUNCTION__);
return 0;
}
ssize_t
dev_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos)
{
char data[] = "brook";
ssize_t ret = 0;
printk("%s():\n", __FUNCTION__);
if (*f_pos >= sizeof(data)) {
goto out;
}
if (count > sizeof(data)) {
count = sizeof(data);
}
if (copy_to_user(buf, data, count) < 0) {
ret = -EFAULT;
goto out;
}
*f_pos += count;
ret = count;
out:
return ret;
}
ssize_t
dev_write(struct file *filp, const char __user *buf, size_t count,
loff_t *f_pos)
{
char *data;
ssize_t ret = 0;
printk("%s():\n", __FUNCTION__);
data = kzalloc(sizeof(char) * DEV_BUFSIZE, GFP_KERNEL);
if (!data) {
return -ENOMEM;
}
if (count > DEV_BUFSIZE) {
count = DEV_BUFSIZE;
}
if (copy_from_user(data, buf, count) < 0) {
ret = -EFAULT;
goto out;
}
printk("%s(): %s\n", __FUNCTION__, data);
*f_pos += count;
ret = count;
out:
kfree(data);
return ret;
}
static int __init init_modules(void)
{
dev_t dev;
int ret;
ret = alloc_chrdev_region(&dev, 0, 1, "brook");
if (ret) {
printk("can't alloc chrdev\n");
return ret;
}
dev_major = MAJOR(dev);
dev_minor = MINOR(dev);
printk("register chrdev(%d,%d)\n", dev_major, dev_minor);
dev_cdevp = kzalloc(sizeof(struct cdev), GFP_KERNEL);
if (dev_cdevp == NULL) {
printk("kzalloc failed\n");
goto failed;
}
cdev_init(dev_cdevp, &dev_fops);
dev_cdevp->owner = THIS_MODULE;
ret = cdev_add(dev_cdevp, MKDEV(dev_major, dev_minor), 1);
if (ret < 0) {
printk("add chr dev failed\n");
goto failed;
}
return 0;
failed:
if (dev_cdevp) {
kfree(dev_cdevp);
dev_cdevp = NULL;
}
return 0;
}
static void __exit exit_modules(void)
{
dev_t dev;
dev = MKDEV(dev_major, dev_minor);
if (dev_cdevp) {
cdev_del(dev_cdevp);
kfree(dev_cdevp);
}
unregister_chrdev_region(dev, 1);
printk("unregister chrdev\n");
}
module_init(init_modules);
module_exit(exit_modules);
當module被load進來時，init_modules()會被執行，而init_modules()做了幾件事情，首先向Linux要求分配char device number "alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count, const char *name)"。dev是分配到的dev_t(output parameter)，baseminor是起始的minor number，當baseminor=0會由系統分配，否則會試圖尋找可以符合baseminor的minor number，count是minor的數量，name是註冊的名稱。
接著設定struct cdev "cdev_init()"，並且註冊到系統"cdev_add()"，在cdev_init()的同時也設定了該device的file operations。
struct file_operations就是device的file operations，簡單的說，UNIX所有的東西都可以視為檔案，當然包含device，既然是檔案，就會有open、close/release、read和write等等操作，而這些操作都會觸發對應的function來反應，這就是file operations的功能了。
我們的read會回應"brook"，而write則是藉由printk()印出來，open和close/release都單純的回應成功而已。
而exit_modules()當然就是負責歸還當初和OS要的資源了"kfree()"，當然包含之前註冊的char device number，現在也要unregister "unregister_chrdev_region()"。
