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static void __exit globalvar_exit(void) { int ret; ret = unregister_chrdev(MAJOR_NUM, "globalvar"); if (ret) { printk("globalvar unregister failure"); } else { printk("globalvar unregister success"); } }
static ssize_t globalvar_read(struct file *filp, char *buf, size_t len, loff_t *off) { //获得信号量 if (down_interruptible(&sem)) { return - ERESTARTSYS; } //将global_var从内核空间复制到用户空间 if (copy_to_user(buf, &global_var, sizeof(int))) { up(&sem); return - EFAULT; } //释放信号量 up(&sem); return sizeof(int); } ssize_t globalvar_write(struct file *filp, const char *buf, size_t len, loff_t *off) { //获得信号量 if (down_interruptible(&sem))//如果返回0,表示获得信号量正常返回 { return - ERESTARTSYS; } //将用户空间的数据复制到内核空间的global_var if (copy_from_user(&global_var, buf, sizeof(int))) { up(&sem); return - EFAULT; } //释放信号量 up(&sem); return sizeof(int); } module_init(globalvar_init); module_exit(globalvar_exit);
接下来,我们给globalvar的驱动程序增加open()和release()函数,并在其中借助自旋锁来保护对全局变量int globalvar_count(记录打开设备的进程数)的访问来实现设备只能被一个进程打开(必须确保globalvar_count最多只能为1):
#include #include #include #include #include
MODULE_LICENSE("GPL");
#define MAJOR_NUM 254
static ssize_t globalvar_read(struct file *, char *, size_t, loff_t*); static ssize_t globalvar_write(struct file *, const char *, size_t, loff_t*); static int globalvar_open(struct inode *inode, struct file *filp); static int globalvar_release(struct inode *inode, struct file *filp);
struct file_operations globalvar_fops = { read : globalvar_read, write : globalvar_write, open : globalvar_open, release: globalvar_release, };
static int global_var = 0; static int globalvar_count = 0; static struct semaphore sem; static spinlock_t spin = SPIN_LOCK_UNLOCKED;
static int __init globalvar_init(void) { int ret; ret = register_chrdev(MAJOR_NUM, "globalvar", &globalvar_fops); if (ret) { printk("globalvar register failure"); } else { printk("globalvar register success"); init_MUTEX(&sem); } return ret; }
static void __exit globalvar_exit(void) { int ret; ret = unregister_chrdev(MAJOR_NUM, "globalvar"); if (ret) { printk("globalvar unregister failure"); } else { printk("globalvar unregister success"); } }
static int globalvar_open(struct inode *inode, struct file *filp) { //获得自选锁 spin_lock(&spin);
//临界资源访问 if (globalvar_count) { spin_unlock(&spin); return - EBUSY; } globalvar_count++;
//释放自选锁 spin_unlock(&spin); return 0; }
static int globalvar_release(struct inode *inode, struct file *filp) { globalvar_count--; return 0; }
static ssize_t globalvar_read(struct file *filp, char *buf, size_t len, loff_t *off) { if (down_interruptible(&sem)) { return - ERESTARTSYS; } if (copy_to_user(buf, &global_var, sizeof(int))) { up(&sem); return - EFAULT; } up(&sem); return sizeof(int); }
static ssize_t globalvar_write(struct file *filp, const char *buf, size_t len,loff_t *off) { if (down_interruptible(&sem)) { return - ERESTARTSYS; } if (copy_from_user(&global_var, buf, sizeof(int))) { up(&sem); return - EFAULT; } up(&sem); return sizeof(int); }
module_init(globalvar_init); module_exit(globalvar_exit);
为了上述驱动程序的效果,我们启动两个进程分别打开/dev/globalvar。在两个终端中调用./globalvartest.o测试程序,当一个进程打开/dev/globalvar后,另外一个进程将打开失败,输出"device open failure"。
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