globalvar.c

Some simple source code to show specific function like mouse and graphic.

#include <linux/module.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <asm/uaccess.h>
#include <linux/wait.h>
#include <asm/semaphore.h> 
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*);

struct file_operations globalvar_fops =
{
read: globalvar_read, 
write: globalvar_write,
};

static int global_var = 0;
static struct semaphore sem;
static wait_queue_head_t outq;
static int flag = 0;

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);
	init_waitqueue_head(&outq);
}
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 ssize_t globalvar_read(struct file *filp, char *buf, size_t len, loff_t *off)
{

if (wait_event_interruptible(outq, flag != 0))
{
	return - ERESTARTSYS;
}

if (down_interruptible(&sem))
{
	return - ERESTARTSYS;
}

flag = 0;
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);
flag = 1;

wake_up_interruptible(&outq);
return sizeof(int);
}

module_init(globalvar_init);
module_exit(globalvar_exit);