一个简单的字符驱动

#include <linux/module.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <asm/uaccess.h>
#include <linux/syscalls.h>

MODULE_LICENSE("GPL");

#define MAJOR_NUM 100

int g_val;

static ssize_t global_read(struct file *filp, char __user *buf, size_t len, loff_t *off)
{
    int ret;
    char val[20];
    printk(KERN_ERR "###### global_read \n");
    sprintf(val, "%d\n", g_val);
    ret = simple_read_from_buffer(buf, len, off, val, strlen(val));
    return ret;
}

static ssize_t global_write(struct file *filp, const char __user *buf, size_t len, loff_t *off)
{
    int  ret, temp;
    char val[20];
    //simple_write_to_buffer(void *to, size_t available, loff_t *ppos,const void __user *from, size_t count);

    ret = simple_write_to_buffer(val, strlen(val), off, buf, len);
    sscanf(val, "%d", &temp);
    g_val = temp;
    return ret;
}

struct file_operations fileops = {
    .read = global_read,
    .write = global_write,
};

static int __init globalvar_init(void)
{

    int reg;
    g_val = 0;
    printk(KERN_ERR "golabvar_init \n");

    reg = register_chrdev(MAJOR_NUM, "globalvar", &fileops);
    if(reg)
    {
        printk(KERN_ERR "register_chrdev fail \n");
    }
    else
    {
        printk(KERN_ERR "register_chrdev sucess \n");
    }

    sys_mknodat(AT_FDCWD, "/dev/xxx", O_RDWR | O_CREAT, 100);
    return 0;
}

static void __exit globalvar_exit(void)
{
    printk("golabvar_exit \n");
    unregister_chrdev(MAJOR_NUM, "globalvar");
}

module_init(globalvar_init);
module_exit(globalvar_exit);