c - 如何将字符串复制到用户并在 Linux 内核读取函数中使用 offp

Question

声明:

static char status[128] = "off\0";

并实现了一个read函数:

static ssize_t read_proc(struct file *filep, char __user *buf,
                    size_t len, loff_t *offp)
{
    ssize_t cnt = strlen(status), ret;

    ret = copy_to_user(buf, status, cnt);
    *offp += cnt;
    return cnt;
}

• 如何考虑 offp？
• 目前它不断地打印 status 到屏幕

Answer 1

感谢这里的评论，我想到了以下实现，我相信这是使用 offp 的正确方法:

static ssize_t read_proc(struct file *filep, char __user *buf,
                    size_t len, loff_t *offp)
{

    ssize_t cnt = strlen(status), ret;

    /* ret contains the amount of chare wasn't successfully written to `buf` */
    ret = copy_to_user(buf, status, cnt);
    *offp += cnt - ret;

    /* Making sure there are no left bytes of data to send user */
    if (*offp > cnt)
         return 0;
    else
         return cnt;
}