Question

I wrote h into driver by doing echo:
echo -n h /dev/mydriver

When I do cat /dev/mydriver, myread function is printing h continuously. I wanted to print once. How to do that.

static char m;

static ssize_t myread(struct file *f, char __user *buf, size_t len, loff_t *off)
{
    printk(KERN_INFO "Read()\n");
    if (copy_to_user(buf, &m, 1) != 0)
        return -EFAULT;
    else
        return 1;
}

static ssize_t my_write(struct file *f, const char __user *buf, size_t len, loff_t *off)
{
    printk(KERN_INFO "Write()\n");
    if (copy_from_user(&c, buf + len – 1, 1) != 0)
        return -EFAULT;
    else
        return len;
}

Answer 1

If you want to use standard tools (such as cat) with your custom drivers, do not forget to set offset (*loff_t off) correctly. Your read function should look something like this:

static ssize_t myread(struct file *f, char __user *buf, size_t len, loff_t *off)
{
    printk(KERN_INFO "Read()\n");

    /* You have just a single char in your buffer, so only 0 offset is valid */
    if(*off > 0)
        return 0; /* End of file */

    if (copy_to_user(buf, &m, 1))
        return -EFAULT;

    *off++;
    return 1;
}