Question

I’ ve got such a function

static ssize_t read_mydevice(struct file *filp, char* buf, size_t count, loff_t* ppos) {

 char *text = "Device is empty\n";
    int len = strlen(text);


    if (*ppos != 0)
            return 0;


    if(count>bytesindev) count=bytesindev;
    if(bytesindev==0) {

        if (copy_to_user(buf, text, len))
                 return -EINVAL;
     } else {
        while(count>0) {
        if (copy_to_user(buf++, msg_Ptr, (unsigned long) 1)) {
                 return -EINVAL;
                } else {
                    strcpy(msg_Ptr, (msg_Ptr+1));
                    bytesindev-=1;
                    *(msg_Ptr+bytesindev) = '\0';
                }
                count-=1;
                printk(KERN_INFO "%d\n", count);
            }
            printk(KERN_INFO "%s\n", buf);
     }
    /*
     * Tell the user how much data we wrote.
     */
    *ppos = len;

    return len;

 }

The problem is that user doesn’t get whole message, it always is a first 13-16 signs of it, a loop is made so many times that it’s supposed to. At first the third argument passed to copy_to_user was the length of whole message, but this solutions also didn’t work properly. Any ideas?
This function should give the user a part of buffer he demanded (number of bytes first written to the buffer, a bit like a FIFO). Data must have been send earlier to the device. It’s the function for writing:

static ssize_t
write_mydevice(struct file *filp, const char *buff, size_t len, loff_t * off)
{
if(bytesindev+len>limit) {
    if(limit-bytesindev<=0) {
        printk(KERN_ALERT "Device is full.\n");
            return -EINVAL;
    } else {
        printk(KERN_INFO "Device almost full.\n");
        strncat(msg_Ptr, buff, limit-bytesindev);
        bytesindev+=limit-bytesindev;
        return limit-bytesindev;
    }
} else {
    //printk(KERN_INFO "Device working\n");
    strncat(msg_Ptr, buff, len);
    bytesindev+=len;
    return len;
} 
}

Answer 1

The problem was that read function returns wrong value, it always returns length of text, not the lenght of the sended message.