Question

I’ve been writing a device dev/my_inc that’s meant to take a positive integer N represented as an ASCII string, and store it internally. Any read from the device should produce the ASCII string representation of the integer (N+1).

However, when I cat /dev/my_inc, I only seem to be getting the first half of the myinc_value message buffer back in user space.

• If myinc_value is 48, cat /dev/my_inc yields 4.

• If myinc_value is 489324, cat /dev/my_inc yields 489.

However, bytes_read indicates the entire message was copied into user space. Here is the output from dmesg:

[54471.381170] my_inc opened with initial value 489324 = 489324.
[54471.381177] my_inc device_read() called with value 489325 and msg 489324.
[54471.381179] my_inc device_read() read 4.
[54471.381182] my_inc device_read() read 8.
[54471.381183] my_inc device_read() read 9.
[54471.381184] my_inc device_read() read 3.
[54471.381185] my_inc device_read() read 2.
[54471.381186] my_inc device_read() read 5. my_inc device_read() returning 7.
[54471.381192] my_inc device_read() called with value 489325 and msg 489325.

And when called from the shell:

root@rbst:/home/rob/myinc_mod# cat /dev/my_inc
489

And the source:

// Read from the device
//
static ssize_t device_read(struct file * filp, char * buffer, 
    size_t length, loff_t * offset)
{
    char c;
    int bytes_read = 0;
    int value = myinc_value + 1;

    printk(KERN_INFO "my_inc device_read() called with value %d and msg %s.\n", 
        value, msg);

    // Check for zero pointer
    if (*msg_ptr == 0) 
    {
        return 0;
    }
    // Put the incremented value in msg 
    snprintf(msg, MAX_LENGTH, "%d", value);

    // Copy msg into user space
    while (length && *msg_ptr) 
    {
        c = *(msg_ptr++);
        printk(KERN_INFO "%s device_read() read %c. ", DEV_NAME, c);
        if(put_user(c, buffer++))
        {
            return -EFAULT;
        }
        length--;
        bytes_read++;
    }

    // Nul-terminate the buffer
    if(put_user('\0', buffer++))
    {
        return -EFAULT;
    }
    bytes_read++;
    printk("my_inc device_read() returning %d.\n", bytes_read);
    return bytes_read;
}

Answer 1

It may be that put_user() is defined as a macro so that the post increment operator in

if(put_user(c, buffer++))

is screwing up – though I don’t see how it explains what you are seeing.

Anyway it would be more convenient and more efficient to use copy_to_user() to copy the whole msg.