On Fri, Sep 10, 2010 at 2:39 PM, John Mahoney <jmahoney@xxxxxxxx> wrote:
> On Fri, Sep 10, 2010 at 1:30 PM, fabio de francesco <fabio@xxxxxxxxxxx> wrote:
>> On Friday 10 September 2010 19:16:31 John Mahoney wrote:
>>> On Fri, Sep 10, 2010 at 1:02 PM, John Mahoney <jmahoney@xxxxxxxx> wrote:
>>> > On Fri, Sep 10, 2010 at 11:51 AM, fabio de francesco <fabio@xxxxxxxxxxx>
>> wrote:
>>> >> Hi all,
>>> >>
>>> >> I have modified a simple character device driver as an exercise from the
>>> >> Cooperstein's Linux Device Drivers book.
>>> >>
>>> >> It seems to work fine except that when I "cat /dev/mycdrv" it provides
>>> >> garbage.
>>> >>
>>> >> This is a trimmed down version of the code:
>>> >>
>>> >> #include <linux/module.h> /* for modules */
>>> >> #include <linux/fs.h> /* file_operations */
>>> >> #include <linux/uaccess.h> /* copy_(to,from)_user */
>>> >> #include <linux/init.h> /* module_init, module_exit */
>>> >> #include <linux/slab.h> /* kmalloc */
>>> >> #include <linux/cdev.h> /* cdev utilities */
>>> >>
>>> >> #define MYDEV_NAME "mycdrv"
>>> >> #define KBUF_SIZE (size_t)( PAGE_SIZE )
>>> >>
>>> >> static char *kbuf;
>>> >> static dev_t first;
>>> >> static unsigned int count = 1;
>>> >> static int my_major = 700, my_minor = 0;
>>> >> static struct cdev *my_cdev;
>>> >> static int counter = 0;
>>> >>
>>> >> static int mycdrv_open (struct inode *inode, struct file *file)
>>> >> {
>>> >> printk( KERN_INFO " open #%d\n", ++counter );
>>> >> kbuf = kmalloc (KBUF_SIZE, GFP_KERNEL);
>>> >> memset( kbuf, '\0', KBUF_SIZE );
>>> >
>>> > First this should be
>>> > memset( kbuf, '0', KBUF_SIZE );
>>> >
>>> > That will print the char 0 instead of the null char.
>>> >
>>> > Second try using "dd if=[dev] count=1" instead to read.
>>> >
>>> > --
>>> > John
>>>
>>> If you actually want to fix it add the following check to the top of
>>> mycdrv_read
>>>
>>> if (*ppos + kbuf > kbuf + KBUF_SIZE) {
>>> printk (KERN_INFO "\n READING function, maxbytes=%d,
>>> bytes_to_do=%d, lbuf=%d\n", maxbytes, bytes_to_do, lbuf);
>>> return 0;
>>> }
>>>
>>> --
>>> John
>>
>> Hi John,
>>
>> I am sorry i am not able to understand how that check could fix the bug that
>> certainly is in my code somewhere. Please do you mind to explain where the bug
>> is located?
>>
>
> You really should do something like this:
> maxbytes = (kbuf + KBUF_SIZE) - (kbuf + *ppos);
>
> I am at work so I just scanned your code real quick, but basically you
> were always reading a min of 4096 bytes and were reading past the end
> of the buffer. You were returning always 4096 instead of how many
> bytes were really left in the buffer.
>
> btw, you can do memset to null if you want.
here is my version of the function:
static ssize_t
mycdrv_read (struct file *file, char __user * buf, size_t lbuf, loff_t * ppos)
{
int nbytes, maxbytes, bytes_to_do;
maxbytes = (kbuf + KBUF_SIZE) - (kbuf + *ppos);
bytes_to_do = lbuf <= maxbytes ? lbuf : maxbytes;
nbytes = bytes_to_do - copy_to_user (buf, kbuf + *ppos, bytes_to_do);
*ppos += nbytes;
printk (KERN_INFO "\n READING function, nbytes=%d, pos=%d\n", nbytes,
(int)*ppos);
return nbytes;
}
--
John
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