On Friday 10 September 2010 20:23:13 Josh Cartwright wrote:
> On Fri, Sep 10, 2010 at 05:51:41PM +0200, fabio de francesco wrote:
> > 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_SIZE - *ppos;
> > bytes_to_do = lbuf <= maxbytes ? lbuf : maxbytes;
> > nbytes = lbuf - 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;
> >
> > }
>
> I'm not going to flat out answer your query, but help you work towards
> the solution yourself.
>
> Think about the termination case. In order to terminate, you'll need to
> return 0 to signal an end of file to the user-mode read.
>
> In what case could your code return 0?
Ok. Good strategy!
Let me think...
When *ppos reaches the end of file it should be equal to KBUF_SIZE. Therefore
KBUF_SIZE - *ppos returns 0 to maxbytes. Immediatelly afterwards bytes_to_do
is set to 0 too. Right?
Ah! I think I see the bug at this point in walking the code... nbytes is set
to lbuf because copy_to_user() returns 0. So I never have nbytes set to 0 in
order to signal an end-of-file.
Is my reasoning correct?
Thanks a lot.
fabio
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