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
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