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 );
return 0;
}
static int mycdrv_release (struct inode *inode, struct file *file)
{
printk (KERN_INFO " CLOSING device: %s:\n\n", MYDEV_NAME);
kfree( kbuf );
--counter;
return 0;
}
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;
}
static const struct file_operations mycdrv_fops = {
.owner = THIS_MODULE,
.read = mycdrv_read,
.write = mycdrv_write,
.open = mycdrv_open,
.release = mycdrv_release,
.llseek = mycdrv_llseek
};
static int __init my_init (void)
{
first = MKDEV (my_major, my_minor);
alloc_chrdev_region( &first, 0, count, MYDEV_NAME );
printk ( KERN_INFO "Device %s: MAJOR %d, MINOR %d\n", MYDEV_NAME, MAJOR(
first ), MINOR( first ) );
my_cdev = cdev_alloc ();
cdev_init (my_cdev, &mycdrv_fops);
cdev_add (my_cdev, first, count);
printk (KERN_INFO "\nSucceeded in registering character device %s\n",
MYDEV_NAME);
return 0;
}
static void __exit my_exit (void)
{
cdev_del (my_cdev);
unregister_chrdev_region (first, count);
printk (KERN_INFO "\ndevice unregistered\n");
}
module_init (my_init);
module_exit (my_exit);
Please, could someone explain why the readings of the device return garbage
data instead of NULL characters as I have set with memset()?
Thanks in advance.
fabio
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