how to write on a device file using copy_from_user()

[karan2386]

Hi ,

I am new to linux and was making a module in which user can read as well as write to a device file.the following is my code where read is working fine but not the write function.

MODULE_LICENSE("DUAL BSD/GPL");

char message[80];
char *msg_ptr;

int dev_major = 0;
int dev_minor = 0;

struct cdev *cdev;

ssize_t dev_read(struct file *filp,char __user *buf,size_t count,loff_t *offset)
{
int i;
i=copy_to_user(buf,msg_ptr,count);
printk(KERN_ALERT"buff:%s",buf);
if(buf == '\0')
{
printk("you idiot dont show your face again");
return 0;
}

return 0;
}
ssize_t dev_write(struct file *filp,const char __user *buf,size_t count,loff_t *offset)
{
int j;
msg_ptr = kmalloc(20*sizeof(char),GFP_KERNEL);
//for(j=0;j<count;j++)
copy_from_user(msg_ptr,buf,count);
//printk(KERN_ALERT"msg_ptr:%s",msg_ptr);
return 0;
}

when i make a char node and then use echo hi >/dev/my_dev then it prints hi but writes infintely as seen in /var/log/messages . Please help.reply as soon as possible.

regards
karan

[Rubberman]

Please move this to the Kernel forum. This is NOT a general programming issue!