Understanding of write file operation in char driver

* Understanding of write file operation in char driver
@ 2015-01-02  2:20 me storage
  2015-01-02 16:02 ` Valdis.Kletnieks at vt.edu
  0 siblings, 1 reply; 6+ messages in thread
From: me storage @ 2015-01-02  2:20 UTC (permalink / raw)
  To: kernelnewbies

Hi All,
Thanks Victor

Can any one please tell me difference between kernal space & user space in
code perspective i.e in write function const char *buffer should point to
address of *hello* in RAM so how it is changing when i am accessing through
some kernel function because it is also accessing the same RAM address?

Thanks & Regards
Prasad

On 1 January 2015 at 21:20, Victor Rodriguez <vm.rod25@gmail.com> wrote:

> On Thu, Jan 1, 2015 at 2:24 AM, me storage <me.storage126@gmail.com>
> wrote:
> > I am learning char drivers.But i didn't understand write operation of
> char
> > device driver properly. the below is my write operation
> >
> > static ssize_t dev_write(struct file *fil,const char __user *buff,size_t
> > len,loff_t *off)
> > {
> >     pr_info("user input string %s\n",buff);
> >     pr_info("user input string len %d\n",len);
> >     return len;
> > }
> >
> > my doubt is if i write into my device like
> > echo "hello" > /dev/myDev
> >
> > it is giving different behaviour like below is the dmesg
> > [20596.975355] user input string hello
> > [20596.975355] 77b9e4
> > [20596.975355] insmod insmod
> > [20596.975355] n/zeitgeist-daemon
> > [20596.975355] atives
> > [20596.975355]
> > [20596.975355] vars "${upargs[@]}"
> > [20596.975355]  cur cword words=();
> > [20596.975355]     local upargs=() upvars=() vcur vcword vprev vwords;
> > [20596.975355]     while getopts "c:i:n:p:w:" flag "$@"; do
> > [20596.975355]         case $flag in
> > [20596.975355]             c)
> > [20596.975355]                 vcur=$OPTARG
> > [20596.975355]             ;;
> > [20596.975355]             i)
> > [20596.975355]                 vcword=$OPTARG
> > [20596.975355]             ;;
> > [20596.975355]             n)
> > [20596.975355]                 exclude=$OPTARG
> > [20596.975355]             ;;
> > [20596.975355]             p)
> > [20596.975355]                 vprev=$OPTARG
> > [20596.975355]             ;;
> > [20596.975355]             w)
> > [20596.975355]                 vwords=$OPTARG
> > [20596.975355]             ;;
> > [20596.975358] user input string len 6
> > [20596.975361] Device closed
> >
> > so i didn't understand what is happening inside .Can any one please
> explain
> > what is happening?
>
> HI Prasad
>
> The problem is ont he part where you try to print the buffer on dmesg.
> I tested this code ( the base full code is on my github repo as
> char_simple.c *):
>
> 106 static ssize_t device_write(struct file *filp,
> 107                             const char *buf,
> 108                             size_t len,
> 109                             loff_t * off)
> 110 {
> 111
> 112         procfs_buffer_size = len;
> 113
> 114
> 115         if ( copy_from_user(buffer_data, buf, procfs_buffer_size)
> ) {
> 116                 return -EFAULT;
> 117         }
> 118          *off += len;
> 119
> 120         pr_info("user input string %s\n",buffer_data);
> 121         //pr_info("user input string len
> %lu\n",procfs_buffer_size);
> 122
> 123         return procfs_buffer_size;
> 124 }
>
> And the dmesg output is :
>
> [ 2735.251589] Hello from the Kernel !!! (how cool is that)
> [ 2735.251600] Major Number = 244
> [ 2735.251604] Name =  mychardriver
> [ 2735.251607] Generate the device file with                mknod
> /dev/mychardriver c 244 0
> [ 2766.806455] user input string hello there
>
> Remember to first do the copy from user space and then print that
> variable instead of just the buff variable. Print the len is fine
> (https://gist.github.com/17twenty/6313566 ) however when you try to
> print the string from the user space , there is where we have the
> problems (I did the experiment )
>
> Here are some useful links:
>
> http://www.ibm.com/developerworks/library/l-kernel-memory-access/
> https://www.kernel.org/doc/htmldocs/kernel-api/ch04s02.html
>
> Hope it helps
>
> Regards
>
> Victor Rodriguez
>
> *char_simple.c :
>
> https://github.com/VictorRodriguez/linux_device_drivers_tutorial/blob/master/char_simple.c
>
>
>
>
>
>
>
>
> > Thanks & Regards
> > Prasad
> >
> >
> > _______________________________________________
> > Kernelnewbies mailing list
> > Kernelnewbies at kernelnewbies.org
> > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
> >
>
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