char driver error

char driver error

[Aravind Vijayan]

Hi all,
       Let me begin this post with thanx all to whom help me on
previous problem.I have completed jobs such as
writing,making,inserting the  module.

i create a character special file using mknod in /dev directory.

i try to write data to my file by" echo -n "abcd" > /dev/memory "

and when i try to read from that file i get  the last char was written
to that file that is "d"

here is my code:http://pastebin.com/9YsJetek

char driver error

[Manohar Vanga]

This line is incorrect. You need to learn how kmalloc (or any *alloc for
that matter) works because it seems like you don't realise what is happening
with this statement.

memory_buffer = kmalloc(1, GFP_KERNEL);


On Mon, May 9, 2011 at 1:26 PM, Aravind Vijayan <aravind1123@gmail.com>wrote:

> Hi all,
>       Let me begin this post with thanx all to whom help me on
> previous problem.I have completed jobs such as
> writing,making,inserting the  module.
>
> i create a character special file using mknod in /dev directory.
>
> i try to write data to my file by" echo -n "abcd" > /dev/memory "
>
> and when i try to read from that file i get  the last char was written
> to that file that is "d"
>
> here is my code:http://pastebin.com/9YsJetek
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>



-- 
/manohar
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char driver error

[Daniel Baluta]

> i create a character special file using mknod in /dev directory.
>
> i try to write data to my file by" echo -n "abcd" > /dev/memory "
>
> and when i try to read from that file i get ?the last char was written
> to that file that is "d"

Hello Aravind,

Can you post the output for:
$ strace -n "abcd" > /dev/memory
$ strace cat /dev/memory a

thanks,
Daniel.

char driver error

[Manohar Vanga]

Google for "kmalloc":
About 182,000 results (0.11 seconds)

http://lwn.net/Kernel/LDD3/

http://tldp.org/LDP/lkmpg/2.6/html/index.html

Make an effort. It usually helps :-)

On Mon, May 9, 2011 at 1:57 PM, Aravind Vijayan <aravind1123@gmail.com>wrote:

> On 5/9/11, Manohar Vanga <manohar.vanga@gmail.com> wrote:
> > This line is incorrect. You need to learn how kmalloc (or any *alloc for
> > that matter) works because it seems like you don't realise what is
> happening
> > with this statement.
> >
> > memory_buffer = kmalloc(1, GFP_KERNEL);
> >
> >
>
> I am totally newbie to kernel and to drivers i wish to study about
> them seeking you all's help.....
>



-- 
/manohar
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char driver error

[Vikash Kumar]

Following are my observations from the code:

1) The driver is maintaining a 1 byte buffer.
memory_buffer = kmalloc(1, GFP_KERNEL);

2) Write operation always copies the last byte of user buffer to driver
buffer.
tmp=buf+count-1;
copy_from_user(memory_buffer,tmp,1)

3) Read operation always returns the copied character from driver buffer on
every read ignoring the count.
copy_to_user(buf,memory_buffer,1);

So, to me it seems everything is working fine.. what's the error here ? what
am I missing something?


Regards,
Vikash Kumar




On Mon, May 9, 2011 at 4:56 PM, Aravind Vijayan <aravind1123@gmail.com>wrote:

> Hi all,
>       Let me begin this post with thanx all to whom help me on
> previous problem.I have completed jobs such as
> writing,making,inserting the  module.
>
> i create a character special file using mknod in /dev directory.
>
> i try to write data to my file by" echo -n "abcd" > /dev/memory "
>
> and when i try to read from that file i get  the last char was written
> to that file that is "d"
>
> here is my code:http://pastebin.com/9YsJetek
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>
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char driver error

[Daniel Baluta]

OK. See how write and read works.

> $strace echo -n "abcd" > /dev/memory(/home/aravind/me
> write(1, "abcd", 4) ? ? ? ? ? ? ? ? ? ? = 1
> write(1, "bcd", 3) ? ? ? ? ? ? ? ? ? ? ?= 1
> write(1, "cd", 2) ? ? ? ? ? ? ? ? ? ? ? = 1
> write(1, "d", 1) ? ? ? ? ? ? ? ? ? ? ? ?= 1

>$ strace cat /dev/memory(/home/aravind/memory)

> read(3, "d", 32768) ? ? ? ? ? ? ? ? ? ? = 1
> write(1, "d", 1d) ? ? ? ? ? ? ? ? ? ? ? ?= 1

Please allocate space for more than 1 char, and also
correct memory_write

  tmp=buf+count-1; -> tmp=buf+*f_ps-1;

thanks,
Daniel.