Understanding of write file operation in char driver

[Victor Rodriguez]

On Thu, Jan 1, 2015 at 2:24 AM, me storage <me.storage126 at 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
>
>
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