date:20150101

Understanding of write file operation in char driver

2015-01-01 Thread me storage

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?

Thanks  Regards
Prasad
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Re: Understanding of write file operation in char driver

2015-01-01 Thread Victor Rodriguez

On Thu, Jan 1, 2015 at 2:24 AM, me storage me.storage...@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 withmknod
/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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 Kernelnewbies@kernelnewbies.org
 http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies


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Re: Understanding of write file operation in char driver

2015-01-01 Thread me storage

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.ro...@gmail.com wrote:

 On Thu, Jan 1, 2015 at 2:24 AM, me storage me.storage...@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 withmknod
 /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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  Kernelnewbies mailing list
  Kernelnewbies@kernelnewbies.org
  http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
 

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Understanding of write file operation in char driver

Re: Understanding of write file operation in char driver

Re: Understanding of write file operation in char driver

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