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...

Thanks & Regards Prasad

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