Regarding how the parameters to the read function is passed in simple char driver

Question

I am newbei to driver programming i am started writing the simple char driver . Then i created special file for my char driver mknod /dev/simple-driver c 250 0 .when it type cat /dev/simple-driver. it shows the string "Hello world from Kernel mode!". i know that function

static const char    g_s_Hello_World_string[] = "Hello world tamil_vanan!\n\0";
    static const ssize_t g_s_Hello_World_size = sizeof(g_s_Hello_World_string);


    static ssize_t device_file_read(
                   struct file *file_ptr
                , char __user *user_buffer
                , size_t count
                , loff_t *possition)
    {
       printk( KERN_NOTICE "Simple-driver: Device file is read at offset = 
           %i, read bytes count = %u", (int)*possition  , (unsigned int)count );

       if( *possition >= g_s_Hello_World_size )
          return 0;

       if( *possition + count > g_s_Hello_World_size )
          count = g_s_Hello_World_size - *possition;

        if( copy_to_user(user_buffer, g_s_Hello_World_string + *possition,         count) != 0              )
          return -EFAULT;   

       *possition += count;
       return count;
    }

is get called . This is mapped to (*read) in file_opreation structure of my driver .My question is how this function is get called , how the parameters like struct file,char,count, offset are passed bcoz is i simply typed cat command ..Please elabroate how this happening

Hi, your module is inserted. As "cat" command is used to read the data from a file and dump it on console. Since in kernel everything is file, "/dev/simple-driver" device node is a special file which as information like major and minor number. In your init function you would have registered your driver with major no, driver name and file operations, so "cat" points to driver's read function. Echo is mapped to driver's write function. Even I think so. — Gautham Kantharaju

kzs kzs · Accepted Answer · 2013-03-24T13:32:25

In Linux all are considered as files. The type of file, whether it is a driver file or normal file depends upon the mount point where it is mounted. For Eg: If we consider your case : cat /dev/simple-driver traverses back to the mount point of device files.

From the device file name simple-driver it retrieves Major and Minor number.
From those number(especially from minor number) it associates the driver file for your character driver.
From the driver it uses struct file ops structure to find the read function, which is nothing but your read function:

static ssize_t device_file_read(struct file *file_ptr, char __user *user_buffer, size_t count, loff_t *possition)

User_buffer will always take sizeof(size_t count).It is better to keep a check of buffer(In some cases it throws warning)
String is copied to User_buffer(copy_to_user is used to check kernel flags during copy operation).
postion is 0 for first copy and it increments in the order of count:position+=count.

Once read function returns the buffer to cat. and cat flushes the buffer contents on std_out which is nothing but your console.

Regarding how the parameters to the read function is passed in simple char driver

2 Answers