Read and Write a /proc File

[Rajaram Suryanarayanan <rajaram_linux@xxxxxxxxx>]

Hi,
 
 I am following "Linux Kernel Module Programming Guide" and have a question on the reading from and writing to a /proc file. Please see the code snippet below, which is taken from the above mentioned guide.
 
 For the full example, please see 
 http://www.tldp.org/LDP/lkmpg/2.6/html/x769.html
 
 My question : 
 In the read function, we just copy a string to the variable "buffer" . We are not using put_user() here.
  But in write function , we see copy_from_user() being used, because we are copying a string from user space to kernel space. 
 Why we are not using put_user() in read function ? Are we not copying something from kernel space to user space there ? Why this difference between read and write for a proc file ? 
 But while reading character device file, we use put_user() in read function.
 
int 
procfile_read(char *buffer,
       char **buffer_location,
      
 off_t offset, int buffer_length, int *eof, void *data)
{
 int ret;
 
 printk(KERN_INFO "procfile_read (/proc/%s) called\n", PROCFS_NAME);
 
 if (offset > 0) {
  /* we have finished to read, return 0 */
  ret  = 0;
 } else {
  /* fill the buffer, return the buffer size */
  memcpy(buffer, procfs_buffer, procfs_buffer_size);
  ret = procfs_buffer_size;
 }

 return ret;
}

/**
 * This function is called with the /proc file is written
 *
 */
int procfile_write(struct file *file, const char *buffer, unsigned long count,
     void *data)
{
 /* get buffer size */
 procfs_buffer_size = count;
 if (procfs_buffer_size > PROCFS_MAX_SIZE ) {
  procfs_buffer_size = PROCFS_MAX_SIZE;
 }
 
 /* write data to the buffer */
 if ( copy_from_user(procfs_buffer, buffer, procfs_buffer_size) ) {
  return -EFAULT;
 }
 
 return
 procfs_buffer_size;
}


Thanks,
Rajaram.

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