Question

I have make a module in which i receive all incoming packets by net filter. I want to print the data (no headers) in /var/log/messages . I have tried the following printk line but its not printing the suitable message .

 struct sk_buff *sock_buff; 
 printk(KERN_INFO"user data :: %x",(sock_buff->data+sizeof(*sock_buff->network_header)+sizeof(*sock_buff->mac_header))  //though i want in character but i cant understand whats it printing not matching the hex of my data
  printk(KERN_INFO"user data :: %s",(sock_buff->data+sizeof(*sock_buff->network_header)+sizeof(*sock_buff->mac_header))

i have even used tail – sizeof(my_data); then also i am not getting desired o/p.

Answer 1

Maybe your forgot take a value by pointer(*), try this

printk(KERN_INFO"user data :: %x",*(sock_buff->data+sizeof(*sock_buff->network_header) +sizeof(*sock_buff->mac_header));  

As I understand, in your structure data field have a pointer type.

typedef struct{
  ...
  unsigned char* data;
  ...
} sk_buff;

Also %x specificator (number in hex) requers numerical agrument, not a pointer, that’s why you should not forgot to put one small (*)