how to reuse the variable in linux kernel?

Question

extern unsigned long current_rx_time;
EXPORT_SYMBOL(current_rx_time);
int netif_rx(struct sk_buff *skb) 
{

current_rx_time = jiffies;

}

I modified the kernel source code in dev.c as shown above. Later I am creating a loadable kernel module in procfs and using the currentrx_time to send it to user space as shown below :

static int my_proc_show(struct seq_file *m, void *v)
{
    //I AM JUST PRINTING THAT VALUE BELOW

    seq_printf(m, "%lu\n", current_rx_time *1000/HZ);

    return 0;
}

but I am getting a error when I compile my module above as current_rx_time is undeclared. Could someone tell me how to solve this problem?

Answer 1

First you need to declare your variable and then you can EXPORT it.

so just declare it as in dev.c

unsigned long current_rx_time;

then export it as in dev.c

EXPORT_SYMBOL(current_rx_time);

and in other loadable module where you want to use it (let's say in temp2.c)...

extern unsigned long current_rx_time;

Now make sure when you going to compile temp2.c at that time dev.c is also getting compile.

Answer 2

The second code needs to declare the external variable, so the linker can know that it's coming from the outside:

extern unsigned long current_rx_time;