Socket Buffer for bpf, __sk_buff

Question

I am playing ebpf code and got a sample like so:

int tc_ingress(struct __sk_buff *skb)
{
    void *data = (void *)(long)skb->data;
    struct ethhdr *eth = data, eth_copy; # what is this line doing? 
}

AS I am new to C and kernel, got stuck on many smaller things. Can some tell me what is this line mean?

struct ethhdr *eth = data, eth_copy;

How "new" are you to C exactly? That is just a cast from `void *` to a variable of type `struct ethhdr *`. Are you sure you don't want to read [a good C book](https://stackoverflow.com/questions/562303/the-definitive-c-book-guide-and-list) first before diving into complex kernel code? — Marco Bonelli, Feb 08 '22 at 04:34

score 1 · Accepted Answer · answered Feb 08 '22 at 09:09

1

struct ethhdr *eth = data, eth_copy;

Is nothing special, it is a more compact version of

struct ethhdr *eth = data; 
struct ethhdr *eth_copy;

So we declare two variables of the same type, but we initialize just the first one.

answered Feb 08 '22 at 09:09

Dylan Reimerink

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Socket Buffer for bpf, __sk_buff

1 Answers1