Libbpf eBPF macro offsetof
The offsetof macro is used to calculate the offset of a member within a struct.
Definition
#define offsetof(type, member) ((unsigned long)&((type *)0)->member)
Usage
This macro can be useful in cases where you need to calculate the offset of a field. This comes up when you need to provide a relative offset or need to do pointer arithmetic.
Example
#define ETH_HLEN 14
SEC("tc")
int example_prog(struct __sk_buff *ctx)
{
__u32 assigned_ip = 0x0a000001; // 10.0.0.1
bpf_skb_store_bytes(skb, ETH_HLEN + offsetof(struct iphdr, daddr), &assigned_ip, sizeof(__u32), 0) < 0)
}