On structure assignment (pack your ETH header structures!)

Program like this :

#include <stdio.h>
#include <stddef.h>


int main()
{
    int i, j;
    char a[] = { 0x0, 0x1, 0x2, 0x4, 0x5, 0x6, 0x7, 0x8};

    typedef struct
    {
        char c;
        short s;
        int k;
    } s_t;

    s_t str;

    printf ("offsetof(str, c) = %d\n", offsetof(s_t, c));
    printf ("offsetof(str, s) = %d\n", offsetof(s_t, s));
    printf ("offsetof(str, k) = %d\n", offsetof(s_t, k));

    str = *( (s_t *)a );

    printf("str.c = %#x\n", str.c);
    printf("str.s = %#x\n", str.s);
    printf("str.k = %#x\n", str.k);

    return 0;
}

Will give output like this :

drasko@bg:~/stuff$ ./struct_ass
offsetof(str, c) = 0
offsetof(str, s) = 2
offsetof(str, k) = 4
str.c = 0
str.s = 0x402
str.k = 0x8070605
drasko@bg:~/stuff$

If, however, structure was packed, like this :

typedef struct
    {
        char c;
        short s;
        int k;
    } __attribute__ ((__packed__)) s_t;

Result would be different :

drasko@bg:~/stuff$ ./struct_ass
offsetof(str, c) = 0
offsetof(str, s) = 1
offsetof(str, k) = 3
str.c = 0
str.s = 0x201
str.k = 0x7060504
drasko@bg:~/stuff$

So - advice for casting your ETH packet stream into ETH header-like pkt_hdr struct for easier manipulation of memcers in ETH frame (like src and dst MAC addr and ethtype) :
pack your header struct to kill the padding!