Question

I want to use this struct in visual c++:

struct iphdr
{
    unsigned int ihl:4;
    unsigned int version:4;

    u_int8_t tos;
    u_int16_t tot_len;
    u_int16_t id;
    u_int16_t frag_off;
    u_int8_t ttl;
    u_int8_t protocol;
    u_int16_t check;
    struct in_addr ip_src;
    struct in_addr ip_dst;

};

but when debugging I found sizeof(struct iphdr) = 24, which expected to be 20
And it is strange that when I replace ihl and version with a char, sizeof(struct iphdr) = 20

Answer 1

The bitfields ihl and version will be placed in an unsigned int field (despite the fact that they would fit into a character). Hence, the field tos will be placed at offset 4, and tot_length at offset 6, due to padding.