C编程：IP标头ID为负

Question

I have following code:

struct ip_header {
        unsigned char ip_hl:4;
        unsigned char ip_ver:4;
        unsigned char ip_dscp:6;
        unsigned char ip_ecn:2;
        short ip_len;
        short ip_id;
...
};
...

const struct ip_header * ip_hdr = (const struct ip_header *)(buffer + ETHERNET_HEADER_SIZE);

...

printf("IP ID: %d", ip_hdr->ip_id);

IP ID I get is negative for most of the packets. Can you guys see a mistake.

Answer 1

%d prints a signed integer (so here it converts your unsigned int into a signed int). Use %u for unsigned integers.