麻烦编译和使用netinet / tcp.h与-std = c99

Question

#include<stdint.h>
#include<netinet/ip.h>
#include<netinet/tcp.h>


int main()
{
        struct tcphdr *tcpHdr = ((struct tcphdr*)NULL) ;
        struct iphdr *ipHdr = ((struct iphdr*)NULL);

        uint8_t offset = (unsigned int)(tcpHdr->doff)*4;
        uint16_t port =  tcpHdr->dest;

        unsigned int hdr_len = (ipHdr->ihl)*4;

        return 0;
}

编译后

gcc -std = c99 test1.c：

test1.c: In function ‘main’:
test1.c:13:40: error: dereferencing pointer to incomplete type
test1.c:14:25: error: dereferencing pointer to incomplete type

问题：

1）。它没有给struct iphdr

带来任何错误

2）。如果我不包括-std = c99，它编译并正常工作。