warning! dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

Question

I have problem with aliasing. I have never faced with it before. I am using Eclipse CDT. I read different solutions but I couldn't find the suitable one for me.

I have the warning

dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

So I have the following peace of code:

timestamp = st0 % 100000000;

for (std::list<struct trace *>::iterator it = frame_list.begin();
     it != frame_list.end();
     ++it) {
  struct my_rtppacket *packet = NULL; 
  packet = new struct my_rtppacket;

  packet->ts = (int32_t)((*it)->time * 1000000);
  packet->seq = count_seq;

  //In the following two strings I have the warnings :(

  *(uint16_t*) (packet->buf + 2) = htons(packet->seq);
  *(int32_t*) (packet->buf + 4) = htonl(packet->ts + timestamp);

  insert_data_to_list(packet); // This function inserts packets to list}
}

As the result there are wrong values in packet->buf + 2 and packet->buf + 4.

Please! Help me to fix this problem!

EDIT I want to understand what was wrong... The structure my_rtppacket is defined in the following way:

{
struct my_rtppacket
{
public:
    my_rtppacket():dump_ts(0), payloadlen(0),packetlen(0),ts(0), seq(0), seq_fr(0), frame_number(0),
    erase(NUM, INIT), path(NUM, INIT), create_time(0), alloc_time(0), before_create(0), sent_flag(0){}
    uint32_t dump_ts;   /*timestamp of RTP dump. It is similar to timestamp of packet generation from the application*/
    int payloadlen;
    int packetlen;
    int32_t ts;         /*timestamp in RTP file*/
    uint16_t seq;       /* Sequеnce number in video sequence*/
    int seq_fr;         /* Sequеnce number in a frame*/
    int frame_number;
    char buf[1600];
    std::vector <path_t> erase;
    std::vector <path_t> path;       //Declare a vector of path_type elements
    char frame_type[10];
    int64_t create_time;
    int64_t alloc_time;
    int64_t before_create;
    int sent_flag;

};

}

Answer 1

You should use something like the following to don't break strict aliasing rule:

const uint16_t nseq = htons(packet->seq);
const uint32_t nts = htonl(packet->ts + timestamp);
memcpy(packet->buf + 2, &nseq, 2);
memcpy(packet->buf + 4, &nts, 4);