aliasing warnings

[Eduard Hasenleithner <eduard at hasenleithner dot at>]

Since gcc 4.4 I get considerably more
"dereferencing type-punned pointer will break strict-aliasing rules"
warnings with my code. The code ist used for message en- and de-capsulation.

The basic message structure is defined like that

struct msg {
  uint8_t msg_type;
  uint8_t msg_len;
  uint8_t msg_data[0];
};

Upon reception of the message by means of a socket the space needed for 
the message is allocated and stored as a pointer to struct msg. 
Depending on the msg_type there can be different contents in the message.

One example of message content are two 32 bit integers. With gcc 4.3 
-Wall and -O3 I could access the contents without a warning:

struct msg *msg = receive();
if (msg->msg_type==5 && msg->msg_len==8) {
  printf("a: %d\n", *(uint32_t*)(msg->msg_data+0));
  printf("b: %d\n", *(uint32_t*)(msg->msg_data+4));
}

Now with gcc 4.4 I get the type-punned pointer warning - but only for 
the (a) printf, the (b) printf appears to be fine. Regardless of the 
warning the printf statements always give the correct text on standard 
output.

Reading several posts about the "strict aliasing rules" in C99 i ask 
myself what gcc optimization (in future) might break the mentioned code.

Furthermore it is strange that the first printf triggers an aliasing 
warning and the second printf not. I cannot find a difference with 
regards to the C99 strict aliasing rules. Could some put any light on this?

Thanks,
Eduard