Re: is portable aliasing possible in C++?

[Andrew Haley <aph@redhat.com>]

On 09/15/2014 02:31 PM, Hei Chan wrote:
> 
> 
> On Monday, September 15, 2014 9:21 PM, Andrew Haley <aph@redhat.com> wrote:
> On 09/15/2014 12:57 PM, Hei Chan wrote:
>> // on a big-endian 64-bit machine
>>
>> struct Message {
>>     int32_t a;
>>     int16_t b;
>>     char c;
>>     char padding;
>> };
>>
>> // send over a socket
>> Message msg = {12345, 678, 'x', 0};
> 
> I'd do this:
> 
> Message msg = {htonl(12345), htons(678), 'x', 0};
> 
> What if I have no control of the sender?

I don't understand this question.  You said that you were sending.

>> send(fd, &msg, sizeof Message, 0);
>>
>>
>> // another machine: a little-endian 64-bit machine
>> char buffer[1024];
>> if (recv(fd, buffer, sizeof buffer, 0)) {
>>     Message msg;
>>     // or we can use the union trick
> 
> Why not read into the Message?
> 
> I think it goes back to the same issue as Andy (the original poster)
> -- the server/sender will send multiple kinds of message types.
> 
> For instances,
> 
> enum class MsgType : int32_t {
> Heartbeat,
> Logon,
> Logout
> };
> 
> struct Header {
>   int32_t MsgType;
>   int32_t padding;
> }
> 
> struct LogonBody {
>     int32_t a;
>     int16_t b;
>     char c;
>     char padding;
> }
> 
> I can read into an instance of Header first (1 system call), then I
> know the message type so I can read into an instance of LogonBody
> (another system call).  But my goal is to avoid latency.  I
> certainly would prefer 1 system call instead of 2 if possible.

But you can either read into a union of the message types and handle
that, or read into a byte array and pick the data straight from there.

Like this:

int
byteswap (int u)
{
  return ((((u) & 0xff000000) >> 24)
          | (((u) & 0x00ff0000) >>  8)
          | (((u) & 0x0000ff00) <<  8)
          | (((u) & 0x000000ff) << 24));
}

int readInt(char *a) {
  int val;
  memcpy(&val, a, sizeof val);
  return byteswap(val);
}

which generates this:

readInt:
	ldr	w0, [x0]
	rev	w0, w0
	ret

> Hope now it makes sense to you.

Not really, no.  I know that there are many bad ways of solving the
problem.  All I'm saying is that you don't have to do it in a bad way.
C provides you with everything you need to do it well.

If you want a way to read from an arbitrary position in an byte array
into any type, in any endianness, you can do that; see readInt above.
If you want to read into a union of all message types, you can do
that.

Andrew.