Re: is portable aliasing possible in C++?

[Hei Chan <structurechart@yahoo.com>]

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?


> 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.

Hope now it makes sense to you.




>     memcpy(&msg, buffer, sizeof Message);    SomeFunctioToConvertFromBigEndianToSmallEndian(msg.a);

I don't know why you'd want to byte-reverse in place if you actually
care about zero-copy network programming.  That makes no sense to me.




Andrew.