On Monday, September 15, 2014 9:21 PM, Andrew Haley <aph@xxxxxxxxxx> 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.
