On 09/15/2014 02:31 PM, Hei Chan wrote:
>
>
> 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?
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.
