Re: [boost] asio networking proposal 0.3.4

From: Martin Bonner (martin.bonner_at_[hidden])
Date: 2005-10-18 03:37:36

> --- Beman Dawes <bdawes_at_[hidden]> wrote:
>> Where did this idea come from that the only way to avoid an
>> additional copy is to use a void *?

From: Christopher Kohlhoff [mailto:chris_at_[hidden]]
> It's not just avoiding a copy, it's avoiding a copy *and* allowing
> arbitrary data structures to be sent. IMHO an important use case for
> C++ and networking is being able to do things like:
> 
>   struct message {
>     int a;
>     double b;
>     int c_size;
>     char c[0];
>   };
>   message* m = ...;
>   sock.write(buffers(m, sizeof(message) + m->c_size));

Oh no! Please!  Don't let us encourage bad code like that.  It will work if
the receiving socket belongs to the same program on the same machine.
Otherwise it depends on:
        - sizeof(int) (16,32,64 bits)
        - endianness
        - representation of double (IEEE or IBM)

... and that's just the obvious, real-world, differences that can be
encountered TODAY.

The only safe datatype for transport over a socket is an array of unsigned
char with a defined layout.  Thus we would need somethink like:
   message* m = ...;
   std::vector<unsigned char> mbuf;
   mbuf.reserver(sizeof(m)+m->c_size);
   mbuf.push_back( m->a & 0xff );
   mbuf.push_back( (m->a >> 8) & 0xff );
   pushDouble( mbuf, m->b );  // pushDouble decodes the double and pushes
                              // the bytes of a IEEE double onto mbuf.
   etc

This looks like an application for something like serialization.  For
example, the result of right shifting a negative number is implementation
defined and something more subtle is required for real portability.
(Something like serialization, rather than the same as, because the user
needs to specify "this field is 16 bits", "this field is 32 bits"
independant of the size of the datatype used to store the field).

(Or have I missed something terribly subtle in the "buffers" code which
handles all this)
-- 
Martin Bonner
Martin.Bonner_at_[hidden]
Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ,
ENGLAND Tel: [phone]