Paddding thing once more

[Havoc Pennington]

On Mon, 2005-08-08 at 15:45 +0200, Jakub Piotr Cłapa wrote:
> Sorry for so many questions about boring things but I hope you excuse 
> three more.
> 
> 1. The padding for every value is basically a constraint on it's start 
> address (counting from the beginning of the header or body) which must 
> be a multiple of the alignment value.
> 
> Example:
> When we marshal "yyyu" then between "yyy" and "u" there must be one nul 
> byte (regardless of the endianess the padding is always before the value?)
> 

Yes. I don't think endianness should matter here, afaik.

> 2. The padding of header is simply adding some nul bytes to the end of 
> the header to make it's length divisible by 8? It has nothing to do with 
> header fileds (which are structs) 8-byte padding? (because their padding 
> always goes before the struct value not after it?)

Right.

> 3. Why? What is gained from using paddings in DBus protocol? (it's 
> probably a question about binary formats design theory) I found in the 
> source that this is borrowed from ORBit2 but haven't seen any 
> explanation why.

The primary reason for the padding is that you can then use arrays and
structs in-place without copying them; just cast to a C type. This is
how dbus_message_iter_get_fixed_array() works. Some CPUs require the
data types to be aligned prior to access.

Havoc