Packing netgraph structs

[Anil Madhavapeddy]

On 1 Jul 2004, at 15:10, Ruslan Ermilov wrote:

> We use Netgraph from within Python, using the netgraph(3) library,
> and we pack/unpack various Netgraph related structs just happily.
>
> Speaking of "struct ng_mesg", if you pack it, it will essentially
> stay the same, because I believe it was created with this in mind,
> as well as most if not all other user-accessible Netgraph structs.

How robust is this solution?  Since ng_mesg uses nested structs, and 
compilers can insert padding at the end of structs at will, this could 
lead to padding between ng_mesg.header.cmdstr and ng_mesg.data.  Alan 
Mycroft, who wrote the ARM C compiler commented that the C spec is 
rather ambiguous about padding, and that compilers are quite free to 
pad as they chose in between structs as well.

In other words - is there any harm in adding a packed attribute here to 
ensure this wont break and lead to hard to track down bugs in the 
future?  It shouldn't even affect the gcc ABI I think ... ?

For the moment, I'll get on with just serialising as if it were 
optimally packed from OCaml, thanks for your help so far!

--
Anil Madhavapeddy                                 http://anil.recoil.org
University of Cambridge                          http://www.cl.cam.ac.uk