Re: [Jack-Devel] Bug#728710: jackd2: Bus error w/ POST_PACKED_STRUCTURE on powerpc G4 32bit

On 22/03/2014 09:17, Nedko Arnaudov wrote:
>
> The jack source code should not force packing that violates alignment
> constraints that are stated in C (and C++ too I guess)
> standard. I.e. accessing variable of size N must be done only through
> pointer aligned to N. Default struct composition ensures this. If
> forced packing is done because of memory shared with process running in
> different model, padding that ensures the above rule must be done too.
>

As the person who recommended structure packing in the first place, let 
me introduce some historical context here.  For (as we should all 
realise) those who don't learn from the mistakes of the past are 
invariably condemned to repeat them!

Structure packing was first introduced to allow clients and servers to 
be built using different compilers.  The problem was that no two 
compilers could agree on what constituted (for example) 8-byte 
alignment.  Was each structure member aligned at a memory location whose 
address was divisible by 8?  Or did this only apply if the next member 
couldn't be accommodated without overshooting the next boundary?  Or was 
each member aligned at an address that was a multiple of 8 bytes from 
the address of the previous member?  In theory, it shouldn't have made 
any difference but in practice it did - because each compiler was 
interpreting the rules differently.  The only packing scheme on which 
every compiler seemed to agree was 1-byte alignment which was therefore 
adopted as the standard for Jack.  As a result of this, an (msvc-built) 
client, for example, can be used reliably with a (mingw-built) server.  
That simply wasn't possible before struct packing was introduced.  
Padding was unreliable too, for largely the same reasons (padding fields 
were subject to the same misinterpretations!!)

So 1-byte packing became the norm.  Having said that...  it turned out 
that some physical architectures simply wouldn't support 1-byte 
alignment (I think ARM might have been one of them).  And as a result, 
struct packing needs to be disabled when building Jack for that 
architecture.  Nevertheless, 1-byte alignment is accepted by the 
majority of platforms as well as by the majority of popular compilers 
and (crucially) wherever it's accepted, it seems to be the only 
alignment scheme on which they all agree.

Anyone who thinks he can come up with a scheme that EVERYTHING will 
agree about is seriously deluding himself IMHO.  Regardless of any C/C++ 
standards, real-world programs need to actually WORK in the real world.  
This means that if compiler designers have interpreted the standards 
differently (which they evidently have) there's never going to be an 
ideal solution.  Programmers need to adopt the most pragmatic solution - 
and this one's been shown to work best in the majority of cases.

John

1395490172.10597_0.ltw:2,a <532D7D75.9090804 at tiscali dot co dot uk>