Re: SPARC tagged data

henry@zoo.toronto.edu (Henry Spencer) writes:
>[It may be difficult to pass structures to C anyway because of
> other design decisions made differently.]


Indeed, it may be difficult to pass structures between C and C if the
fragments were compiled with different compilers (e.g., libraries).
Problems include:


* Structure passing (most notably return) conventions
* Alignment
* Padding


So, for example, a compiler for a machine that allows unaligned fetches
(e.g., the VAX) might implement


struct { char c; int i; }


as:


one byte followed by 4 bytes, non-padded
one byte followed by 4 bytes, padded to 8 bytes
1 byte, padded to 4 bytes, followed by 4 bytes


I think these choices are all legal, but I wouldn't swear to it. The point
is that a compiler may legitimately derive different struct layouts from one
hadware specification. The second one is pretty silly, but the third may
actually improve performance by reducing the number of unaligned fetches.


So even if all the world programmed in C, we still wouldn't have solved
the interoperability problem :-)


;-D on ( Inter-city commuting? Inter-face computing ) Pardo
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