Hi
Any unaligned struct members *and* structures themselves on
RISC generally, i guess. Better to ask developers. There is an
alternative -- byte by byte access.
I don't understand. An "alternative" to what? It's what gcc
does on other targets when accessing misaligned packed struct
members.
I noticed some time ago that there is some problem with GCC and pointers
to structures and indexes, its a bug in how gcc handles structures and
should compile to the correct code, but does not. The compiler should
compile what you ask of it, if it does not its a bug. The original post
this seemed to be the case. Give us a test case of code that does not
work and it might be fixable, without that I don't know what is wrong.
This is independent of how data is aligned in memory.
But how efficient it can be? Better to have good _design_
right from the start.
If you don't get to define the layout of the data in memory, I
don't see how you can "design" your way around doing byte by
byte access of misaligned values.
ok,
char array[128] is always in sequence and you have control of how that
is aligned in memory. You put them in in sequence and incrementing a
pointer to them always gets you the characters in sequence.
now,
struct
{
char x;
int y;
char z;
long b;
char v[128];
}
Can be in any sequence the compiler likes to lay it out, even 'packed'
(use minimum memory). Incrementing a pointer to this struct will not
always get you x then y high, y low etc... It depends on the machine,
the compiler, the final alignment in memory etc. C says thats its upto
the compiler how to lay this out in memory. Its non portable, yes a lot
of examples make this mistake, but its not how ANSI C is specified.
If you want to control the layout in memory then use an array. It should
not matter what the layout is in memory for your code to work. If your
using pointers to type cast between variable types (or structs) then you
should be carful, type casting with pointers is dangerous, and the
compiler tries to warn you of this where it can.
My 2C's worth.
--
Peter Jansen
STS
Antarctic Division
203 Channel Highway
Kingston
TAS 7050
AUSTRALIA
Phone +61 3 6232 3533
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