word alignment for structures in UNIX...

[Michael GALLOP]

In article <4046 at quiche.cs.mcgill.ca> adr at quiche.cs.mcgill.ca (Alain DURAND) writes:
>Hello,
>
>       I've troubles with word alignments on different unix machine:
>sizeof(struct foo) doesn't return the same thing on a sun3, sun386i
>and sparc. Is there an option to set at compilation time or somewere
>in the kernel to fix that?
>thanks for any answer, please reply by E-Mail at:
>
>adr at quiche.cs.mcgill.ca
>


Please note: The poster is only a grad student at Mcgill and does not reflect
 the undergrad education :-)

Well Alain. There are several different factors. First off when dealing with
any two machines, your request will likely return different values.

One of the reasons-perhaps the main one- is the different architectures in
the CPUs of the machines. From what I remember 80X86 swaps the bytes around
from the way the SPARC deals with them


More importantly is the difference in word size. An 80386 is a 32 bit machine
-in some modes. In fact all of these are 32 bit machines. However the 
register addressing is different on each one. The Sparc uses circular
addressing, whereas the i386 uses a form of virtual addressing.

Also, an important factor is the overhead of your C compiler, your OS.
These _MIGHT_ (probably won't) affect the result returned....
This, coupled with the different ways that each OS allocates memory
will result in your observations.

Why, though are you asking ? You can't magic number a (Not can't- shouldn't)
malloc. So you should if ((malloc (sizeof (foo))==NULL) or whatever is needed.

As for patching it in the kernel, I couldn't possibly see why, or how, you'd 
do that. By hard wiring a value like that in the kernel, you cause havoc in 
the rest of the OS. Especially in a 386. If you were to do that on a 386 in 
386 protected mode, or, worse in virtual 8086 mode, the page frame would 
cause a memory fault- I should know, I spent many hours chasing my mistake
on that one I tried to align with a magic number :-( -and dump the machine.


-- 
| mikey at quiche.cs.mcgill.ca |  Mike Gallop     				   |
|"Stealing from one author is plagarism....Stealing from many is research" |
I shall walk through the valley of Death and I shall fear no evil.......
..Except, perhaps, a sadistics assignment