Proc structure in 4.3

[Dave Grubbs]

A couple weeks ago, I found this in unix-wizards and couldn't believe it, so
I saved it for the day I would have time to check it out:

> Path: ci-dandelion!talcott! ... !brl-smoke!smoke!ron at BRL.ARPA
> From: ron at BRL.ARPA
> Newsgroups: net.unix-wizards
> Subject: brl-vgr Bug Report
> Date: 13 Oct 86 23:03:55 GMT
> 
> Subject:  Warning to those who would change proc.h
> Index:	sys/h/proc.h 4.3BSD
> 
> Description:
> 	Changing the length of the proc structure so that it is not
> 	double word aligned anymore will cause the system to crash
> 	in bizarre ways.
> Repeat-By:
> 	Add a word to the proc structure and then put a load on the
> 	system.  Soon things like ps will stop working and then the
> 	whole machine will experience a strange trap.
> Fix:
> 	1.  Don't change the proc structure.
> 	2.  If you do, pad it out to the next double word.
> 


I still don't believe it.  I worked with System V for a year as a developer, I
worked on Project Athena for two years and changed all sorts of things without
showing this sort of problem.  I now work on Ultrix source code, which has
been hacked even more, by DEC and by me and it works perfectly.  The reason is
pretty obvious.  The C compiler on all the SysV, 4.2, 4.3 and Ultrix systems I
have worked on have all aligned every field in a structure to their own size.
(i.e. longs on double word, words on word boundaries)

The proc structure starts with a (struct proc *), which means that all the
array elements (each of which is a structure) start on long word boundaries,
no matter what the structure ends with or what size the last element is.

Run the following program:  Note the results.

struct a {
  struct a *ap;
  char ac;
};

struct b {
  struct b *bp;
  short bc;
};

struct c {
  struct c *cp;
  long cc;
};

main()
{
    struct a aa[10];
    struct b bb[10];
    struct c cc[10];

    printf ("A: Element size(%d) array size(%d)\n", sizeof(aa[0]), sizeof(aa));
    printf ("B: Element size(%d) array size(%d)\n", sizeof(bb[0]), sizeof(bb));
    printf ("C: Element size(%d) array size(%d)\n", sizeof(cc[0]), sizeof(cc));
}

Output:
A: Element size(8) array size(80)
B: Element size(8) array size(80)
C: Element size(8) array size(80)

Something else must be wrong with whatever you did to the proc structure.