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Date: Fri, 1 Mar 2002 13:34:01 -0500 (EST)
From: Cliff Woolley <jwoolley@virginia.edu>
To: Aaron Bannert <aaron@clove.org>
cc: dev@httpd.apache.org, <dev@apr.apache.org>
Subject: Re: [PATCH] fix alignment on shared memory
In-Reply-To: <20020301102927.Z15055@clove.org>
Message-ID: 
 <Pine.LNX.4.44.0203011331550.16457-100000@deepthought.cs.virginia.edu>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII

On Fri, 1 Mar 2002, Aaron Bannert wrote:

> Yes, I completely agree that each structure needs to be 64-bit aligned.
> I don't see how the shared memory code itself is incorrect. It must be
> mapping the segment at a properly aligned boundary, so as long as the
> contents of that segment are accessed in an aligned manner than we're
> safe, right?

That's what I'm thinking, too.  I don't see the problem there.  It looks
to me like you (Aaron) were right on when you suggested that it's
ap_calc_scoreboard_size() and ap_init_scoreboard()'s fault.  Try this, for
example:

#include <stddef.h>
#include <stdio.h>

int main(void)
{
    void *foo;
    struct s1 {
        char a;
        char b;
    } *s1;
    struct s2 {
        char a;
        int b;
    } *s2;
    struct s3 {
        char a;
        long long b;
    } *s3;
    struct s4 {
        char a;
        double b;
    } *s4;

    printf("%d %d %d %d\n",
           offsetof(struct s1,b),
           offsetof(struct s2,b),
           offsetof(struct s3,b),
           offsetof(struct s4,b));
    printf("  sizes: %d %d %d %d\n",
           sizeof(struct s1),
           sizeof(struct s2),
           sizeof(struct s3),
           sizeof(struct s4));

    foo = malloc(100);
    s1 = (struct s1 *)foo;
    s2 = (struct s2 *)(((char *)s1)+sizeof(s1)*4);
    s3 = (struct s3 *)(((char *)s2)+sizeof(s2));
    printf("%p\n%p\n%p\n",s3,&s3->a,&s3->b);
    s3->a = 'A';
    s3->b = 10241024LL;
    return 0;
}


jcw5q@cobra:/uf2/jcw5q$ ./test
1 4 8 8
  sizes: 2 8 16 16
20c44
20c44
20c4c
Bus Error


Except for the times 4, this is very similar to what ap_init_scoreboard()
is giving us.  Each of the sizeof()'s needs to be padded out because they
can interfere with one another when you place the structs back-to-back in
memory.

--Cliff


--------------------------------------------------------------
   Cliff Woolley
   cliffwoolley@yahoo.com
   Charlottesville, VA