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From: sah@batman (Steve Hanson)
Newsgroups: comp.sys.mips
Subject: Re: Unaligned struct access (how to)
Message-ID: <45928@mips.mips.COM>
Date: 14 Feb 91 23:01:55 GMT
References: <5054@lure.latrobe.edu.au>
Sender: news@mips.COM
Reply-To: sah@batman (Steve Hanson)
Organization: MIPS Computer Systems Inc.
Lines: 78
In-reply-to: CCHD@lure.latrobe.edu.au (Huw Davies - La Trobe University Computer Centre)

In article <5054@lure.latrobe.edu.au>, CCHD@lure (Huw Davies - La Trobe University Computer Centre) writes:
>I have a slight problem with the c compiler (on  MIPS 120/5 running RISC/os 
>4.5.1).
>
>The following code doesn't do what I want :-) Please note that this
>is an example of the error extracted from debugging 6K lines of c code
>(It took a while to find....).
>
>$ more test_struct.c
>#include <stdio.h>
>
>struct test
>{
>    short a;
>    int b;
>} c;
>
>main()
>{
>        printf("a: %x\nb: %x\nSize of a: %d\n", &c.a, &c.b, 
>                                                  (int) &c.b - (int) &c.a);
>}
>
>$ ./test
>a: 2003e460
>b: 2003e464
>Size of a: 4
>$ 
>
>I would have liked the difference in address to be 2, not 4. I know
>that there are efficiency reasons for aligned access, but I would like
>to be able to override the default.
>
>For those few of you who read comp.unix.aix as well (there's got to be
>someone else who believes in vendor independence...) I have the
>same problem with our RS/6000 systems. (Different implementation - same
>bugs :-)
>
>Huw Davies
>Computing Services
>
>e-mail: cchd@lucifer.latrobe.edu.au


Taken from the 2.20, in Beta now,  release notes:

o  #pragma pack(n)
   #pragma pack()
        The first form is used to change the strictest alignment of
        structure members within a structure. n is the new alignment
        restriction in bytes. If n is omitted, then it defaults to the
        compiler option -Zp, if present, or the compiler default,
        which is the strictest member alignment.



Your code would look like:

#include <stdio.h>
#pragma pack(2)
struct test
{
    short a;
    int b;
} c;



main()
{
        printf("a: %x\nb: %x\nSize of a: %d\n", &c.a, &c.b, 
                                                  (int) &c.b - (int) &c.a);
}

with the ouput:
a: 10000b7c
b: 10000b7e
Size of a: 2