Re: Arrays and Alignment

[Andreas Schwab <schwab at suse dot de>]

"Michael N. Moran" <mnmoran@bellsouth.net> writes:

> I am declaring an array of an aligned type. However
> it would appear that when used in an array, the alignment
> is not maintained as I would expect. Here's a test-case.
>
> #include <stdio.h>
>
> typedef unsigned char achar __attribute__ ((__aligned__(32)));
>
> achar           a[2];
>
> int main(int argc,char* argv[]){
>     printf("__alignof__(achar):%u\n",__alignof__(achar));
>     printf("__alignof__(a[1]):%u\n",__alignof__(a[1]));
>     printf("&a: %8.8lX\n",(unsigned long)&a);
>     printf("sizeof(a):%u\n",sizeof(a));
>     printf("sizeof(a[0]):%u\n",sizeof(a[0]));
>     printf("&a[0]: %8.8lX\n",(unsigned long)&a[0]);
>     printf("&a[1]: %8.8lX\n",(unsigned long)&a[1]);
>     return 0;
>     }
>
> % gcc -o main main.c ; main
>
> __alignof__(achar):32
> __alignof__(a[1]):1
> &a: 08049740
> sizeof(a):32
> sizeof(a[0]):1
> &a[0]: 08049740
> &a[1]: 08049741         <<<<<<<<<<<< I expected this to be 08049760

You can't have both.  That would require to put padding between array
elements, which is not allowed (unlike structure members).

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
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