Re: Arrays and Alignment

["Michael N. Moran" <mnmoran at bellsouth dot net>]

Andreas Schwab wrote:

> "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,

Thanks for the instantaneous reply :-)

I must confess, however, that I don't understand what you mean
by "both". Do you mean (1) array alignment and (2) array element
alignment? In this case it would seem that the type "achar"
requires an alignment of 32bytes. Why would that not extend
to the elements of the array?

mike