On Tue, Jul 13, 2004 at 17:17:41 +0530, amith wrote:
> hi all,
>
> main()
> {
> void **blkPtr;
> char *Buffer;
> char *ptr;
> Buffer = (char *) malloc(10000);
>
> if( ((unsigned long)Buffer % 4) == 0)
> printf(" Buffer = %u is divisible by 4 \n",Buffer);
>
> // Buffer++; /* if this line is commented ,the code works as
> expected */
> blkPtr = (void **)Buffer;
>
>
> *blkPtr = ((void *)0x11223344);
> ptr = Buffer;
> printf(" value = %X address = %X\n",*ptr,ptr);
> ptr++;
> printf(" value = %X address = %X\n",*ptr,ptr);
> ptr++;
> printf(" value = %X address = %X\n",*ptr,ptr);
> ptr++;
> printf(" value = %X address = %X\n",*ptr,ptr);
> }
> [...]
> could someone bail me out of this ?
I fear that the code is invalid. It violates the aliasing rules. The
compiler is allowed to expect, that a char * never ever points to the
same location as void **.
It's not said anywhere that something that is valid pointer to character
is also a valid pointer to pointer. It seems to be the case that on ARM
it really isn't, because there is alignment requirement for pointers.
When there is alignment requirement, the compiler makes sure to never
violate it -- and emmits instructions that only work when it's
satisfied. When you deliberately break the rules, you have to keep the
pieces.
-------------------------------------------------------------------------------
Jan 'Bulb' Hudec <bulb@ucw.cz>
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