[XviD-devel] mem_align.c bug

[pikatu]

On Thursday 12 December 2002 13:12, suxen_drol wrote:
> On Thu, 12 Dec 2002 10:13:17 +0000 pikatu <pikatu@012.net.il> wrote:
> > You still have a bug there since in the case of no alignment the free
> > routine will fail.
>
> for those who want to know:
>
> the memory ptr passed to xvid_free() does NOT point to the address of
> the the "allocated block", instead, it points to an
> "aligned address" WITHIN the allocated block.
>
> to free the allocated block, we must first calculate the "allocated
> address" from the "aligned address". to do this, xvid stores an "offset"
> located one-byte before the "aligned address": eg.
>
> void xvid_free(void * mem_ptr)
> {
> 	uint8_t * p = (uint8_t*) mem_ptr;
> 	p -= p[-1];
> 	free(p);
> }
>
> currently, when xvid_malloc() is called with alignment=0, the offset
> value is set to zero. e.g:
>
> void * xvid_alloc(int num_bytes, int alignment)
> {
> 	if (alignment == 0)
> 	{
> 		uint8_t * mem_ptr = malloc(num_bytes + alignment + 1);
> 		*mem_ptr = 0;
> 		return mem_ptr++;
> 	}else{
> 		/* ... */
> 	}
> }
>
> however when we call xvid_free() the value of  p[-1] is zero, causing
> free() to be called on the "aligned address" and not the "allocated
> address".
>
> the solution to this problem, use:  *mem_ptr = 1;
>
> ed: assuming my above rant is correct, i will let you commit this fix.
>

That was my original suggestion .... 

> > By the way, why not set the default alignment to something more
> > reasonable
> > than 1 ?
>
> one??
> alignment is specified in #define CACHE_LINE, xvidcore/src/portab.h
>

o.k. I meant why not use CACHE_LINE alignment(instead of 1, which is what 
happens in practice for the case of no alignment)
Probably default was not the right word here.

> -- pete; life is like a box of ammo
>
>
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