Function malloc_usable_size

freebsd/kern/kern_malloc.c:1037–1068

* malloc_usable_size: returns the usable size of the allocation. */

 *	malloc_usable_size: returns the usable size of the allocation.
 */
size_t
malloc_usable_size(const void *addr)
{
#ifndef DEBUG_REDZONE
	uma_zone_t zone;
	uma_slab_t slab;
#endif
	u_long size;

	if (addr == NULL)
		return (0);

#ifdef DEBUG_MEMGUARD
	if (is_memguard_addr(__DECONST(void *, addr)))
		return (memguard_get_req_size(addr));
#endif

#ifdef DEBUG_REDZONE
	size = redzone_get_size(__DECONST(void *, addr));
#else
	vtozoneslab((vm_offset_t)addr & (~UMA_SLAB_MASK), &zone, &slab);
	if (slab == NULL)
		panic("malloc_usable_size: address %p(%p) is not allocated.\n",
		    addr, (void *)((u_long)addr & (~UMA_SLAB_MASK)));

	if (!malloc_large_slab(slab))
		size = zone->uz_size;
	else
		size = malloc_large_size(slab);
#endif
	return (size);
}

CTASSERT(VM_KMEM_SIZE_SCALE >= 1);