[RFC] how to get the size of a malloc(9) block ?

[dt71 at gmx.com]

John-Mark Gurney wrote, On 12/01/2013 03:20:
> Either it happens rarely, and always doing a realloc won't hurt
> performance, or it happens often, and then you should be using a larger
> buffer in the first place..

What if a size-elastic implementation of a dynamic data structure would be able to adjust to the malloc implementation, such as agreeing to allocate regions of size (2^k - 8)? Much like the use of getpagesize() (yes, I know it's not part of a technical standard).