libs have it, it is a function that is considered harmful, so:
Delete malloc_usable_size(). It is a function that blurs the line
between malloc managed memory and application managed memory and
exposes some of the internal workings of malloc.  If an application
relies on that, it is likely to break using another implementation
of malloc.  If you want usable size x, just allocate x bytes. ok
deraadt@ and other devs