Re: qsort_r

[Alan Barrett <apb%cequrux.com@localhost>]

On Sun, 08 Dec 2013, David Holland wrote:
> My irritation with not being able to pass a data pointer through 
> qsort() boiled over just now. Apparently Linux and/or GNU 
> has a qsort_r() that supports this; so, following is a patch 
> that gives us a compatible qsort_r() plus mergesort_r(), and 
> heapsort_r().

Apparently FreeBSD [1] and GNU [2] have incompatible versions 
of qsort_r, passing the extra 'thunk' or 'data' argument in a 
different position.

[1]: FreeBSD qsort_r <http://www.manpagez.com/man/3/qsort_r/>
[2]: Linux qsort_r  <http://man7.org/linux/man-pages/man3/qsort.3.html>

If we have to pick one, let's pick the FreeBSD version.

> I have done it by having the original, non-_r functions 
> provide a thunk for the comparison function, as this is least 
> invasive. If we think this is too expensive, an alternative is 
> generating a union of function pointers and making tests at the 
> call sites; another option is to duplicate the code (hopefully 
> with cpp rather than C&P) but that seems like a bad plan.

I'd probably duplicate the code via CPP, to trade time for space, 
but your way is fine.

> Note that the thunks use an extra struct to hold the function 
> pointer; this is to satisfy C standards pedantry about void 
> pointers vs. function pointers, and if we decide not to care it 
> could be simplified.

That adds more run-time overhead.  Could you make it conditional 
on whether it's really necessary?  All existing NetBSD platforms 
can convert back and forth between void * and function pointers 
without any trouble.

--apb (Alan Barrett)