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Date: Sun, 8 Aug 2021 13:42:27 +0100
From: Jon Chesterfield <jonathanchesterfield@...il.com>
To: musl@...ts.openwall.com
Cc: Olivier Galibert <galibert@...ox.com>
Subject: Re: [PATCH] stdlib: implement qsort_r
On Sun, Aug 8, 2021 at 12:29 PM Olivier Galibert <galibert@...ox.com> wrote:
> The extension qsort_r allows calling qsort on a list of indices
> without having a global variable to hold the data to sort.
>
qsort_r is a strong improvement on qsort. I think it's available outside of
glibc.
I remember doing something similar locally. Just looked it up and I
renamed & mutated qsort to pass the context along. Therefore typed into
email, I think something like this would provide an implementation of
qsort in terms of qsort_r.
// declare qsort_r
typedef int (*cmp_r_t)(const void *, const void *, void * context);
void qsort_r(void *base, size_t nel, size_t width, cmp_r_t cmp, void*
context);
// pass it a function that extracts the comparator for qsort from the
// context
typedef int (*cmp_t)(const void *, const void *);
static int compare_adapter(const void *l, const void *r, void * context)
{
cmpt_t c = (cmpt_t) context;
return c(l,r);
}
// tail call
void qsort(void *base, size_t nel, size_t width, cmp_t c)
{
return qsort_r(base, nel, width, compare_adapter, (cmp_t_t)c);
}
Given optimism about inlining or an always inline annotation
it should turn into the same machine code as the macro
instantiation approach. Alternatively it's a tail call into qsort_r, so
a couple of indirections in exchange for minimal code size growth.
I haven't compiled or tested that (or looked up the coding conventions
for musl) so this is a drive by suggestion, written in pseudocode
instead of prose for clarity.
Thanks all!
Jon
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