Re: [PATCH 1/3] compat: add qsort_s()

["René Scharfe" <l.s.r@web.de>]

Am 01.12.2016 um 21:19 schrieb Jeff King:
> On Thu, Dec 01, 2016 at 12:14:42PM -0800, Junio C Hamano wrote:
>
>> Jeff King <peff@peff.net> writes:
>>
>>> To make matters more fun, apparently[1] there are multiple variants of
>>> qsort_r with different argument orders. _And_ apparently Microsoft
>>> defines qsort_s, but it's not quite the same thing. But all of that can
>>> be dealt with by having more specific flags (HAVE_GNU_QSORT_R, etc).

AFAIU it went like this:

// FreeBSD 5.0 (2003)
void qsort_r(void *base, size_t nmemb, size_t size,
	void *context,
	int (*compar)(void *context, const void *x, const void *y));

// Microsoft Visual Studio 2005
void qsort_s(void *base, size_t nmemb, size_t size,
	int (*compar)(void *context, const void *x, const void *y),
	void *context);

// glibc 2.8 (2008)
void qsort_r(void *base, size_t nmemb, size_t size,
	int (*compar)(const void *x, const void *y, void *context),
	void *context);

// C11 Annex K (2011)
errno_t qsort_s(void *base, rsize_t nmemb, rsize_t size,
	int (*compar)(const void *x, const void *y, void *context),
	void *context);

>>> It just seems like we should be able to do a better job of using the
>>> system qsort in many cases.

Sure, platform-specific implementations can be shorter.

>> If we were to go that route, perhaps we shouldn't have HAVE_QSORT_S
>> so that Microsoft folks won't define it by mistake (instead perhaps
>> call it HAVE_ISO_QSORT_S or something).

OK.

>> I like your suggestion in general.  The body of git_qsort_s() on
>> systems without ISO_QSORT_S can do
>>
>>  - GNU qsort_r() without any change in the parameters,
>>
>>  - Microsoft qsort_s() with parameter reordered, or
>>
>>  - Apple/BSD qsort_r() with parameter reordered.
>>
>> and that would cover the major platforms.

Yes.

However, for MSys INTERNAL_QSORT is defined for some reason, so the 
platform's qsort(3) is not used there; I guess the same reason applies 
to qsort_s().  If it doesn't then an implementation may want to convert 
a call to the invalid parameter handler (which may show a dialog 
offering to Retry, Continue or Abort) into a non-zero return value.

>> Eh, wait.  BSD and Microsoft have paramters reordered in the
>> callback comparison function.  I suspect that would not fly very
>> well.
>
> You can hack around it by passing a wrapper callback that flips the
> arguments. Since we have a "void *" data pointer, that would point to a
> struct holding the "real" callback and chaining to the original data
> pointer.
>
> It does incur the cost of an extra level of indirection for each
> comparison, though (not just for each qsort call).

Indeed.  We'd need a perf test to measure that overhead before we could 
determine if that's a problem, though.

> You could do it as zero-cost if you were willing to turn the comparison
> function definition into a macro.

Ugh.  That either requires changing the signature of qsort_s() based on 
the underlying native function as well, or using a void pointer to pass 
the comparison function, no?  Let's not do that, at least not without a 
good reason.

René