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Re: [PATCH] manual: Remove incorrect claim that qsort() can be stabilized
- From: ams at gnu dot org (Alfred M. Szmidt)
- To: Anders Kaseorg <andersk at MIT dot EDU>
- Cc: libc-alpha at sourceware dot org
- Date: Tue, 01 Jul 2014 04:58:23 -0400
- Subject: Re: [PATCH] manual: Remove incorrect claim that qsort() can be stabilized
- Authentication-results: sourceware.org; auth=none
- References: <alpine dot DEB dot 2 dot 02 dot 1407010413160 dot 28890 at all-night-tool dot MIT dot EDU>
- Reply-to: ams at gnu dot org
-If you want the effect of a stable sort, you can get this result by
-writing the comparison function so that, lacking other reason
-distinguish between two elements, it compares them by their addresses.
-Note that doing this may make the sorting algorithm less efficient, so
-do it only if necessary.
+Previous versions of this manual incorrectly claimed that you can get
+the effect of a stable sort by writing the comparison function to
+compare the element addresses as a last resort. This does not work
+because, under certain conditions, @var{qsort} falls back to an in-place
+algorithm that compares some elements after moving them around in
+memory. The only way to perform a stable sort with @var{qsort} is to
+first augment the objects with a monotonic counter of some kind.
Personally, I think this blurb should be put in the NEWS file not
here. To some extent it is a user-visible change.