manual: Remove incorrect claim that qsort() can be stabilized
Commit Message
Under certain conditions on the size of the array and its items,
qsort() may fall back to an in-place quicksort if it cannot allocate
memory for a temporary array with malloc(). This algorithm is not a
stable sort even if the comparison function is written in the
described manner.
Fixes #10672.
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
---
manual/search.texi | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)
Comments
On 07/01/2014 10:13 AM, Anders Kaseorg wrote:
> Under certain conditions on the size of the array and its items,
> qsort() may fall back to an in-place quicksort if it cannot allocate
> memory for a temporary array with malloc(). This algorithm is not a
> stable sort even if the comparison function is written in the
> described manner.
> -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.
I would suggest to remove the entire paragraph instead, or perhaps a
warning that the pointers with which the comparison function is called
do not necessarily have to reside within the input array.
-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.
On Tue, 1 Jul 2014, Alfred M. Szmidt wrote:
> -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.
You understand that this is not a behavior change, right? qsort has
always behaved this way; this paragraph has never been correct. We could
just remove it (let me know if you want me to send such a patch), but I
thought it might be worthwhile to say something to discourage any further
propagation of whatever misconception led to its addition in the first
place.
Anders
@@ -180,11 +180,13 @@ This can make a difference when the comparison considers only part of
the elements. Two elements with the same sort key may differ in other
respects.
-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.
Here is a simple example of sorting an array of doubles in numerical
order, using the comparison function defined above (@pxref{Comparison