diff mbox

[v2] manual: Remove incorrect claim that qsort() can be stabilized

Message ID alpine.DEB.2.02.1407022115210.28890@all-night-tool.MIT.EDU
State Committed
Headers show

Commit Message

Anders Kaseorg July 3, 2014, 1:17 a.m. UTC
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>
---

On Tue, 1 Jul 2014, Florian Weimer wrote:
> 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.

What do you think about this wording, then?  (The problem is not actually 
about pointers residing outside the input array, because it happens that 
the algorithm where that occurs is stable.  But I agree it’s worth 
mentioning too.)

Anders

 manual/search.texi | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)
diff mbox

Patch

diff --git a/manual/search.texi b/manual/search.texi
index 509a543..8aff574 100644
--- a/manual/search.texi
+++ b/manual/search.texi
@@ -180,11 +180,10 @@  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.
+The addresses passed to the comparison function need not correspond with
+the original location of the objects, and need not even lie within the
+original array.  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