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authorAnders Kaseorg <andersk@mit.edu>2014-12-11 05:33:22 -0500
committerOndřej Bílka <neleai@seznam.cz>2014-12-21 14:11:32 +0100
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manual: Correct guarantee about pointers compared by qsort()
C99, C11, POSIX, and the glibc implementation do guarantee that the pointers passed to the qsort comparison function lie within the array. Signed-off-by: Anders Kaseorg <andersk@mit.edu>
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@@ -164,8 +164,8 @@ To sort an array using an arbitrary comparison function, use the
@comment ISO
@deftypefun void qsort (void *@var{array}, size_t @var{count}, size_t @var{size}, comparison_fn_t @var{compare})
@safety{@prelim{}@mtsafe{}@assafe{}@acunsafe{@acucorrupt{}}}
-The @var{qsort} function sorts the array @var{array}. The array contains
-@var{count} elements, each of which is of size @var{size}.
+The @code{qsort} function sorts the array @var{array}. The array
+contains @var{count} elements, each of which is of size @var{size}.
The @var{compare} function is used to perform the comparison on the
array elements. This function is called with two pointer arguments and
@@ -180,10 +180,12 @@ 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.
-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.
+Although the object addresses passed to the comparison function lie
+within the array, they need not correspond with the original locations
+of those objects because the sorting algorithm may swap around objects
+in the array before making some comparisons. The only way to perform
+a stable sort with @code{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