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manual: Correct guarantee about pointers compared by qsort()

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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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Anders Kaseorg 2014-12-11 05:33:22 -05:00 committed by Ondřej Bílka
parent d12455f596
commit 98fe149e34
2 changed files with 13 additions and 6 deletions
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@ -1,3 +1,8 @@
2014-12-21 Anders Kaseorg <andersk@mit.edu>
* manual/search.texi: (Array Sort Function): Clarify stable sorting
guarantees.
2014-12-20 Chris Metcalf <cmetcalf@ezchip.com>
* sysdeps/unix/sysv/linux/tile/localplt.data: New file.

14
manual/search.texi
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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