I see nothing in the C standard specification of qsort() that says sorting an array with zero members leads to undefined behaviour.

http://port70.net/~nsz/c/c11/n1570.html#7.22.5.2

The POSIX specification for qsort() is quite clear that sorting zero elements is safe.

https://pubs.opengroup.org/onlinepubs/9699919799/functions/qsort.html

The manual pages for Linux and macOS (BSD) do not specify that sorting zero elements leads to undefined behaviour.

Is there a platform where there is a known problem? If so, it would be worth documenting which platform/version explicitly so that the change can be undone in future if the recalcitrant platform(s) change their behaviour.

With all that said, the change is trivial — but could be improved by using the test if (nlinks > 1) so that the code doesn't call qsort for a single-element array either; that is already in sorted order, of course.

On Tue, Oct 25, 2022 at 10:10 PM Paul Eggert via tz <tz@iana.org> wrote:

* zic.c (make_links): Don't call qsort(NULL, 0, ...) as this has
undefined behavior in C. This fixes a bug introduced five days ago.
---
zic.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/zic.c b/zic.c
index 78afa67..a699285 100644
--- a/zic.c
+++ b/zic.c
@@ -683,7 +683,8 @@ static void
make_links(void)
{
ptrdiff_t i, j, nalinks;
- qsort(links, nlinks, sizeof *links, qsort_linkcmp);
+ if (nlinks)
+ qsort(links, nlinks, sizeof *links, qsort_linkcmp);

/* Ignore each link superseded by a later link with the same name. */
j = 0;
--
2.37.3

Jonathan Leffler <jonathan.leffler@gmail.com> #include <disclaimer.h>
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"Blessed are we who can laugh at ourselves, for we shall never cease to be amused."