On 01/10/2016 19:19, René Scharfe wrote:
It's hard to imagine an implementation of qsort(3) that can't handle
zero elements. QSORT's safety feature is that it prevents the compiler
from removing NULL checks for the array pointer. E.g. the last two
lines in the following example could be optimized away:
qsort(ptr, n, sizeof(*ptr), fn);
if (!ptr)
do_stuff();
You can see that on https://godbolt.org/g/JwS99b -- an awesome website
for exploring compilation results for small snippets, by the way.
Ah, second attempt. Originally misread the original code, and didn't
understand what it was doing.
I get it now.
A nasty trap I hadn't been aware of - I was under the impression NULL +
zero length was generally legal, but the C standard does indeed not give
you a specific out for NULL to library functions in that case.
As such, NULL checks can still be elided even with your change. If you
effectively change your example to:
if (nmemb > 1)
qsort(array, nmemb, size, cmp);
if (!array)
printf("array is NULL\n");
array may only be checked for NULL if nmemb <= 1. You can see GCC doing
that in the compiler explorer - it effectively turns that into "else
if". To make that check really work, you have to do:
if (array)
qsort(array, nmemb, size, cmp);
else
printf("array is NULL\n");
So maybe your "sane_qsort" should be checking array, not nmemb.
Kevin