Nothing in this article is wrong. What adds to your knowledge of C is that certain operations on C pointers that would make sense if they were just bit pattern are actually undefined behavior. Undefined behavior may lead to subtle bugs due to how compilers do optimizations.
> I don't see how [. . .] is undefined behaviour.
This is a perfectly meaningless sentence. "Undefined behavior" doesn't mean "things which don't make sense in a particular implementation", it literally means behavior not defined by the spec. The spec declines to define the meaning of p==q when p and/or q is a freed pointer. There's nothing to "see" here: it's just not defined.
#include <stdio.h>
int main(void)
{
int x, y;
int *p = &x + 1;
int *q = &y;
printf("%p %p %d\n", p, q, p == q);
return 0;
}
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