Question

What is the expected behaviour for this code snippet?

char * aNullPointer = 0;
snprintf (res, 128, "Testing %s null pointer",aNullPointer);

Note that I am deliberately trying to get it to de-reference my null pointer aNullPointer.

Behaviour 1) res points to a string “Testing (null) null pointer”

Behaviour 2) Seg Fault

It seems I get differing behaviours depending on my platform. Some snprintf implementations perform a sanity check, whereas others do not.

What is the most common behaviour?

Answer 1

It’s undefined behavior – there’s nothing to expect. The fact that some implementations check for NULL and replace it with “nil” or “null” is just a nicety, you can’t rely on it at all.