A lot of compilers will optimize out a NULL pointer check because dereferencing a NULL pointer is UB.

Because assert will not run the following code in the case of a NULL pointer, AFAIK this exact code is still defined behavior, but if for some reason some code dereferenced the NULL pointer before, it would be optimized out - there are some corner cases that aren't obvious on the surface.

This kind of thing was always theoretically allowed, but really started to become insidious within the past 5-10 years. It's probably one of the more surprising UB things that bites people in the field.

GCC has a flag "-fno-delete-null-pointer-checks" to specifically turn off this behavior.

https://qinsb.blogspot.com/2018/03/ub-will-delete-your-null-...

This is an actual Linux kernel exploit caused by this behavior where the compiler optimized out code that checked for a NULL pointer and returned an error.

https://lwn.net/Articles/342330/

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IshKebab a day ago | parent [-]

Sure, but none of that is relevant to just the code snippet that was posted. The compiler can exploit UB in other code to do weird things, but that's just C being C. There's nothing unexpected in the snippet posted.

The issue is cause by C declaring that dereferencing a null pointer is UB. It's not really an issue with assertions.

You can get the same optimisation-removes-code for any UB.

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maccard a day ago | parent [-]

> There's nothing unexpected in the snippet posted.

> The issue is cause by C declaring that dereferencing a null pointer is UB. It's not really an issue with assertions. > You can get the same optimisation-removes-code for any UB.

I disagree - It’s a 4 line toy example but in a 30-40 line function these things are not always clear. The actual problem is if you compile with NDEBUG=1, the nullptr check is removed and the optimiser can (and will, currently) do unexpected things.

The printf sample above is a good example of the side effects.

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IshKebab a day ago | parent [-]

> The actual problem is if you compile with NDEBUG=1

That is entirely expected by any C programmer. Sure they named things wrong - it should have been something like `assert` (always enabled) and `debug_assert` (controlled by NDEBUG), as Rust did. And I have actually done that in my C++ code before.

But I don't think the mere fact that assertions can be disabled was the issue that was being alluded to.

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maccard 20 hours ago | parent [-]

I wrote the comment, assertions being disabled was exactly what was being alluded to.

> that is entirely expected by any C programmer

That’s great. Every C programmer also knows to avoid all the footguns and nasties - yet we still have issues like this come up all the time. I’ve worked as a C++ programmer for 12 years and I’d say it’s probably 50/50 in practice how many people would spot that in a code review.

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IshKebab 17 hours ago | parent [-]

It's definitely a footgun, but the compiler isn't doing weird stuff because the assertions can be disabled. It's doing weird stuff because there's UB all over the place and it expects programmers to magically not make any mistakes. Completely orthogonal to this particular (fairly minor IMO) footgun.

> I’ve worked as a C++ programmer for 12 years and I’d say it’s probably 50/50 in practice how many people would spot that in a code review.

Spot what? There's absolutely nothing wrong with the code you posted.

	▲	jmalicki 15 hours ago \| parent [-]
		That assert could completely fail to fire if inlined into another function that did a dereference first.