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Seg faults are "safe" in the programming language sense:

A program fragment is safe if it does not cause untrapped errors to occur. Languages where all program fragments are safe are called safe languages. Therefore, safe languages rule out the most insidious form of execution errors: the ones that may go unnoticed.

...

It is useful to distinguish between two kinds of execution errors: the ones that cause the computation to stop immediately, and the ones that go unnoticed (for a while) and later cause arbitrary behavior. The former are called trapped errors, whereas the latter are untrapped errors.

Type Systems, Luca Cardelli

https://scholar.google.com/scholar?cluster=90442457768317510...

And practically speaking seg faults are easy to debug and fix.

I see a lot of abuse of the terms "safe" and "safe language" lately.

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The problem is that segfaults are frequently the latter type of problem. You do some unsafe thing and corrupt your state, but keep on going without obvious issue. Then at some point in the future, your corrupted state causes an otherwise bug-free portion of you program to segfault.

The problem isn't the segfault -- it's the earlier unsafe behavior. That is, the untrapped error that eventually caused the segfault.

If the language only has trapped errors, which are indicated by seg faults, it's a safe language.

Every language has such errors. What does divide by zero do?

What does blowing the stack / infinite recursion do in Rust? It seg faults.

The seg fault is the safe behavior. If the stack overflow overwrote heap data structures or global data structures and the program kept running, that would be unsafe.

> What does divide by zero do?

Returns a value, of course! (And that said, JavaScript does have some trapped errors, such as (1/0).foo.foo (and yes you need the second .foo…))

IMO, the execution "error" here (in this thread) is accessing memory illegally. Sometimes the runtime traps it, but sometimes it does not, and "sometimes" isn't always, so its effectively untrapped as we cannot depend on the trap. (Especially in adversarial circumstances.)

And further, the original quote talks about languages — the behavior of a language like C is that memory access is not necessarily trapped; the behavior is not well defined. Given the lack of a requirement in the C language for a trap, I think it is fair to call C "unsafe" given the above definition of safe/unsafe.

> What does blowing the stack / infinite recursion do in Rust? It seg faults.

Somewhat interestingly, it detects it and SIGABRTs, which technically isn't a segfault. And that's now some black magic that I'm curious about as I really thought it would have segfaulted.

> such as (1/0).foo.foo (and yes you need the second .foo…)

I don't understand the nuance here. In my Firefox developer tools, I can do the following:

> (1/0).foo.foo

---> TypeError: (intermediate value).foo is undefined

> (1/0).foo

---> undefined

> (1/1).foo.foo

---> TypeError: 1.foo is undefined

> Infinity.foo.foo

---> TypeError: Infinity.foo is undefined

> undefined.foo

---> TypeError: undefined has no properties

While these are all different errors, I don't really understand why the '(1/0).foo.foo' case is an example of a _trapped_ error, but the others are not.

The point they were making is that (1/0).foo is not an error, as it returns a value.

None of those are trapped errors.

In Python, Java, C, OCaml, and most other languages, 1 / 0 aborts the program. That is, division by zero is a trapped error.

In JavaScript, it's not. It keeps going and lets you do stuff like access nonexistent properties .foo on the result, which are also untrapped errors.

So JavaScript is unsafe in Cardelli's terminology. It gives you untrapped errors rather than trapped ones. The program keeps chugging along until you find out later and have to trace backwards to the bug.

It's Infinity in Haskell

    Prelude> 1.0 / 0.0 :: Double
    Infinity

Because / is floating point division, and infinity is a defined value in floating point numbers.

Try it again with div for integer division.

Thanks, removed.

I’m sort of surprised by that since my memory is that infinity isn’t a real number, rational, etc. That is, does infinity in Haskell obey some algebraic laws?

Infinity is a valid floating point number in the ISO standard.

However integer division by 0 isn’t. div 1 0 will fail.

Yeah it's an interesting case. It appears that Inf is in floating point to AVOID a trapped error.

This answer has an interesting way of looking at it. If you go on the theory that floating points are supposed to represent reals, then in floating point, you can't tell if a value is actually zero or just indistinguishably close to zero.

In the case of "indistinguishably close to zero", you're getting the wrong answer, and the program doesn't halt. It keeps on chugging doing bad math. So that's an untrapped error, and it's UNSAFE by Cardelli's definition.

https://cs.stackexchange.com/questions/82811/why-do-floating...

The key point is that "safe" sometimes means "crashes" and sometimes means "doesn't crash". It's an auto-antonym in that sense.

A broader definition is "errors are flagged as early as possible", including with seg faults / hardware exceptions.

Floats aren’t reals, at best they are an approximation for certain calculations. +0 and -0 are defined and distinct floating point numbers. Floating point math is known to be problematic and does not evenly distribute numbers on the real line either. There are numerical methods used for reducing error on floating point operations when it is acceptable. Otherwise fixed point or intervals may be used.

This is false. Integer division by zero is undefined, but floating point division is perfectly fine. Many languages have different operators to distinguish floating point and integer division, like pythons / for floats and // for integers.

> Somewhat interestingly, it detects it and SIGABRTs, which technically isn't a segfault. And that's now some black magic that I'm curious about as I really thought it would have segfaulted.

It's a signal handler, of course: https://github.com/rust-lang/rust/blob/d8bdb3fdcbd88eb16e1a6...

> I think it is fair to call C "unsafe" given the above definition of safe/unsafe.

I don't think anyone would disagree with this. What they're saying is that a segfault is safe and that's because a segfault is essentially your OS's version of an out-of-bounds error, one of the reasons that C is not safe is because an out-of-bounds access will not necessarily cause a segfault.

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