Hacker News Reader

Comment by Spiritus | Hacker News Reader

>But in most cases, you can encapsulate the low-level hardware inside a safe API

I'm probably missing something obvious. But isn't that true for most languages?

In C and C++ you have to treat everything as unsafe, because you have neither a GC nor a compiler to help tell you when you have accidentally violated some memory management invariant that some API was depending on. Rust's type system gives you the tools to define those safe APIs and have them checked by the compiler, even if you need to do some unsafe shenanigans under the hood.

Not really. I mean let's say you program in C, how will you enforce some pointer is never null? In Rust you can say &Object and that reference is never null (modulo any unsafe shenanigans).

Check it when you use it?

Experience tells us that this idea doesn't work so well in practice without a compiler yelling if you don't do it.

Compiler doesn't yell, because it's unnecessary in most cases.

My experience is that huge amount of C code running on my computer had exactly zero issues like this today. It has been working perfectly fine.

I run GNOME which uses a paradigm of checking all inputs to a function on entry via g_return(_val)_if_fail(assertion_expression). It helps the programmer do the right thing when it comes to using APIs that are disallowing NULL input.

Two days ago, the code on my workstation hit one of those "assertions":

Sep 26 23:40:31 core transmission-gt[1084]: g_file_test: assertion 'filename != NULL' failed

No oops in the kernel recently.

Millions of lines of desktop and kernel code, and one failed assertion in two days for using NULL incorrectly in the API.

So unless your standard is absolute perfection, it works fine as is.

thankfully C++ has references.

References can still be null.

In C++, null references are Very Bad, and they trigger undefined behavior: https://stackoverflow.com/questions/4364536/is-null-referenc...

I don't think I've ever run into a null reference in the real world. I'm sure it happens, especially if people write "&*some_function_that_might_return_null()". But it shouldn't be a normal thing. There are lots of other issues with C++, but this has never been a major one in my experience.

> In C++, null references are Very Bad,

They are!

And while they are not a normal thing, they are a thing and I've run in to them a handful of times in the real world, almost always the result of someone not checking for null before dereferencing a pointer.

Rust does not really have this issue.

Rust also has this issue, unless you fully validate every single pointer coming out of unsafe blocks.

The difference being that all c++ code is 'unsafe' in the rust sense, whereas a typical rust program will have only a small portion of unsafe code (or none), making it easier to fully validate - hence 'doesn't really have this problem'.

I agree, but it depends how seriously unsafe blocks get reviewed.

Because I can assure you, unsafe blocks in enterprise Rust will be reviewed as much as C and C++ code currently are in most Big Corps™.

We do have such problems with native libraries killing Java and .NET processes, with unsafe being the FFI boundary.

C++ references aren't exactly safe either.

    std::vector<int> x;
    x.push_back(4);
    int& val = x.back();
    x.push_back(5); // ahh, val may be garbage now.

    Foo f;
    Foo& fRef = f;
    Foo g = std::move(f);

Null is not really the most pertinent example in this context - at least you get a segfault. What is more important is that in C or C++ (even C++17), it is trivially easy to produce buffer overruns, use after frees, dangling pointers, invalidated iterators, data races etc. That is the unsafety that we are talking about here. Opt-in nullability via Option<T> is nice to have though.

Yeah, I went with familiarity/simplicity in that example.

My point was similar C/C++ don't have a safe subset.

Not a safe subset anyone would want to use at least. You could just not use pointers in your code and you'd have memory safety.

Your code, or any of the code it calls; iterator invalidation, for example, wouldn't force you to use pointers directly, but can still cause memory unsafety.

> at least you get a segfault

If the platform has an MMU, if you've properly set up your page table mappings, etc. We are talking about writing an OS here!

Not to mention all the UB around null pointers that can cause miscompilation. Well, not technically miscompilation, but stuff like https://news.ycombinator.com/item?id=15324414

> at least you get a segfault.

If there were a list "falsehoods software engineers believe about memory safety" this should be there.

https://cansecwest.com/slides07/Vector-Rewrite-Attack.pdf

Reply on news.ycombinator.com