Comment by AndyKelley 19 hours ago
If you wanted to match Ghostty's performance in Rust, you'd need to use unsafe in order to use these memory mapping APIs, then you'd be in the exact same boat. Actually you'd be in a worse boat because Zig is safer than unsafe Rust.
Thank you for the kind words.
> every line of C/Zig is unsafe
This is trivially false... for instance here's a line:
const pi = 3.14;
Also IMO the word "safety" should include integer overflow. I don't agree that those kind of bugs are so unimportant as to not be checked in safe builds.
> Thank you for the kind words.
Absolutely, I meant them.
> This is trivially false... for instance here's a line:
Yep, that was really wrongly stated on my part -- what I meant is that the kind of protections that "safe" Rust provides are not available anywhere in average lines of Zig code (though they can be detected with tooling, etc).
What I should have written is that I could easily write unsafe code anywhere in Zig (as in C). In practice of course most people don't because they're not trying to destroy their own computers, and most code is benign. Rust will at least save me from myself some of the time.
> Also IMO the word "safety" should include integer overflow. I don't agree that those kind of bugs are so unimportant as to not be checked in safe builds.
Rust does do some work to catch trivial overflows, but you're right that it does not catch any slightly more complex overflows, and that is certainly unsafe in a sense. I don't think any reasonable person would disagree with that.
Rust's answer to this of course is checked_{op}/wrapping_{op}/etc options, and that's what I often see in high quality codebases where it matters. Of course, this is a footgun that could have had a safety applied and it's too late now (AFAIK) to change the default to be always wrapping or something (also, I think people may oppose always checked for perf reasons).
[EDIT] Just to compare/make this more concrete, playgrounds:
https://zig.fly.dev/p/LGnrBGXPlVJ
https://play.rust-lang.org/?version=stable&mode=release&edit...
Rust in this case of doing something obviously wrong is at least a little more helpful -- the obvious overflow does not compile.
And of course you can get rust to do it like it allows (and what would be present in any codebase with real complexity):
https://play.rust-lang.org/?version=stable&mode=release&edit...
It's just that little bit of safety that makes it easy for me (personally) to default to Rust. Very possible that someday that won't be true.
[EDIT2] Also, somewhat under-discussed, but if Zig supported a bolt-on a "safety check compile mode" that ran with some stricter (maybe not quite borrow checking level) semantics, that would be pretty dope. Of course not something anyone should devote any real time to for a long time (or ever?) BUT it would trivialize a lot of these discussions maybe.
But in the mean time people just using what they're comfortable with/the feel they want is obviously fine.
I don't buy your theory that a Rust terminal would need to directly use mmap to deliver matching performance. In fact I doubt Ghostty's author would endorse this claim either, they've never tried any alternatives, they tried this and it works for their purpose which is a long way from other ways wouldn't work or would all be slower or whatever.
Please don’t get defensive and spread silly FUD. You can be proud of what you’ve accomplished without feeling sad that a different language has strengths that yours doesn’t.
Calling unsafe mmap APIs not only is unlikely to run into the corner cases where unsafe Rust is tricky to get right, there’s “millions” of crates that offer safe APIs to do so and it’s fundamentally not hard to write it safely (it would be very hard to write it to have any issues).
And fundamentally I think Rust is much more likely to be easier to get high performance because the vast majority of safe code you write is amenable to the compiler performing safe optimizations that Zig just can’t do regarding pointer aliasing (or if it does brings all the risks of of unsafe Rust when the user annotates something incorrectly).
I don't think this is silly FUD. The article describes a scenario where the low-level abstractions itself was buggy in a subtle way, the comparison to "unsafe" Rust seems entirely fair to me. (edited for typos)
With Rust you always could unsafely do whatever went wrong in somebody's C or Zig or whatever, but the question is whether you would. Rust's technical design reinforces a culture where the answer is usually "No".
I don't find the claim that weird low level mmap tricks here are perf critical at all persuasive. The page recycling makes sense - I can see why that's helping performance, but the bare metal mmap calls smell to me like somebody wanted to learn about mmap and this was their excuse. Which is fine - I need to be clear about that - but it's not actually crucial to end users being happy with this software.
I think we can agree that Mitchell knows what he’s doing and isn’t playing around with mmap just because. It’s probably quite important to ensure a low memory footprint. But mmap in rust is not extra risky in some weird mystical way. It’s just a normal FFI function to get a pointer back and you can trivially build safe abstractions around it to ensure the lifetime of a slice doesn’t exceed the lifetime of the underlying map. It’s rust 101 and there’s nothing weird here that can cause the unsafe bits here to be extra dangerous (in general unsafe rust can be difficult to get right with certain constructs, but it doesn’t apply here).
I actually don't think I agree about mmap. Reading around it seems as though Mitchell had clever ideas for abusing mmap and those didn't work out. Now he's got mmap for the pages and it works so why replace it, but that does not mean you need mmap to deliver this performance and in fact I'd be extremely surprised if that were true as somebody who spent about a decade of his life mostly writing close-to-metal database code in C using mmap...
If you want a whole lot of bytes and you ask your allocator, do you know what almost any popular general purpose allocator will do on a vaguely decent modern Unix? Call mmap to get them for you. So at most you're cutting out a few CPU instructions worth of middle man.
Also in C or Zig you do not need to create your own memory management using mmap. Whether this is necessary in this case or not is a different question.
In the end, if the Rust advantage is that "Rust's technical design reinforces a culture" where one tries to avoid this, then this is a rather weak argument. We will see how this turns out in the long run though.
The low level abstraction was buggy because they forgot to free memory because they confused types, not because of mmap.
Thats completely orthogonal to the question and less likely in Rust because you would generally use an enum with Drop implemented for the interior of the variants to guarantee correct release.
And mmap is no more difficult to call in Rust nor more magically unsafe - that’s the FUD. The vast majority of Ghostty wouldn’t even need unsafe meaning the vast majority of code gets optimized more due to no aliasing being automatic everywhere and why the argument that “zig is safer than unsafe rust” is disingenuous about performance or safety of the overall program.
> If you wanted to match Ghostty's performance in Rust, you'd need to use unsafe in order to use these memory mapping APIs, then you'd be in the exact same boat.
Yea, but not for all the parts — being able to isolate the unsafe and build abstractions that ensure certain usage parts of the unsafe stuff is a key part of high quality rust code that uses unsafe.
In this case though I think the emphasis is on the fact that there is a place where that code should have been in Rust land, and writing that function would have made it clear and likely avoided the confusion.
Less about unsafe and more about the resulting structure of code.
> Actually you'd be in a worse boat because Zig is safer than unsafe Rust
Other people have mentioned it but I disagree with this assertion.
Its a bit simplistic but I view it this way — every line of C/Zig is unsafe (lots of quibbling to do about what “unsafe” means of course) while some lines of rust are unsafe. Really hard for that assertion to make sense under that world view.
That said, I’m not gonna miss this chance to thank you and the Zig foundation and ecosystem for creating and continuously improving Zig! Thanks for all the hard work and thoughtful API design that has sparked conversation and progress.