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0 pointswongarsu1y ago0 comments

Which can be an issue if you want to initialize a 2MB large heap-allocated object (e.g. heap-allocating a large nested struct or a big array).

Without guaranteed “placement new” that can mean that your 2MB object gets constructed on the stack and copied to the heap. And while Linux defaults to a 4MB stack, Windows defaults to 1MB and will crash your program. Or it might work if the compiler optimizes in your favor.

It's not something you encounter frequently, it can be worked around, and Rust will eventually solve it ergonomically without introducing constructor hell (probably with just a keyword). But finding the best language-level solution isn't straightforward (efforts to fix this for rust are ongoing for 9 years)

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godshatter1y ago

>Which can be an issue if you want to initialize a 2MB large heap-allocated object (e.g. heap-allocating a large nested struct or a big array).

>Without guaranteed “placement new” that can mean that your 2MB object gets constructed on the stack and copied to the heap. And while Linux defaults to a 4MB stack, Windows defaults to 1MB and will crash your program. Or it might work if the compiler optimizes in your favor.

C gets a lot of hate, often for good reasons, but at least you know where your memory is coming from when you are allocating it yourself. If you're allocating a large heap-allocated object, you're grabbing the memory directly from the heap.

wongarsuOP1y ago

Memory allocation is one of the areas where currently C/C++ has or had genuine advantages over Rust. Custom allocators took Rust years, and giving standard library constructs like a Vector a custom allocator that isn't the global allocator is still experimental (=opt-in nightly-only). Similarly while Rust gives you good control over where the data ends up being stored, there is no way to make sure it isn't also put on the stack during function execution. One of the implicit assumptions underlying the language seems to be that the stack is cheap and effectively infinite while the heap is expensive. So you have a lot of control over what touches the heap, but less control over what touches the stack.

Those are temporary pains that have remedies in the works. Rust is a fairly young language, and a lot of good-enough solutions get thrown out before ever getting beyond the experimental stage. But if you are writing software today then needing absolute control over where exactly your data touches is a good reason to prefer C/C++ today. Not that that's a very common need.

tialaramex1y ago

I'm not persuaded that scribbling on a Box<MaybeUninit<T>> until it's initialised is less ergonomic than the C. Which isn't to say it's a desirable end state, I just don't see C as a more ergonomic alternative even for this application.

bennettnate51y ago

It can also be an issue if you want to wrap any API that requires fixed memory locations for objects (such as POSIX semaphores). It's UB to call a POSIX semaphore from any other memory location than where it was initialized, so making a `Semaphore::new()` API is just asking for trouble. You can deal with it by `Box`ing the semaphore, but then you can't construct the semaphore in a shared memory segment (one of the stronger use cases for process-shared semaphores).

I have a hunch this is why there's no Semaphore implementation in the Rust standard library, though it could be due to fundamental inconsistencies in semaphore APIs across OSs as well ¯\_(ツ)_/¯

masklinn1y ago

No, Rust doesn't have semaphores in the stdlib[0] because it was not clear what precise semantics should be supported, or what purpose they would serve since by definition they can't mitigate exclusive and thus write access to a resource and mitigating access to code isn't much of a rust convention. And nobody has really championed their addition since.

Furthermore, they still present a fair amount of design challenges in the specific context of Rust: https://neosmart.net/blog/implementing-truly-safe-semaphores...

[0] technically they were there, added in 0.4, never stabilised, deprecated in 1.7, and removed in 1.8

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