There Is Life Before Main in Rust

Posted by mmastrac 5 days ago

Comments

Comment by mmastrac 4 days ago

Author here, happy to answer any questions. I've been working on building some higher-level abstractions on link sections (specifically, link-time optimized collections like maps (1) and sorted slices (2)) and wanted to share the hard-fought knowledge from the last couple of months.

There's a decent amount of knowledge around pre-main work in Rust, but I think this is one of the first attempts to walk through mutable link sections, which open up a pretty wide world of optimization, IMO. Even without mutability, I figured there isn't nearly enough documentation on these approaches out there.

(1) https://docs.rs/scattered-collect/0.20.0/scattered_collect/m...

(2) https://docs.rs/scattered-collect/0.20.0/scattered_collect/s...

Comment by 4 days ago

Comment by jeffbee 4 days ago

The general lesson of these things is main is not that special and it pays to understand how your program actually starts. This has little/nothing to do with Rust or other language tools. On Linux, given a static ELF program, the kernel returns to the IP given by e_entry, which can proceed to do anything. If the program is dynamic (has a .interp) then it loads the interpreter and returns to its e_entry instead. The interpreter, in turn, can do absolutely whatever.

Comment by ameliaquining 4 days ago

The relevance to Rust is precisely that it doesn't have life before main at the language level; therefore, if you need it*, you need to use these kinds of linker hacks (which fortunately are amenable to encapsulation through macros). By contrast, if the article were about C++, the focus would be on "what happens under the hood when you use static initialization, in case you were curious" rather than "how to use these low-level mechanisms to do something not otherwise possible".

* Which you should think very carefully before concluding is the case, as it's responsible for rather a lot of bugs in C++. I think in Rust it is mostly used for registry-pattern type stuff since the const system can't currently(?) handle that.

Comment by Animats 4 days ago

Yes. There isn't Rust language support for this.

Order of initialization can be supported at various levels:

- Completely random (OK if interdependence are locked out, otherwise bad)

- Consistent, but sorted by something such as alphabetical name (meh.)

- Manual, controlled in linker scripts (headache)

- True dependency tree order, including diagnosing loops (seen in the Modula family).

General comment: yes, you can, and you probably shouldn't unless you have profiling data that indicates a significant performance improvement for a critical use case.

Comment by ameliaquining 4 days ago

I think these things are used more for developer experience than for performance, since you can always just do the initialization in main if you really have to.

Comment by Animats 4 days ago

It's something of an issue if you have some crate that needs to set itself up at load time without a call from main. But those are very rare. Even "simplelog" needs a call at startup to do anything.

Comment by ameliaquining 4 days ago

Yeah, I think of avoiding the call from main as a devex consideration rather than a performance one, since either way the initialization code runs once at process startup.

Comment by rcxdude 3 days ago

But it can cause problems when there's multiple crates trying to do it. If the call is explicit, the application developer can sequence the calls appropriately (or at least deterministically), as opposed to having the order determined by details of the implementation (something that was learned from the C++ 'static initialization order fiasco')

Comment by Animats 3 days ago

I'm trying to think of any useful crate that really uses this. "Tracy", the profiling crate, used to self-start on loading, but that was changed to require a call to get it going.

Comment by kibwen 4 days ago

You don't need linker hacks to control what happens before main in Rust. You can disable the default runtime setup with `#![no_main]` in your crate root, and then manually designate a starting point via an unmangled function named appropriately for your specific platform (e.g. `_start`).

Comment by smy20011 4 days ago

> This post is 100% human-written. Claude was used for feedback and to assist with the linker symbol diagram. Cursor was used for feedback and to ensure examples were compilable.

Love this, I hope every blog have the same disclaimer about how AI is used.

Comment by 4 days ago

Comment by rootnod3 4 days ago

If Claude gave feedback then it’s not really 100% human written is it?

Comment by frakt0x90 4 days ago

I'm pretty much hardline anti-AI and even I would say this is too far. If I read documentation or ask my wife to review something, those people did not write the final product. Perhaps it would be mentioned in a citation, like this person has.

Comment by ronsor 4 days ago

If you merely get feedback from a human, are they now a co-author?

Comment by wahern 4 days ago

If the feedback or involvement is substantive it used to be common to mention this, even if just in a prologue, epilogue, or footnote. You still see this is some academic writing and some journalism, where authors mention with whom they consulted. Books and other literature have tended to dissociate people from sources of knowledge, and the Internet furthered the dissociation. But honest writing should disclose all the sources of substantive claims, preferably traced back to primary sources. Legal writing and scientific papers are perhaps the last bastions where this is still done, or at least expected to be done, fairly rigorously, but the manner in which AI is used seems qualitatively more problematic for maintaining any kind of rigor in citation.

Comment by mmastrac 4 days ago

FWIW this post has both a "thanks" section for the human reviewers, and numerous footnotes linking to more authoritative sources.

Comment by vitally3643 4 days ago

It was written on a computer with a keyboard, so clearly it's 0% human written

Comment by orthecreedence 4 days ago

Show me the cave drawing version of this post or I will absolutely not be reading it.

Comment by hresvelgr 4 days ago

Most human editors are not given credit, why would it be any different for an LLM?

Comment by khuey 4 days ago

If you run spell check on a document is it no longer 100% human written?

Comment by sophacles 4 days ago

Yeah! It was typed into a computer and never even put on paper. How can you say it was written at all?

Further, can anything be "100% human writt even if it uses pen and paper? No of course not! Unless it is created by pricking a finger and put on human vellum, it's only partially human written.

Seriously though - if you want to do stupid purity test games, at least be properly pure about it. This half-assed nonsense is just trite.

Comment by Sharlin 4 days ago

Editors (as in, the human kind) are not co-writers either.

Comment by wild_pointer 4 days ago

I thought it was about prehistoric Rust before the main branch was created :)

Comment by ryanshrott 3 days ago

I ran into this with embedded Rust: put alloc in a .init_array function, but the global allocator also uses .init_array, and there's no ordering guarantee. Took me hours to figure out why I was getting garbage from the heap before main.

Comment by Surac 4 days ago

everything abount Rust MUST have some AI in it nowerdays. even this article