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There Is Life Before Main in Rust

91 points by mmastrac

by mmastrac

1 subcomments

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...

by smy20011

2 subcomments

> 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.

by ryanshrott

0 subcomment

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.

by wild_pointer

0 subcomment

by jeffbee

1 subcomments

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.

by Surac

0 subcomment