The crate is part of the linktime project.

crate	docs	version
linktime
ctor
dtor
link-section

link-section

A crate for defining link sections in Rust.

Sections are defined using the #[section] macro. This creates an associated data and text section, and items decorated with the #[in_section] macro are placed into the associated section.

Platform Support

(§1) Orphan section handling is a feature of the linker that allows sections to be defined without a pre-defined name.

(§2) WASM requires const items, and uses ctor-like initialization to copy data to a contiguous section. To access link-section slices in WASM in #[ctor] functions, make sure to use at least #[ctor(priority = 1)].

(§3) Host environment support (by calling the exported register_link_section function) is required to register each section with the runtime.

(§4) AIX requires -C link-arg=-bdbg:namedsects:ss which enables functionality similar to LLVM/GCC's orphan section handling.

Platform Details

Each platform has a slightly different implementation of section control.

Linux and other LLVM/GCC platforms

• Has start/end symbols: ✅ (C-compatible names only)
• Supports linker sorting: ❌

On Linux and other LLVM/GCC platforms, the linker supports orphan sections, which allow sections to be defined without a pre-defined name. These sections are emitted as if they were r/w .data. For sections with C-compatible names, the linker will emit start/end symbols for the section.

Orphan sections are not sorted via numeric suffix (e.g.: SECTION.1, SECTION.2, etc.) with the default linker script.

macOS

• Has start/end symbols: ✅
• Supports linker sorting: ❌

On macOS, sections are configured via __DATA or __TEXT prefix and option suffixes (regular, no_dead_strip, etc.). The linker emits start and stop symbols, but Rust requires a (somewhat-stable) \x01 prefix to avoid mangling the section name. macOS does not support ordering in the linker.

Windows

• Has start/end symbols: ❌
• Supports linker sorting: ✅

On Windows, the linker does not emit start/end symbols, but all sections with a common prefix are automatically sorted by suffix, allowing us to use suffixes to control placement of start/stop symbols that we emit.

See this blog post and this blog post for more details about the alphabetical sorting rule.

WASM

• Has start/end symbols: ❌
• Supports linker sorting: ❌

On WASM platforms, Rust emits data into custom sections which do not support ordering, and are stored out-of-band. The host environment is responsible for registering this out-of-band section with this library as this data is not accessible by the WASM runtime.

Normally, WASM does not support placing arbitrary data in link sections - only non-pointer data is supported. However, the WASM support uses const items and pre-main construction functions to copy each entry into a contiguous section allocated at startup. The number of items in a link-section is computed by generating a custom data section containing one byte per item.

The WASM support expects a function in the module's environment with the following signature and functionality. The wasm import only passes the four usize / pointer parameters; the embedder should close over WebAssembly.Module and WebAssembly.Memory from compile/instantiate when installing the import.

/**
 * Support function for `link-section` crate.
 */
export function readCustomSection(
  wasmModule: WebAssembly.Module,
  wasmInstance: WebAssembly.Instance,
  namePtr: number,
  nameLength: number,
  targetPtr: number,
  targetLength: number,
): number {
    const memory = wasmInstance.exports.memory as WebAssembly.Memory;
    const nameBytes = new Uint8Array(memory.buffer, namePtr, nameLength);
    const sectionName = new TextDecoder().decode(nameBytes);

    const sections = WebAssembly.Module.customSections(wasmModule, sectionName);
    if (sections.length === 0) {
        return 0;
    }

    const section = sections[0];
    const need = section.byteLength;
    if (targetLength < need) {
        return need;
    }

    new Uint8Array(memory.buffer, targetPtr, need).set(new Uint8Array(section));
    return need;
}

AIX

• Has start/end symbols: ✅
• Supports linker sorting: ❌

AIX maps Rust's #[link_section] to csects (Control Sections), which act like subsections of the larger .text and .data sections ^↳. A csect is the smallest, indivisible unit of code or data.

By default, AIX does not have section start/stop symbols, but the most recent versions of the linker added a new -bdbg:namedsects:ss flag which enables section start/stop symbols ^↳.

This flag can be set with -C link-arg=-bdbg:namedsects:ss (or by upgrading to a recent Rust version that sets this automatically ^↳ to support link sections.

The linker will report an error like this if the start/stop symbols are not found:

  = note: ld: 0711-317 ERROR: Undefined symbol: __start__data_link_section_DATABASES
          ld: 0711-317 ERROR: Undefined symbol: __stop__data_link_section_DATABASES
          ld: 0711-345 Use the -bloadmap or -bnoquiet option to obtain more information.

For debugging AIX link-section issues, -C link-arg=-bmap:[path]/linker.out and -C link-arg=-bnoquiet may also be useful.

AIX supports a special mode to strip (strip -r) that preserves structural symbols like csects and exports. A future version of link-section may add support for loading csect bounds from the binary's symbol table.

[target.powerpc64-ibm-aix]
rustflags = [
    "-C", "link-arg=-bdbg:namedsects:ss",   # required
    "-C", "link-arg=-bmap:linker.out",      # for debugging
    "-C", "link-arg=-bnoquiet",             # for debugging
]

Typed Sections

Typed sections provide a section where all items are of a specific, sized type. The typed section may be accessed as a slice of the type at zero cost if desired.

A typed section can be created from either static or const items.

For const items: a copy of the const is materialized at link time, while the constant itself remains available for use as a constant in const contexts.

For static items: the static is stored directly in the link section.

fn items are special-cased and stored as function pointers in the typed section.

Usage

Create an untyped section using the #[section] macro that keeps related items in close proximity:

use link_section::{in_section, section};

#[section]
pub static CODE_SECTION: link_section::Section;

#[in_section(CODE_SECTION)]
pub fn link_section_function() {
    println!("link_section_function");
}

Create a typed section using the #[section] macro that stores items of a specific, sized type from static or const items:

mod my_registry {
    use link_section::{in_section, section};

    pub struct MyStruct {
        name: &'static str,
    }

    #[section]
    pub static MY_REGISTRY: link_section::TypedSection<MyStruct>;

    // Registers a `const` item.
    mod register_a_constant {
        use super::*;

        // A copy of this constant is registered in the link section.
        #[in_section(MY_REGISTRY)]
        pub const LINKED_MY_STRUCT: MyStruct = MyStruct { name: "my_struct" };
    }

    // Registers a `static` item.
    mod register_a_static {
        use super::*;

        // This static lives directly in the link section.
        #[in_section(MY_REGISTRY)]
        pub static LINKED_MY_STRUCT: MyStruct = MyStruct { name: "my_struct_2" };
    }
}

Inspiration

link-section was originally inspired by the linkme project.