ctor_bare

Module initialization functions for Rust (like attribute((constructor)) in C/C++) under no_std.

After registering a constructor function, a function pointer pointing to it will be stored in the .init_array section.

It can support Linux, MacOS and other systems, and can be also used in no_std environments when developing your own kernel.

In Linux, Windows, MacOS and other systems, the .init_array section is a default section to store initialization functions. When the program starts, the system will call all functions in the .init_array section in order.

When you are running your own operating system, you can call ctor_bare::call_ctors to invoke all registered constructor functions.

Usage

use ctor_bare::register_ctor;
#[register_ctor]
fn hello_world() {
    println!("Hello, world!");
}

static MAX_NUM: std::sync::atomic::AtomicUsize = std::sync::atomic::AtomicUsize::new(0);

#[register_ctor]
fn set_max_num() {
    MAX_NUM.store(20, std::sync::atomic::Ordering::Relaxed);
}

fn main() {
    assert_eq!(MAX_NUM.load(std::sync::atomic::Ordering::Relaxed), 20);
}

Because the .init_array section is a default section to store initialization functions in Linux and some other systems, it will be included in the linker script of compilers like GCC and Clang.

However, if you are using a custom linker script, you need to add the .init_array section and map them in the page table manually, so that these functions can be executed correctly. You can add the following line to your linker script as a reference:

.init_array : ALIGN(4K) {
    PROVIDE_HIDDEN (__init_array_start = .);
    *(.init_array .init_array.*)
    PROVIDE_HIDDEN (__init_array_end = .);
    . = ALIGN(4K);
}

Notes

To avoid section-related symbols being optimized by the compiler, you need to add "-z nostart-stop-gc" to the compile flags (see https://lld.llvm.org/ELF/start-stop-gc).

For example, in .cargo/config.toml:

[build]
rustflags = ["-C", "link-arg=-z", "link-arg=nostart-stop-gc"]
rustdocflags = ["-C", "link-arg=-z", "-C", "link-arg=nostart-stop-gc"]