# ctor_bare
[](https://crates.io/crates/ctor_bare)
[](https://docs.rs/ctor_bare)
[](https://github.com/arceos-org/ctor_bare/actions/workflows/ci.yml)
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
```rust
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:
```test, ignore
.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`:
```toml
[build]
rustflags = ["-C", "link-arg=-z", "link-arg=nostart-stop-gc"]
rustdocflags = ["-C", "link-arg=-z", "-C", "link-arg=nostart-stop-gc"]
```