Crate ctor_lite

The ctor crate reimplemented using procedural macros.

In some cases it is necessary to run code at the very start or the very end of the program. This crate provides a macro that can be used to run code at the very beginning of program execution, along with some extra features.

Advantages over ctor

• Completely dependency free, thanks to relying on procedural macros instead of proc macros.
• Supports all of the same use cases as the ctor crate.
• Supports all of the same platforms as the ctor crate.
• Fixes a couple of warts in ctor’s API, such as:
  • unsafe is required when it is used, see the “Safety” section below.
  • Global variables are required to be Sync.
  • Global variables use MaybeUninit instead of Option.
  • Functions set up with the ctor or dtor macros cannot be called in other Rust code.

Disadvantages

• The API has a slightly different form factor that can be inconvenient in some cases.
• The MSRV has been raised to 1.36.0.

Functional Usage

The ctor macro can be used to run a function at program startup time.

use std::sync::atomic::{AtomicUsize, Ordering};

static INITIALIZED: AtomicUsize = AtomicUsize::new(0);

ctor_lite::ctor! {
    unsafe fn set_value() {
        INITIALIZED.store(1, Ordering::Relaxed);
    }
}

assert_eq!(INITIALIZED.load(Ordering::Relaxed), 1);

Note that this macro is a procedural block rather than an attribute macro. If you prefer the old way of using the macro you can use the macro-rules-attribute crate.

use macro_rules_attribute::apply;
use std::sync::atomic::{AtomicUsize, Ordering};

static INITIALIZED: AtomicUsize = AtomicUsize::new(0);

#[apply(ctor_lite::ctor!)]
unsafe fn set_value() {
    INITIALIZED.store(1, Ordering::Relaxed);
}

assert_eq!(INITIALIZED.load(Ordering::Relaxed), 1);

Static Usage

The ctor macro can be used to create a static variable initialized to a default value. At startup time, the function is used to initialize the static variable.

fn value() -> i32 {
    6
}

ctor_lite::ctor! {
    unsafe static VALUE: i32 = value();
}

assert_eq!(*VALUE, 6);

Destructor

This crate can also be used to run a function at program exit as well. The dtor macro can be used to run a function when the program ends.

use macro_rules_attribute::apply;

#[apply(ctor_lite::dtor!)]
unsafe fn run_at_exit() {
    do_some_cleanup();
}

Safety

Macros from this crate must be used with care. In general Rust code is run with the assumption that no other code is run before program startup, and no other code is run after program shutdown. Specifically, libstd sets up some global variables before the main function and then assumes these variables are set throughout its runtime. Therefore, calling libstd functions that use these variables will lead to undefined behavior.

Generally, functions from core or alloc are safe to call in these functions. In addition, functions from libc should be able to be called freely, as well as most of the functions contained in rustix. Other crates should be used only when it is understood what other calls they contain.

In addition, no ordering is guaranteed for functions ran in the ctor or dtor macros.

Implementation

The ctor macro works by creating a function with linker attributes that place it into a special section in the file. When the C runtime starts the program, it reads function pointers from this section and runs them.

This function call…

ctor_lite::ctor! {
    unsafe fn foo() { /* ... */ }
}

…is translated to code that looks like this:

#[used]
#[cfg_attr(any(target_os = "linux", target_os = "android"), link_section = ".init_array")]
#[cfg_attr(target_os = "freebsd", link_section = ".init_array")]
#[cfg_attr(target_os = "netbsd", link_section = ".init_array")]
#[cfg_attr(target_os = "openbsd", link_section = ".init_array")]
#[cfg_attr(target_os = "illumos", link_section = ".init_array")]
#[cfg_attr(any(target_os = "macos", target_os = "ios", target_os = "tvos"), link_section = "__DATA_CONST,__mod_init_func")]
#[cfg_attr(target_os = "windows", link_section = ".CRT$XCU")]
static FOO: extern fn() = {
  #[cfg_attr(any(target_os = "linux", target_os = "android"), link_section = ".text.startup")]
  extern fn foo() { /* ... */ };
  foo
};

When creating a global constant with the ctor macro it writes code that runs the function then writes the value into a global constant.

This code…

ctor_lite::ctor! {
    unsafe static FOO: i32 = foo();
}

…is translated to code that looks like this, with modifications that allow for FOO to be used from safe code:

static mut FOO: i32 = core::mem::uninitialized();
ctor_lite::ctor! {
    unsafe fn init_storage() {
        FOO = foo();
    }
}
# fn foo() -> i32 { 1 }

When functions are put into dtor, it runs ctor with the libc::atexit function to ensure that the function is run at program exit.

This code…

ctor_lite::dtor! {
    unsafe fn foo() {
        /* ... */
    }
}

…is translated to code that looks like this, with modifications that let us avoid a dependency on the libc crate:

unsafe fn foo() {
    /* ... */
}

ctor_lite::ctor! {
    unsafe fn run_dtor() {
        libc::atexit(foo);
    }
}

Macros

ctor

Run a function on program startup or initialize a constant.

dtor

Run a function on program shutdown.