rust-ctor

ctor

dtor

Module initialization/teardown functions for Rust (like __attribute__((constructor)) in C/C++) for Linux, OSX, FreeBSD, NetBSD, Illumos, OpenBSD, DragonFlyBSD, Android, iOS, WASM, and Windows.

This library currently requires Rust > 1.56.0 at a minimum for edition 2021 support. Library versions 0.2.x should work for edition 2018, and 1.0 is planned to be released as 2021-only.

Zero Dependency

As of ctor 0.3.0+, ctor has no dependencies (other than the ctor-proc-macro crate). The proc macro in this crate calls into the declarative macro that does the majority of the work.

Support

This library works and is regularly tested on Linux, OSX, Windows, and FreeBSD, with both +crt-static and -crt-static where possible. Other platforms are supported but not tested as part of the automatic builds. This library will also work as expected in both bin and cdylib outputs, ie: the ctor and dtor will run at executable or library startup/shutdown respectively.

This library supports WASM targets, but the MSRV for this target is 1.85.

Features

Feature	Description	Default
std	Enable support for the standard library. This is required for static ctor variables, but not for functions.	Yes
proc_macro	Enable support for the proc macro. Required for #[ctor] and #[dtor] macros, but not for ctor! and dtor! forms.	Yes
dtor	Include #[dtor] support in the ctor crate.	Yes
used_linker	Enable support for #[used(linker)] (nightly only).	No

Warnings

Rust's philosophy is that nothing happens before or after main and this library explicitly subverts that. The code that runs in the ctor and dtor functions should be careful to limit itself to libc functions and code that does not rely on Rust's stdlib services.

For example, using stdout in a dtor function is a guaranteed panic. Consider using the libc-print crate for output to stderr/stdout during #[ctor] and #[dtor] methods. Other issues may involve signal processing or panic handling in that early code.

Some linker configurations may cause #[ctor] and #[dtor] functions to be stripped from the final binary. The used_linker feature may prevent this, but is not supported outside of nightly Rust. Often, a simple use module_with_ctor is sufficient to ensure the linker does not strip the function.

On some platforms, unloading of shared libraries may not actually happen until process exit, even if explicitly unloaded. The rules for this are arcane and difficult to understand. For example, thread-local storage on OSX will affect this (see this comment).

Examples

Marks the function foo as a module constructor, called when a static library is loaded or an executable is started:

    static INITED: AtomicBool = AtomicBool::new(false);

    #[ctor]
    fn foo() {
        INITED.store(true, Ordering::SeqCst);
    }

Creates a HashMap populated with strings when a static library is loaded or an executable is started (new in 0.1.7):

static items are equivalent to std::sync::OnceLock, with an automatic deref implementation and eager initialization at startup time. #[ctor] on static items requires the default std feature.

#[ctor]
/// This is an immutable static, evaluated at init time
static STATIC_CTOR: HashMap<u32, &'static str> = {
    let mut m = HashMap::new();
    m.insert(0, "foo");
    m.insert(1, "bar");
    m.insert(2, "baz");
    m
};

Print a message at shutdown time. Note that Rust may have shut down some stdlib services at this time.

#[dtor]
unsafe fn shutdown() {
    // Using println or eprintln here will panic as Rust has shut down
    libc::printf("Shutting down!\n\0".as_ptr() as *const i8);
}

Under the Hood

The #[ctor] macro makes use of linker sections to ensure that a function is run at startup time.

The above example translates into the following Rust code (approximately):

#[used]
#[cfg_attr(target_os = "linux", link_section = ".init_array")]
#[cfg_attr(target_vendor = "apple", link_section = "__DATA,__mod_init_func,mod_init_funcs")]
#[cfg_attr(target_os = "windows", link_section = ".CRT$XCU")]
/* ... other platforms elided ... */
static FOO: extern fn() = {
    extern fn foo() { /* ... */ };
    foo
};

The #[dtor] macro effectively creates a constructor that calls libc::atexit with the provided function, ie roughly equivalent to:

#[ctor]
fn dtor_atexit() {
    libc::atexit(dtor);
}

Inspiration

Idea inspired by this code in the Neon project.