The crate is part of the [`linktime`](https://crates.io/crates/linktime) project.

[![GitHub](https://img.shields.io/badge/repo-github-blue)](https://github.com/mmastrac/linktime) [![Crates.io License](https://img.shields.io/crates/l/link-section)](https://crates.io/crates/link-section) [![Build Status](https://github.com/mmastrac/linktime/actions/workflows/rust.yml/badge.svg)](https://github.com/mmastrac/linktime/actions/workflows/rust.yml) 


| crate | |
| --- | --- |
| `linktime`<br>[![docs.rs](https://docs.rs/linktime/badge.svg)](https://docs.rs/linktime) [![crates.io](https://img.shields.io/crates/v/linktime.svg)](https://crates.io/crates/linktime) | Convenience crate for `ctor`, `dtor` and `link-section` |
| `ctor`<br>[![docs.rs](https://docs.rs/ctor/badge.svg)](https://docs.rs/ctor) [![crates.io](https://img.shields.io/crates/v/ctor.svg)](https://crates.io/crates/ctor) | Module initialization functions before main |
| `dtor`<br>[![docs.rs](https://docs.rs/dtor/badge.svg)](https://docs.rs/dtor) [![crates.io](https://img.shields.io/crates/v/dtor.svg)](https://crates.io/crates/dtor) | Module shutdown functions before main |
| `link-section`<br>[![docs.rs](https://docs.rs/link-section/badge.svg)](https://docs.rs/link-section) [![crates.io](https://img.shields.io/crates/v/link-section.svg)](https://crates.io/crates/link-section) | Linker-managed typed (slices) and untyped sections |
| `scattered-collect`<br>[![docs.rs](https://docs.rs/scattered-collect/badge.svg)](https://docs.rs/scattered-collect) [![crates.io](https://img.shields.io/crates/v/scattered-collect.svg)](https://crates.io/crates/scattered-collect) | Linker-managed collections: slices, sorted slices, maps |
# dtor
Shutdown functions for Rust (like `__attribute__((destructor))` in C/C++) for
Linux, macOS, Windows, mobile (iOS/Android), WASM, BSD/BSD-likes and many other
platforms.

```rust
use dtor::dtor;
use libc_print::*;

#[dtor(unsafe)]
fn foo() {
    libc_println!("Life after main!");
}
```

# Examples

Print a message at shutdown time.

```rust
#[dtor(unsafe)]
fn shutdown() {
    // Using println! or eprintln! here may panic as Rust may have
    // shut down some stdlib services at this time.
    libc_println!("Shutting down!");
}
```

# Platform Support

| Platform                   | Link Section                              | at_binary_exit | at_module_exit       |
| -------------------------- | ----------------------------------------- | -------------- | -------------------- |
| Linux                      | `.fini_array`                             | Yes (`atexit`) | Yes (`__cxa_atexit`) |
| macOS                      | `.mod_term_func` <sup><sup>🍎</sup></sup> | Yes (`atexit`) | Yes (`__cxa_atexit`) |
| Windows                    | `.CRT$XPU` <sup><sup>🪟</sup></sup>       | No             | Yes (`atexit`)       |
| WASM 🕸️                    | No                                        | Yes            | No                   |
| AIX                        | "Kind of" <sup><sup>🔵</sup></sup>        | Yes            | Yes                  |
| Other POSIX-like platforms | `.fini_array`/`.dtors`                    | Yes (`atexit`) | Yes (`__cxa_atexit`) |

Notes:

- <sup><sup>🍎</sup></sup> Not recommended. Apple platforms no longer call
  `mod_term_func` functions.
- <sup><sup>🪟</sup></sup> Not recommended. Windows platforms may not reliably
  call functions in link sections, unless a binary is built with a static CRT.
- <sup><sup>🔵</sup></sup> Link sections are not supported on AIX, but the
  platform calls functions with the prefix `__sinit` and `__sterm` at startup
  and shutdown respectively. `__sterm`-prefixed functions are used when the
  method is specified as `linker`.
- <sup><sup>🕸️</sup></sup> WASM `wasm-unknown-unknown`, `wasm-wasip1`,
  `wasm-wasip2` are supported.
  - Rust does not currently allow linking into `.fini_array` sections on WASM,
    regardless of target, so `at_binary_exit` is the only supported method on
    all WASM targets.
  - `wasm-unknown-unknown` requires host environment support for `atexit`.
  - `wasm-wasip2` may require you to manually call `__wasm_call_ctors` and
    `__wasm_call_dtors` at the appropriate times.

# Shutdown Method (`#[dtor(method = ...)]`)

The `#[dtor]` macro supports multiple registration strategies via
`#[dtor(method = ...)]`. The best choice is platform-dependent:

- `#[dtor]` (no method specified): Use the platform's most reliable method:
  `at_module_exit` on Windows and Apple platforms, and `linker` on others.
- `unload`: Run on _module unload_ (library unload or process exit) using the
  platform's default unload method.
- `term`: Run on _process termination only_ using the platform's default
  termination method. Not recommended: code may be unloaded before the dtor
  runs.
- `at_module_exit`: Register using `__cxa_atexit` (non-Windows) or `atexit`
  (Windows) so the dtor runs when the module unloads. Unsupported on WASM.
- `at_binary_exit`: Register to run at process exit (unsupported on Windows).
- `linker`: Register using the platform's linker mechanism (`link_section` on
  all platforms with the exception of `export_name_prefix` on AIX). Unsupported
  on Apple platforms.

Default:

- Apple and Windows default to `at_module_exit`
- WASM defaults to `at_binary_exit` (note that you will need to provide your own
  atexit implementation for `wasm32-unknown-unknown`)
- Most other platforms default to `linker`

Examples:

```rust
use dtor::dtor;
/// Use `at_module_exit` on all platforms
#[dtor(unsafe, method = at_module_exit)]
fn shutdown() {}
```

```rust
use dtor::dtor;

/// Use `link_section` with a section name of `.dtors` on most platforms,
/// and `export_name_prefix` on AIX.
/// 
/// Platform note: this will fail to compile on Apple platforms.
#[dtor(unsafe, method = linker, link_section = ".dtors")]
fn shutdown() {}
```

# 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.

See [`::life_before_main`](crate::life_before_main) for more information.

# Under the Hood

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

```rust,ignore
#[ctor]
fn dtor_atexit() {
    libc::atexit(dtor);
}
```
# Re-exporting from another crate

The macros assume this crate is available as a direct dependency, resolving their
support paths through the crate's own name. If you re-export this crate's items as
part of your own crate (so that downstream users don't need to depend on it
directly), you have two options:

- (preferred) use the declarative macro form. It resolves its support paths
  relative to your re-export, so no extra configuration is required.
- Alternatively, pass the `crate_path` attribute to redirect the macro's
  generated output to the path where this crate has been re-exported.

See the `crate_path` entry in the *Macro Attributes* section below for the exact
syntax for this crate.
# Crate Features

| Cargo feature | Description |
| --- | --- |
| `proc_macro` |  Enable support for the proc-macro `#[dtor]` attribute. The declarative form (`dtor!(...)`) is always available. It is recommended that crates re-exporting the `dtor` macro disable this feature and only use the declarative form. |
| `std` |  Enable support for the standard library. |

# Macro Attributes

<table><tr><th>Attribute</th><th>Description</th></tr>
<tr><td><code>anonymous</code></td><td>

 Do not give the destructor's registration entry a name in the generated
 code (allows for multiple items with the same name). Equivalent to
 wrapping the registration in an anonymous const (i.e.: `const _ = { ... };`).


</td></tr>
<tr><td><code>crate_path = ::path::to::dtor::crate</code></td><td>

 The path to the `dtor` crate containing the support macros. If you
 re-export `dtor` items as part of your crate, you can use this to
 redirect the macro's output to the correct crate.

 Using the declarative [`dtor!`][d] form is
 preferred over this parameter.

 [d]: crate::declarative::dtor!


</td></tr>
<tr><td><code>ctor(export_name_prefix = "ctor_")</code></td><td>

 Specify a custom export name prefix for the generated constructor
 function.

 If specified, an export with the given prefix will be generated in the
 form:

 `<prefix>_<unique_id>`


</td></tr>
<tr><td><code>ctor(link_section = ".ctors")</code></td><td>

 Place the generated registration constructor's function pointer in a
 custom link section.


</td></tr>
<tr><td><code>export_name_prefix = "ctor_"</code></td><td>

 Specify a custom export name prefix for the destructor function.

 If specified, an export with the given prefix will be generated in the form:

 `<prefix>_<unique_id>`


</td></tr>
<tr><td><code>link_section = ".dtors"</code></td><td>

 Place the destructor function pointer in a custom link section.


</td></tr>
<tr><td><code>method = term|unload|at_module_exit|at_binary_exit|linker</code></td><td>

 Specify the dtor method.

  - `term`: Run the dtor on binary termination using the platform's
    [default_term_method](#default_term_method). Not recommended as code
    may be unloaded before the dtor is called.
  - `unload`: Run the dtor on module unload (library or binary) using the
    platform's [default_unload_method](#default_unload_method).
  - `at_module_exit`: Run the dtor using the platform's
    [`at_module_exit`][at_module_exit] (`__cxa_atexit` on all platforms
    other than Windows, `atexit` on Windows).
  - `at_binary_exit`: Run the dtor using the platform's
    [`at_binary_exit`][at_binary_exit] (unsupported on Windows
    platforms).
  - `linker`: Register the dtor using the platform's
    [link_section](#link_section) or
    [export_name_prefix](#export_name_prefix) (unsupported on Apple
    platforms).

 [at_module_exit]: crate::native::at_module_exit
 [at_binary_exit]: crate::native::at_binary_exit


</td></tr>
<tr><td><code>unsafe</code></td><td>


 Marks a dtor as unsafe. Required.

 The `dtor` crate rejects `#[dtor]` without marking the item unsafe;
 that error can be suppressed by passing
 `RUSTFLAGS="--cfg linktime_no_fail_on_missing_unsafe"` to Cargo.


</td></tr>
<tr><td><code>used(linker)</code></td><td>


 Mark generated function pointers `used(linker)`. Requires nightly
 for the nightly-only feature `feature(used_with_arg)` (see
 <https://github.com/rust-lang/rust/issues/93798>).

 This can be made the default by using the `cfg` flag
 `linktime_used_linker` (`RUSTFLAGS="--cfg linktime_used_linker"`).

 For a crate using this macro to function correctly with and without
 this flag, it is recommended to add the following line to the top of
 lib.rs in the crate root:

 `#![cfg_attr(linktime_used_linker, feature(used_with_arg))]`



</td></tr>
</table>

# Defaults

## `ctor_export_name_prefix`

 ```rust
#[cfg(target_os = "aix")]
ctor_export_name_prefix = "__sinit80000000"

 // default
ctor_export_name_prefix = ()
 ```

## `ctor_link_section`

 ```rust
#[cfg(target_vendor = "apple")]
ctor_link_section = "__DATA,__mod_init_func,mod_init_funcs"

#[cfg(any(target_os = "linux", target_os = "android", target_os = "freebsd",
target_os = "netbsd", target_os = "openbsd", target_os = "dragonfly",
target_os = "illumos", target_os = "haiku", target_os = "vxworks", target_os =
"nto", target_family = "wasm"))]
ctor_link_section = ".init_array"

#[cfg(target_os = "none")]
ctor_link_section = ".init_array"

#[cfg(target_arch = "xtensa")]
ctor_link_section = ".ctors"

#[cfg(all(target_os = "windows", any(target_env = "gnu", target_env = "msvc")))]
ctor_link_section = ".CRT$XCU"

#[cfg(all(target_os = "windows", not(any(target_env = "gnu", target_env = "msvc"))))]
ctor_link_section = ".ctors"

#[cfg(target_os = "uefi")]
ctor_link_section = ".init_array"

#[cfg(target_os = "aix")]
ctor_link_section = ()

 // default
ctor_link_section = ".init_array"
 ```

## `default_term_method`

 ```rust
#[cfg(target_os = "windows")]
default_term_method = at_module_exit

 // default
default_term_method = at_binary_exit
 ```

## `default_unload_method`

 ```rust
 // default
default_unload_method = at_module_exit
 ```

## `export_name_prefix`

 ```rust
#[cfg(target_os = "aix")]
export_name_prefix = "__sterm80000000"

 // default
export_name_prefix = ()
 ```

## `link_section`

 ```rust
#[cfg(target_vendor = "apple")]
link_section = "__DATA,__mod_term_func,mod_term_funcs"

#[cfg(any(target_os = "linux", target_os = "android", target_os = "freebsd",
target_os = "netbsd", target_os = "dragonfly", target_os = "illumos",
target_os = "haiku", target_os = "vxworks", target_os = "nto", target_family =
"wasm"))]
link_section = ".fini_array"

#[cfg(target_os = "openbsd")]
link_section = ".dtors"

#[cfg(target_os = "none")]
link_section = ".fini_array"

#[cfg(target_arch = "xtensa")]
link_section = ".dtors"

#[cfg(all(target_os = "windows", any(target_env = "gnu", target_env = "msvc")))]
link_section = ".CRT$XPU"

#[cfg(all(target_os = "windows", not(any(target_env = "gnu", target_env = "msvc"))))]
link_section = ".dtors"

#[cfg(target_os = "uefi")]
link_section = ".fini_array"

#[cfg(target_os = "aix")]
link_section = ()

 // default
link_section = ".fini_array"
 ```

## `method`

 ```rust
#[cfg(target_vendor = "apple")]
method = at_module_exit

#[cfg(target_os = "windows")]
method = at_module_exit

#[cfg(target_family = "wasm")]
method = at_binary_exit

#[cfg(target_os = "openbsd")]
method = at_module_exit

 // default
method = linker
 ```

## `r#unsafe`

 ```rust
#[cfg(linktime_no_fail_on_missing_unsafe)]
r#unsafe = (no_fail_on_missing_unsafe)

 // default
r#unsafe = ()
 ```

## `used_linker`

 ```rust
#[cfg(linktime_used_linker)]
used_linker = used_linker

 // default
used_linker = ()
 ```
