Beneath std

This section documents features that are normally provided by the std crate and that #![no_std] developers have to deal with (i.e. provide) to build #![no_std] binary crates.

Using libc

In order to build a #[no_std] executable we will need libc as a dependency. We can specify this using our Cargo.toml file:

[dependencies]
libc = { version = "0.2.146", default-features = false }

Note that the default features have been disabled. This is a critical step - the default features of libc include the std crate and so must be disabled.

Alternatively, we can use the unstable rustc_private private feature together with an extern crate libc; declaration as shown in the examples below. Note that windows-msvc targets do not require a libc, and correspondingly there is no libc crate in their sysroot. We do not need the extern crate libc; below, and having it on a windows-msvc target would be a compile error.

Writing an executable without std

We will probably need a nightly version of the compiler to produce a #![no_std] executable because on many platforms, we have to provide the eh_personality lang item, which is unstable.

You will need to define a symbol for the entry point that is suitable for your target. For example, main, _start, WinMain, or whatever starting point is relevant for your target. Additionally, you need to use the #![no_main] attribute to prevent the compiler from attempting to generate an entry point itself.

Additionally, it's required to define a panic handler function.

#![feature(lang_items, core_intrinsics, rustc_private)]
#![allow(internal_features)]
#![no_std]
#![no_main]

// Necessary for `panic = "unwind"` builds on cfg(unix) platforms.
#![feature(panic_unwind)]
extern crate unwind;

// Pull in the system libc library for what crt0.o likely requires.
#[cfg(not(windows))]
extern crate libc;

use core::ffi::{c_char, c_int};
use core::panic::PanicInfo;

// Entry point for this program.
#[unsafe(no_mangle)] // ensure that this symbol is included in the output as `main`
extern "C" fn main(_argc: c_int, _argv: *const *const c_char) -> c_int {
    0
}

// These functions are used by the compiler, but not for an empty program like this.
// They are normally provided by `std`.
#[lang = "eh_personality"]
fn rust_eh_personality() {}
#[panic_handler]
fn panic_handler(_info: &PanicInfo) -> ! { core::intrinsics::abort() }

If you are working with a target that doesn't have binary releases of the standard library available via rustup (this probably means you are building the core crate yourself) and need compiler-rt intrinsics (i.e. you are probably getting linker errors when building an executable: undefined reference to `__aeabi_memcpy'), you need to manually link to the compiler_builtins crate to get those intrinsics and solve the linker errors.