[WIP] Rust's standard library, free of C dependencies, for Linux systems
Rust Other
Switch branches/tags
Nothing to show
Clone or download
homunkulus Auto merge of #157 - japaric:c-int-width, r=japaric
add the target-c-int-width field to the target specifications

this is a new mandatory field in the target specification
Latest commit 23882a1 Oct 20, 2017



[WIP] Rust's standard library, free of C dependencies, for Linux systems

It's very early days. Very little functionality has been ported over.


The ultimate goal is achieve truly hassle free cross compilation from any system (macOS, Windows, *BSD, etc.) to any Linux system, be it x86 or ARM hardware. That is:

$ cargo build --target aarch64-unknown-linux-steed

Should work without having to install a C toolchain or cross compiled C libraries, and without having to run the command inside a Docker container / VM.

By removing all the C dependencies, steed solves half of the problem. The other half will be solved by embedding lld, LLVM's multi-arch linker, into rustc.

The short term goal is to provide the exact same functionality and API as the std crate. Hopefully, by the time that's complete, PAL and std-aware Cargo will be around and we'll be able to plug a pal_steed crate into the normal std to get C free Rust programs.

Although, IMO, it would make more sense (*) and it would be more ergonomic to land chunks of steed in rust-lang/rust and add a new set of built-in target, e.g. aarch64-linux. That way, compiling C free Rust programs would be as simple as e.g. cargo build --target aarch64-linux.

(*) Because linking requires different linker arguments and programs are always statically linked.


  • Supporting other unix like systems, like the *BSD ones. Mainly because we have no (easy) way to test them inside Travis and also to reduce the amount of work required. After we are done with the C free port of std for Linux then we can start thinking about supporting other OSes.


  • Zero C code. Zero C dependencies. Not even startup objects are required to link a steed binary. steed programs should still be able to interoperate with or link to C code though (This last part is currently untested).

  • Small, statically linked binaries:

// examples/hello.rs
use std::{io, process};
use std::io::Write;

fn main() {
    if io::stdout().write_all(b"Hello, world!\n").is_err() {
# xargo rustc --target x86_64-unknown-linux-steed --release --example hello -- -C lto
$ ./hello
Hello, world!

$ strip -s hello

$ file hello
hello: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, stripped, with debug_info

$ size hello
   text    data     bss     dec     hex filename
    173       0       0     173      ad hello

$ ls -l hello
-rwxr-xr-x 2 japaric japaric 4712 Apr 11 00:00 hello

Disclaimer The binary size will inevitably go up after we add missing startup/runtime features like stack overflow protection and unwinding.

Supported architectures

Turns out that writing architecture specific code is tricky! We were trying to provide "if it compiles, it works" level of support for every architecture that std supports but that stalled development as it's hard to get some features working on a bunch of different architectures.

So, we have adopted Rust's tier system to unstuck development. Our platform support is split in two tiers:

Tier 1

"If it compiles, it works" level of support. PRs won't land if CI tests don't pass on these platforms:

  • aarch64-unknown-linux-steed

  • arm-unknown-linux-steedeabi

  • armv7-unknown-linux-steedeabihf

  • i686-unknown-linux-steed

  • x86_64-unknown-linux-steed

Tier 2

"Best effort" level of support. Using some features that require architecture specific code, like threads, may panic at runtime if the work to support that feature has not been done yet (i.e. unimplemented!()). We don't block PRs if CI tests don't pass on these platforms:

  • mips-unknown-linux-steed
  • mipsel-unknown-linux-steed

  • powerpc-unknown-linux-steed

  • powerpc64-unknown-linux-steed

We eventually hope to move all targets into the tier 1 but we'll need help from people more familiar with the non-x86 architectures. If you'd like to help, feel free to contact us on IRC (#rust-steed @ irc.mozilla.org) or via the issue tracker, or directly tackle the architecture specific issues, issue tagged with e.g. A-powerpc, listed on the issue tracker.


To compile your library / application against steed, follow these steps:

DISCLAIMER steed has not achieved feature parity with std yet, so it's likely that your crate won't compile against steed. However, if your crate compiles against steed and then crashes or doesn't behave as expected at runtime, that's a bug and we would appreciate a bug report.

Using cross

To easiest way to use steed is to use the cross tool:

NOTE cross depends on Docker and only works on x86_64 Linux

# Always use the latest version
$ cargo install cross

# instead of this step, just go to the crate you want to build
$ cargo new --bin hello && cd $_

# Xargo magic to replace `std` with `steed`
# (if you want to run tests, fetch `Xargo.test.toml` instead of `Xargo.std.toml`)
$ curl -L https://raw.githubusercontent.com/japaric/steed/master/Xargo.std.toml > Xargo.toml
# NOTE `steed` uses its own set of targets; these have `steed` instead of `gnu`
# in their triples
$ cross run --target x86_64-unknown-linux-steed
Hello, world!

$ file target/x86_64-unknown-linux-steed/debug/hello
hello: ELF 64-bit LSB executable, x86-64, version 1 (SYSV),statically linked, not stripped, with debug_info

You can use cross to cross compile your crate to other architectures as well.

# continuation from the previous example
$ cross build --target aarch64-unknown-linux-steed

$ file target/aarch64-unknown-linux-steed/debug/hello
hello: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, not stripped, with debug_info

cross can even transparently execute those cross compiled binaries using QEMU. QEMU doesn't need to be installed on the host system.

$ cross run --target aarch64-unknown-linux-steed -v
       Fresh hello v0.1.0 (file:///project)
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `/target/aarch64-unknown-linux-steed/debug/hello`
Hello, world!

NOTE cross test works as well but you have to use the other Xargo.toml

Using lld

If you are not running x86_64 Linux or don't want to install / use Docker. You can compile steed programs using xargo and lld.

# This is for Ubuntu, adjust as necessary
$ sudo apt-get install lld-4.0

# Xargo v0.3.4 or newer is required
$ cargo install xargo

# OMITTED: fetching Xargo.toml

# Fetch the target definition
$ curl -LO https://raw.githubusercontent.com/japaric/steed/master/docker/x86_64-unknown-linux-steed.json

$ xargo run --target x86_64-unknown-linux-steed
Hello, world!

Cross compilation works out of the box; there's no need to install a cross C toolchain:

# fetch another target definition
$ curl -LO https://raw.githubusercontent.com/japaric/steed/master/docker/aarch64-unknown-linux-steed.json

$ xargo build --target aarch64-unknown-linux-steed

$ file target/aarch64-unknown-linux-steed/debug/examples/hello
hello: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, not stripped, with debug_info

To execute foreign binaries you can use QEMU:

# This is for Ubuntu, adjust as necessary
$ sudo apt-get install qemu-user

$ qemu-aarch64 target/aarch64-unknown-linux-steed/debug/examples/hello
Hello, world!

Using gcc

If you don't want to install lld, you can link steed programs using gcc, which you probably already have installed.

$ xargo rustc --target x86_64-unknown-linux-steed -- -C linker=gcc -Z linker-flavor=gcc

$ file target/x86_64-unknown-linux-steed/debug/hello
hello: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, BuildID[sha1]=ac4fce139edd9741b818cd73123be6c934718f78, not stripped, with debug_info

Current functionality

Check the API docs, but to summarize the functionality that interfaces with the Linux kernel:

  • Standard I/O (stdin, stdout, stderr)

  • File I/O

  • Filesystem operations (std::fs)

  • std::net: TCP, UDP. lookup_host is missing.

  • Dynamic memory allocation (thanks to ralloc!)

  • std::time

  • Minimal thread support.

Yup, that's all! I did say it was very early days, didn't I?


There's plenty of work to do (spoilers most of it is copy pasting code from the rust-lang/rust repo) and you can help! Check out the issue tracker. We have tagged the issues according to their difficulty from D-easy to D-hard. Each issue description has instructions on how to proceed but keep these general guidelines in mind:

  • We have an IRC channel on Mozilla network, #rust-steed, if you have any question!

  • We can't depend on the libc crate because that will make all our programs link to libc, libm, etc.

  • We still don't support the #[test] attribute so, if you add new functionality, please add a smoke test in the form of an example that exercises the new functionality to the examples directory and to the list in ci/script.sh.

  • Some functionality, like std::Path, is architecture independent. Re-implementing that functionality in steed is as simple as copy pasting std's code. When that happens, make sure to copy from a stable Rust release, e.g. 1.14.0, and to also add a comment indicates from which Rust version the code came from.

  • Try to mimic the layout of rust-lang/rust's src/libstd directory as much as possible. I expect that, except for the sys and linux modules, everything else will be very similar in layout and contents to the rust-lang/rust repo.

  • Keep all the code that directly interfaces with the Linux kernel in the private linux module.

  • Some code will involve constants / flags like EBADF or O_CLOEXEC. Get the values of those constants from the Linux kernel source code. Be careful with architecture dependent values! Look for O_CLOEXEC inside the linux module to see how those are handled.

# cargo install ripgrep
$ rg 'define[ \t]+\bO_CLOEXEC[ \t]+'
51:# define O_CLOEXEC           02000000

62:#define O_CLOEXEC    02000000        /* set close_on_exec */

20:#define O_CLOEXEC    0x400000

16:#define O_CLOEXEC    010000000 /* set close_on_exec */

17:#define O_CLOEXEC    010000000 /* set close_on_exec */
  • Some code will require doing system calls. Instead of directly using the syscall! macro, create a wrapper function to add type checking. As for the signature of the wrapper function, base it on the signature used in the Linux kernel:
$ rg '\bSYSCALL_DEFINE.\(open,'
1066:SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, umode_t, mode)

Would become:

// fs/open.c
pub unsafe fn open(filename: *const c_char, flags: c_int, mode: umode_t) { .. }
  • Some of those signatures will involve type aliases like umode_t. Get those from the Linux kernel source as well and add the type aliases to the linux module.
$ rg 'typedef.+umode_t'
18:typedef unsigned short               umode_t;
  • man 2 is your friend. Most system calls are documented in there. For example, man 2 write, documents the write system call.


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.