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scripts Initial commit May 31, 2015 SUPERSEDED by Feb 6, 2016


This document has been SUPERSEDED by rust-cross, an extensive cross compilation guide.

This document won't receive updates or fixes in the case it has wrong information.

P.S. There is some wrong information here. The correct triple for OpenWRT 15.05 and older is mips-unknown-linux-uclibc (which is not currently a supported target) and not mips-unknown-linux-gnu. Now because there is some (or many?) ABI similarities between uClibc and glibc, some programs cross compiled to mips-unknown-linux-uclibc may work on OpenWRT but not all programs are guaranteed to work.

P.P.S. The trunk version of OpenWRT, which is covered by the triple mips-unknown-linux-musl, is a supported target.

-- @japaric, 2016/02/05

Rust on OpenWRT (MIPS edition)

This how-to covers:

  • Setting up a cross-compilation environment
  • Cross compiling a "Hello, world!" Rust program
  • Configuring cargo for cross-compilation

Although this how-to uses a MIPS based router as the target device, the steps outlined here should be applicable to other targets/architectures.

Cross compilation requirements

In general to cross compile Rust programs you need four things:

  • Know what's the rustc target triple for your device, e.g. arm-unknown-linux-gnueabi or mips-unknown-linux-gnu.
  • A gcc cross-compiler, because rustc uses gcc as a linker
  • Cross compiled C dependencies (libraries) that will be linked to your program, at the very least libc
  • Rust dependencies (crates) that will be linked to your program, most likely the std crate will be one of them.

Once you have all those, cross compiling is as easy as passing --target=$TRIPLE to rustc.

We can get the first three things from the OpenWRT SDK, so let's install that.

Installing the OpenWRT SDK

If you already have the SDK installed in your system, you can skip this section.

The SDK can be downloaded from, but you need to know which OpenWRT release is running on your device and what's your device "codename". You can find this information by looking at the /etc/openwrt_release file in your OpenWRT device:

# On your OpenWRT device
$ cat /etc/openwrt_release
DISTRIB_RELEASE="14.07"  # <-- this is the release
DISTRIB_TARGET="ar71xx/generic"  # <-- this is the codename
DISTRIB_DESCRIPTION="OpenWrt Barrier Breaker 14.07"

The SDK for your device will be under the folder $RELEASE/$CODENAME of the download website. In my case the full URL to the right SDK is:

After downloading the SDK, extract it using tar.

$ pwd

$ ls *.tar.bz2

$ tar jxf *.tar.bz2 --strip-components=1

Verifying that the SDK works

To verify that you got the right SDK, we'll compile a "Hello, world!" C program, and run it on the OpenWRT device.

When working with the OpenWRT SDK you'll need to set these two environment variables, and be sure to keep them in your environment for the rest of this how-to.

# Make sure you are in the OpenWRT SDK folder
$ pwd

$ export STAGING_DIR="$PWD/staging_dir"

$ export PATH="$PWD/$(echo staging_dir/toolchain-*/bin):$PATH"

You should now be able to call the cross compiler, which should be in your PATH:

$ mips-openwrt-linux-gcc -v
gcc version 4.8.3 (OpenWrt/Linaro GCC 4.8-2014.04 r42625)

Now let's compile a "Hello, world!" C program:

$ cat hello.c
#include <stdio.h>

int main() {
    printf("Hello, world!");

$ mips-openwrt-linux-gcc hello.c

$ file a.out
a.out: ELF 32-bit MSB executable, MIPS, MIPS32 rel2 version 1, dynamically linked, interpreter /lib/, not stripped

Let's test this program on the OpenWRT device:

$ scp a.out root@openwrt:~

$ ssh root@openwrt ./a.out
Hello, world!

So far, so good.

The SDK contains the toolchain, (uC)libc and other C libraries cross compiled for the target device. Now we must find out...

What's the rustc target triple for my device?

The easiest way to get the target triple for your device is to look at the prefix of the OpenWRT toolchain and "translate" that to a triple that rustc understands. In my case, the prefix is mips-openwrt-linux-(gcc), this means that the rustc target triple for my device is mips-unknown-linux-gnu.

Here's a "dictionary" for other toolchains prefixes:

# Toolchain prefix                -> `rustc` target triple
arm-openwrt-linux-uclibcgnueabi-  -> arm-unknown-linux-gnueabi
mips-openwrt-linux-               -> mips-unknown-linux-gnu
mipsel-openwrt-linux-             -> mipsel-unknown-linux-gnu

And here's a list of all the triples that rustc supports (as of 1.0.0).

Getting a cross-compiled std crate

Most Rust programs depend on the std crate, so we'll need a version of std that has been cross compiled for the mips-unknown-linux-gnu target, that's our last requirement.

There are two ways to get the std crate:

  • You can compile it yourself from rust source, or

  • You can use one of my pre-compiled versions.

The first option is the sure way to get a std crate that will work on your device, but is also the most time-consuming. On the other hand, the second option is the easiest but may not work for your device (because it was compiled for a specific device).

In this how-to we'll pick the second route, if that doesn't work for you or if you want to try the other route, then check the scripts folder for more information about how to cross compile the std crate from source.

UPDATE: Another way to get a cross compiled std crate is to compile it yourself using cargo and the rustc you already have installed, this is way faster that using the Rust build system, it takes less than one minute. Check this repository for more information.

You can get the pre-compiled crates from here.

It's very important that the rustc version that you have installed in your host matches the version of the cross-compiled crates that you will download. In this how-to we'll use the 1.0.0 version of rustc. So make sure your rustc version is the 1.0.0 one:

rustc 1.0.0 (a59de37e9 2015-05-13) (built 2015-05-14)

Next fetch the 1.0.0 version of the cross compiled crates:

$ wget $SOME_URL/1.0.0/rust-$DATE-$HASH-mips-unknown-linux-gnu-$HASH.tar.gz

You'll need to extract the tarball in the rustlib directory of your Rust distribution. For users that would be the /usr/local/lib/rustlib path, and for multirust users that would be the ~/.multirust/toolchains/1.0.0/lib/rustlib path.

Ultimately your lib folder should look like this:

# I'm using multirust, use the right path for your setup
$ tree ~/.multirust/toolchains/1.0.0/lib
├── (..)
└── rustlib
    ├── mips-unknown-linux-gnu  <- this the folder that you just extracted
    │   └── lib
    │       ├── libarena-4e7c5e5c.rlib
    │       └── (..)
    └── x86_64-unknown-linux-gnu  <- this is part of the original distribution
        └── lib
            ├── libarena-4e7c5e5c.rlib
            └── (..)

Hello, Rust!

Alright, after a very long setup, we can finally cross compile a "Hello, world!" Rust program.

$ cat
fn main() {
    println!("Hello, world!");

I mentioned in the requirements that rustc will use gcc as linker when compiling programs, so we'll need to tell rustc what's the right gcc to use when cross compiling, otherwise it will, by default, use the cc linker and fail spectacularly:

$ rustc --target=mips-unknown-linux-gnu -C linker=mips-openwrt-linux-gcc
$ file hello
hello: ELF 32-bit MSB shared object, MIPS, MIPS32 rel2 version 1 (SYSV), dynamically linked, interpreter /lib/, not stripped

Now, let's test the binary on the OpenWRT device. You may need to install libpthread and librt on OpenWRT your device if you don't have them installed:

$ scp hello root@openwrt:~
$ ssh root@openwrt

# On the OpenWRT device
$ opkg install libpthread
$ opkg install librt
$ ./hello
Hello, world!

It's interesting to compare the shared libraries required by the Rust program vs the ones required by the C program. You can do this using the LD_TRACE_LOADED_OBJECTS environment variable:

# On the OpenWRT device
$ LD_TRACE_LOADED_OBJECTS=1 ./a.out => /lib/ (0x77426000) => /lib/ (0x773b9000) => /lib/ (0x7744a000)

$ LD_TRACE_LOADED_OBJECTS=1 ./hello => /lib/ (0x77330000) => /lib/ (0x7730a000) => /lib/ (0x772f6000) => /lib/ (0x772d2000) => /lib/ (0x77265000) => /lib/ (0x77344000) => /lib/ (0x7723f000)

NOTE: If you compile with -C lto, the binary won't depend on libm or librt.

Cargo all the things

For non-toy programs, you'll want to use cargo to handle your program dependencies and the multiple rustc calls required to build it.

Just like with rustc, to cross compile you just need to pass the --target=$TRIPLE flag to cargo, but there is one extra thing that we must do before it just works.

By default, cargo will use cc as linker and ar as archiver for native and cross compilation. We'll have to instruct cargo to use the right prefixed tools for cross compilation; that's done with a config file.

$ cat ~/.cargo/config
ar = "mips-openwrt-linux-ar"
linker = "mips-openwrt-linux-gcc"

Now we can use cargo to cross compile.

$ cargo new --bin hello
$ cd hello
$ cargo build --target=mips-unknown-linux-gnu
Compiling hello v0.1.0 (file:///home/japaric/tmp/hello)

The final binary will be under the target/mips-unknown-linux-gnu/debug folder.

$ file target/mips-unknown-linux-gnu/debug/hello
target/mips-unknown-linux-gnu/debug/hello: ELF 32-bit MSB shared object, MIPS, MIPS32 rel2 version 1 (SYSV), dynamically linked, interpreter /lib/, not stripped

Finally, let's check that the binary actually works:

$ scp target/mips-unknown-linux-gnu/debug/hello root@openwrt:~
$ ssh root@openwrt ./hello
Hello, world!

Reducing binary size

Rust binaries can be quite large, here are some ways to make them smaller.

Increase optimization level

With rustc: pass the -O or --opt-level=3 flag

With cargo: build in release mode cargo build --target=$TRIPLE --release

Use Link Time Optimization (LTO)

With rustc: pass the -C lto flag

With cargo, you'll need to enable it per profile:

// Cargo.toml
lto = true

Enabling LTO greatly increases build time, so I recommend using it only for release, and keeping it disabled during development for faster edit-compile-test loops.

Strip symbol information

With rustc: pass the -C link-args=-s flag

With cargo: use cargo rustc instead of cargo build, example:

cargo rustc --target=$TRIPLE --release -- -C link-args=-s

Remove non-essential features

If you are cross compiling the std crate using cargo, then you can disable RUST_BACKTRACE support or switch to malloc (instead of jemalloc), this can easily reduce binary sizes by 200KB. Check this repository for more information about cross compiling the std crate using cargo.

That's all for this how-to, happy cross compiling!