"Zero setup" cross compilation and "cross testing" of Rust crates
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"Zero setup" cross compilation and "cross testing" of Rust crates

This project is developed and maintained by the Tools team.

`cross test`ing a crate for the aarch64-unknown-linux-gnu target
`cross test`ing a crate for the aarch64-unknown-linux-gnu target

Disclaimer: Only works on a x86_64 Linux host (e.g. Travis CI is supported)


  • cross will provide all the ingredients needed for cross compilation without touching your system installation.

  • cross provides an environment, cross toolchain and cross compiled libraries (e.g. OpenSSL), that produces the most portable binaries.

  • "cross testing", cross can test crates for architectures other than i686 and x86_64.

  • The stable, beta and nightly channels are supported.


  • A Linux kernel with binfmt_misc support is required for cross testing.


$ cargo install cross


cross has the exact same CLI as Cargo but as it relies on Docker you'll have to start the daemon before you can use it.

# Start the Docker daemon, if it's not already running
$ sudo systemctl start docker

# MAGIC! This Just Works
$ cross build --target aarch64-unknown-linux-gnu

# EVEN MORE MAGICAL! This also Just Works
$ cross test --target mips64-unknown-linux-gnuabi64

# Obviously, this also Just Works
$ cross rustc --target powerpc-unknown-linux-gnu --release -- -C lto


You can place a Cross.toml file in the root of your Cargo project to tweak cross's behavior:

Custom Docker images

The default Docker image that cross uses provides a C environment that tries to cover the most common cross compilation cases. However, it can't cover every single use case out there. When the default image is not enough, you can use the target.$TARGET.image field in Cross.toml to use custom Docker image for a specific target:

image = "my/image:tag"

In the example above, cross will use a image named my/image:tag instead of the default one. Normal Docker behavior applies, so:

  • Docker will first look for a local image named my/image:tag

  • If it doesn't find a local image, then it will look in Docker Hub.

  • If only image:tag is specified, then Docker won't look in Docker Hub.

  • If only tag is omitted, then Docker will use the latest tag.

It's recommended to base your custom image on the default Docker image that cross uses: japaric/$TARGET:$VERSION (where $VERSION is cross's version). This way you won't have to figure out how to install a cross C toolchain in your custom image. Example below:

FROM japaric/aarch64-unknown-linux-gnu:v0.1.4

RUN dpkg --add-architecture arm64 && \
    apt-get update && \
    apt-get install libfoo:arm64
$ docker build -t my/image:tag path/to/where/the/Dockerfile/resides

Passing environment variables into the build environment

By default, cross does not pass any environment variables into the build environment from the calling shell. This is chosen as a safe default as most use cases will not want the calling environment leaking into the inner execution environment.

In the instances that you do want to pass through environment variables, this can be done via build.env.passthrough in your Cross.toml:

passthrough = [

To pass variables through for one target but not others, you can use this syntax instead:

passthrough = [

Use Xargo instead of Cargo

By default, cross uses cargo to build your Cargo project unless you are building for one of the thumbv*-none-eabi* targets; in that case, it uses xargo. However, you can use the build.xargo or target.$TARGET.xargo field in Cross.toml to force the use of xargo:

# all the targets will use `xargo`
xargo = true


# only this target will use `xargo`
xargo = true

Note that xargo = false has no effect as you can't use cargo with targets that only support xargo.

Supported targets

A target is considered as "supported" if cross can cross compile a "non-trivial" (binary) crate, usually Cargo, for that target.

Testing support is more complicated. It relies on QEMU user emulation, so testing may sometimes fail due to QEMU bug sand not because there's a bug in the crate. That being said, cross test is assumed to "work" (test column in the table below) if it can successfully run compiler-builtins test suite.

Also, testing is very slow. cross will actually run units tests sequentially because QEMU gets upset when you spawn several threads. This also means that, if one of your unit tests spawns several threads then it's more likely to fail or, worst, "hang" (never terminate).

Target libc GCC OpenSSL C++ QEMU test
aarch64-linux-android [5] N/A 4.9 1.0.2m N/A
aarch64-unknown-linux-gnu 2.19 4.8.2 1.0.2m 2.8.0
arm-linux-androideabi [5] N/A 4.9 1.0.2m N/A
arm-unknown-linux-gnueabi 2.19 4.8.2 1.0.2m 2.8.0
arm-unknown-linux-musleabi 1.1.15 5.3.1 N/A 2.8.0
armv7-linux-androideabi [5] N/A 4.9 1.0.2m N/A
armv7-unknown-linux-gnueabihf 2.15 4.6.2 1.0.2m 2.8.0
armv7-unknown-linux-musleabihf 1.1.15 5.3.1 N/A 2.8.0
asmjs-unknown-emscripten [4] 1.1.15 1.37.13 N/A N/A
i586-unknown-linux-gnu 2.23 5.3.1 1.0.2m N/A
i686-linux-android [5] N/A 4.9 1.0.2m N/A
i686-pc-windows-gnu N/A 6.2.0 N/A N/A
i686-unknown-freebsd [1] 10.2 5.3.0 1.0.2m N/A
i686-unknown-linux-gnu 2.15 4.6.2 1.0.2m N/A
i686-unknown-linux-musl 1.1.15 5.3.1 1.0.2m N/A
mips-unknown-linux-gnu 2.23 5.3.1 1.0.2m 2.8.0
mips64-unknown-linux-gnuabi64 2.23 5.3.1 1.0.2m 2.8.0
mips64el-unknown-linux-gnuabi64 2.23 5.3.1 1.0.2m 2.8.0
mipsel-unknown-linux-gnu 2.23 5.3.1 1.0.2m 2.8.0
powerpc-unknown-linux-gnu 2.19 4.8.2 1.0.2m 2.7.1
powerpc64-unknown-linux-gnu 2.19 4.8.2 1.0.2m 2.7.1
powerpc64le-unknown-linux-gnu 2.19 4.8.2 1.0.2m 2.7.1
s390x-unknown-linux-gnu 2.23 5.3.1 1.0.2m 2.8.0
sparc64-unknown-linux-gnu [2] 2.23 5.3.1 1.0.2m 2.8.0
sparcv9-sun-solaris [1] 2.11 5.3.0 1.0.2m N/A
thumbv6m-none-eabi [3] 2.2.0 5.3.1 N/A N/A
thumbv7em-none-eabi [3] 2.2.0 5.3.1 N/A N/A
thumbv7em-none-eabihf [3] 2.2.0 5.3.1 N/A N/A
thumbv7m-none-eabi [3] 2.2.0 5.3.1 N/A N/A
wasm32-unknown-emscripten [4] 1.1.15 1.37.13 N/A N/A
x86_64-linux-android [5] N/A 4.9 1.0.2m N/A
x86_64-pc-windows-gnu N/A 6.2.0 N/A N/A
x86_64-sun-solaris [1] 2.11 5.3.0 1.0.2m N/A
x86_64-unknown-dragonfly [1] [2] 4.6.0 5.3.0 1.0.2m N/A
x86_64-unknown-freebsd [1] 10.2 5.3.0 1.0.2m N/A
x86_64-unknown-linux-gnu 2.15 4.6.2 1.0.2m N/A
x86_64-unknown-linux-musl 1.1.15 5.3.1 1.0.2m N/A
x86_64-unknown-netbsd[1] 7.0 5.3.0 1.0.2m N/A

[1] For *BSD and Solaris targets, the libc column indicates the OS release version from where libc was extracted.

[2] No std component available as of 2017-01-10

[3] libc = newlib

[4] libc = musl, gcc = emcc; Some projects that use libc may fail due to wrong definitions (will be fixed by https://github.com/rust-lang/libc/pull/610)

[5] Only works with native tests, that is, tests that do not depends on the Android Runtime. For i686 some tests may fails with the error assertion failed: signal(libc::SIGPIPE, libc::SIG_IGN) != libc::SIG_ERR, see issue #140 for more information.


QEMU_STRACE (v0.1.9+)

You can set the QEMU_STRACE variable when you use cross run to get a backtrace of system calls from "foreign" (non x86_64) binaries.

$ cargo new --bin hello && cd $_

$ QEMU_STRACE=1 cross run --target aarch64-unknown-linux-gnu
9 brk(NULL) = 0x0000004000023000
9 uname(0x4000823128) = 0
9 write(1,0xa06320,14)Hello, world!
 = 14
9 sigaltstack(0x4000823588,(nil)) = 0
9 munmap(0x0000004000b16000,16384) = 0
9 exit_group(0)

Caveats / gotchas

  • path dependencies (in Cargo.toml) that point outside the Cargo project won't work because cross use docker containers only mounts the Cargo project so the container doesn't have access to the rest of the filesystem.

  • cross will mount the Cargo project as READ ONLY. Thus, if any crate attempts to modify its "source", the build will fail. Well behaved crates should only ever write to $OUT_DIR and never modify $CARGO_MANIFEST_DIR though.


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.

Code of Conduct

Contribution to this crate is organized under the terms of the Rust Code of Conduct, the maintainer of this crate, the Tools team, promises to intervene to uphold that code of conduct.