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https://github.com/japaric/xargo

The sysroot manager that lets you build and customize `std`
https://github.com/japaric/xargo

bare-metal cargo cross-compilation no-std rust sysroot

Last synced: 7 days ago
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The sysroot manager that lets you build and customize `std`

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# [PSA: Xargo is in maintenance mode](https://github.com/japaric/xargo/issues/193)



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# `xargo`



> The sysroot manager that lets you build and customize `std`





Cross compiling `std` for i686-unknown-linux-gnu




Cross compiling `std` for i686-unknown-linux-gnu




Xargo builds and manages "sysroots" (cf. `rustc --print sysroot`). Making it

easy to cross compile Rust crates for targets that *don't* have binary

releases of the standard crates, like the `thumbv*m-none-eabi*` targets. And

it also lets you build a customized `std` crate, e.g. compiled with `-C

panic=abort`, for your target.



## Dependencies



- The `rust-src` component, which you can install with `rustup component add

  rust-src`.



- Rust and Cargo.



## Installation



```

$ cargo install xargo

```



## Usage



### `no_std`



`xargo` has the exact same CLI as `cargo`.



```

# This Just Works

$ xargo build --target thumbv6m-none-eabi

   Compiling core v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/libcore)

    Finished release [optimized] target(s) in 11.61 secs

   Compiling lib v0.1.0 (file://$PWD)

    Finished debug [unoptimized + debuginfo] target(s) in 0.5 secs

```



`xargo` will cache the sysroot, in this case the `core` crate, so the next

`build` command will be (very) fast.



```

$ xargo build --target thumbv6m-none-eabi

    Finished debug [unoptimized + debuginfo] target(s) in 0.0 secs

```



By default, `xargo` will only compile the `core` crate for the target. If you

need a bigger subset of the standard crates, specify the dependencies in a

`Xargo.toml` at the root of your Cargo project (right next to `Cargo.toml`).



```

$ cat Xargo.toml

# Alternatively you can use [build.dependencies]

# the syntax is the same as Cargo.toml's; you don't need to specify path or git

[target.thumbv6m-none-eabi.dependencies]

collections = {}



$ xargo build --target thumbv6m-none-eabi

   Compiling core v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/libcore)

   Compiling alloc v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/liballoc)

   Compiling std_unicode v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/libstd_unicode)

   Compiling collections v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/libcollections)

    Finished release [optimized] target(s) in 15.26 secs

   Compiling lib v0.1.0 (file://$PWD)

    Finished debug [unoptimized + debuginfo] target(s) in 0.5 secs

```



### `std`



You can compile a customized `std` crate as well, just specify which Cargo

features to enable.



```

# Build `std` with `-C panic=abort` (default) and with jemalloc as the default

# allocator

$ cat Xargo.toml

[target.i686-unknown-linux-gnu.dependencies.std]

features = ["jemalloc"]



# Needed to compile `std` with `-C panic=abort`

$ tail -n2 Cargo.toml

[profile.release]

panic = "abort"



$ xargo run --target i686-unknown-linux-gnu --release

    Updating registry `https://github.com/rust-lang/crates.io-index`

   Compiling libc v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/rustc/libc_shim)

   Compiling core v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/libcore)

   Compiling build_helper v0.1.0 (file://$SYSROOT/lib/rustlib/src/rust/src/build_helper)

   Compiling gcc v0.3.41

   Compiling unwind v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/libunwind)

   Compiling std v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/libstd)

   Compiling compiler_builtins v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/libcompiler_builtins)

   Compiling alloc_jemalloc v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/liballoc_jemalloc)

   Compiling alloc v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/liballoc)

   Compiling rand v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/librand)

   Compiling std_unicode v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/libstd_unicode)

   Compiling alloc_system v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/liballoc_system)

   Compiling panic_abort v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/libpanic_abort)

   Compiling collections v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/libcollections)

    Finished release [optimized] target(s) in 33.49 secs

   Compiling hello v0.1.0 (file://$PWD)

    Finished release [optimized] target(s) in 0.28 secs

     Running `target/i686-unknown-linux-gnu/release/hello`

Hello, world!

```



If you'd like to know what `xargo` is doing under the hood, pass the verbose,

`-v`, flag to it.



```

$ xargo build --target thumbv6m-none-eabi -v

+ "rustc" "--print" "target-list"

+ "rustc" "--print" "sysroot"

+ "cargo" "build" "--release" "--manifest-path" "/tmp/xargo.lTBXKnaUGicV/Cargo.toml" "--target" "thumbv6m-none-eabi" "-v" "-p" "core"

   Compiling core v0.0.0 (file://$SYSROOT/lib/rustlib/src/rust/src/libcore)

     Running `rustc --crate-name core $SYSROOT/lib/rustlib/src/rust/src/libcore/lib.rs --crate-type lib -C opt-level=3 -C metadata=a5c596f87f7d486b -C extra-filename=-a5c596f87f7d486b --out-dir /tmp/xargo.lTBXKnaUGicV/target/thumbv6m-none-eabi/release/deps --emit=dep-info,link --target thumbv6m-none-eabi -L dependency=/tmp/xargo.lTBXKnaUGicV/target/thumbv6m-none-eabi/release/deps -L dependency=/tmp/xargo.lTBXKnaUGicV/target/release/deps`

    Finished release [optimized] target(s) in 11.50 secs

+ "cargo" "build" "--target" "thumbv6m-none-eabi" "-v"

   Compiling lib v0.1.0 (file://$PWD)

     Running `rustc --crate-name lib src/lib.rs --crate-type lib -g -C metadata=461fd0b398821543 -C extra-filename=-461fd0b398821543 --out-dir $PWD/target/thumbv6m-none-eabi/debug/deps --emit=dep-info,link --target thumbv6m-none-eabi -L dependency=$PWD/target/thumbv6m-none-eabi/debug/deps -L dependency=$PWD/lib/target/debug/deps --sysroot $HOME/.xargo`

    Finished debug [unoptimized + debuginfo] target(s) in 0.5 secs

```



### Dev channel



Oh, and if you want to use `xargo` to compile `std` using a "dev" `rustc`, a

rust compiled from source, you can use the `XARGO_RUST_SRC` environment variable

to tell `xargo` where the Rust source is.



```

# `$XARGO_RUST_SRC` must point to the `library` subfolder of a Rust checkout.

$ export XARGO_RUST_SRC=/path/to/rust/library



$ xargo build --target msp430-none-elf

```



**NOTE** This also works with the nightly channel but it's not recommended as

the Rust source may diverge from what your compiler is able to compile as it may

make use of newer features that your compiler doesn't understand.



### Compiling the sysroot with custom rustc flags



Xargo uses the same custom rustc flags that apply to the target Cargo project.

So you can use either the `RUSTFLAGS` env variable or a `.cargo/config`

configuration file to specify custom rustc flags.



```

# build the sysroot with debug information

$ RUSTFLAGS='-g' xargo build --target x86_64-unknown-linux-gnu



# Alternatively

$ edit .cargo/config && cat $_

[build]

rustflags = ["-g"]



# Then you can omit RUSTFLAGS

$ xargo build --target x86_64-unknown-linux-gnu

```



### Compiling the sysroot for a custom target



At some point you may want to develop a program for a target that's not

officially supported by rustc. Xargo's got your back! It supports custom targets

via target specifications files, which are not really documented anywhere other

than in the [compiler source code][spec-docs]. Luckily you don't need to write

a specification file from scratch; you can start from an existing one.



[spec-docs]: https://github.com/rust-lang/rust/blob/256e497fe63bf4b13f7c0b58fa17360ca849c54d/src/librustc_back/target/mod.rs#L228-L409



For example, let's say that you want to cross compile a program for a PowerPC

Linux systems that uses uclibc instead of glibc. There's a similarly looking

target in the list of targets supported by the compiler -- see `rustc --print

target-list` -- and that is `powerpc-unknown-linux-gnu`. So you can start by

dumping the specification of that target into a file:



``` console

$ rustc -Z unstable-options --print target-spec-json --target powerpc-unknown-linux-gnu | tee powerpc-unknown-linux-uclibc.json

```



``` js

{

  "arch": "powerpc",

  "data-layout": "E-m:e-p:32:32-i64:64-n32",

  "dynamic-linking": true,

  "env": "gnu",

  "executables": true,

  "has-elf-tls": true,

  "has-rpath": true,

  "is-builtin": true,

  "linker-flavor": "gcc",

  "linker-is-gnu": true,

  "llvm-target": "powerpc-unknown-linux-gnu",

  "max-atomic-width": 32,

  "os": "linux",

  "position-independent-executables": true,

  "pre-link-args": {

    "gcc": [

      "-Wl,--as-needed",

      "-Wl,-z,noexecstack",

      "-m32"

    ]

  },

  "target-endian": "big",

  "target-family": "unix",

  "target-pointer-width": "32",

  "vendor": "unknown"

}

```



One of the things you'll definitively want to do is drop the `is-builtin` field

as that's reserved for targets that are defined in the compiler itself. Apart

from that the only modification you would have to in this case is change the

`env` field from `gnu` (glibc) to `uclibc`.



``` diff

   "arch": "powerpc",

   "data-layout": "E-m:e-p:32:32-i64:64-n32",

   "dynamic-linking": true,

-  "env": "gnu",

+  "env": "uclibc",

   "executables": true,

   "has-elf-tls": true,

   "has-rpath": true,

-  "is-builtin": true,

   "linker-flavor": "gcc",

   "linker-is-gnu": true,

   "llvm-target": "powerpc-unknown-linux-gnu",

```



Once you have your target specification file you only have to call Xargo with

the right target triple; make sure that the specification file is the same

folder from where you invoke Xargo because that's where rustc expects it to be.



``` console

$ ls powerpc-unknown-linux-uclibc.json

powerpc-unknown-linux-uclibc.json



$ xargo build --target powerpc-unknown-linux-uclibc

```



Your build may fail because if rustc doesn't support your target then it's

likely that the standard library doesn't support it either. In that case you

will have to modify the source of the standard library. Xargo helps with that

too because you can make a copy of the original source -- see `rustc --print

sysroot`, modify it and then point Xargo to it using the `XARGO_RUST_SRC` env

variable.



### Multi-stage builds



Some standard crates have implicit dependencies between them. For example, the

`test` crate implicitly depends on the `std`. Implicit here means that the test

crate Cargo.toml [doesn't list std as its dependency][test]. To compile a

sysroot that contains such crates you can perform the build in stages by

specifying which crates belong to each stage in the Xargo.toml file:



[test]: https://github.com/rust-lang/rust/blob/1.17.0/src/libtest/Cargo.toml



``` toml

[dependencies.std]

stage = 0



[dependencies.test]

stage = 1

```



This will compile an intermediate sysroot, the stage 0 sysroot, containing the

`std` crate, and then it will compile the `test` crate against that intermediate

sysroot. The final sysroot, the stage 1 sysroot, will contain both the `std` and

`test` crates, and their dependencies.



### Creating a sysroot with custom crates



Xargo lets you create a sysroot with custom crates. You can virtually put any

crate in the sysroot. However, this feature is mainly used to create [alternative

`std` facades][rust-3ds], and to replace the `test` crate with [one that supports

`no_std` targets][utest]. To specify the contents of the sysroot simply list the

dependencies in the Xargo.toml file as you would do with Cargo.toml:



[steed]: https://github.com/rust3ds/rust3ds-template

[utest]: https://github.com/japaric/utest



``` toml

# First build some standard crates.

[dependencies.alloc]

[dependencies.panic_abort]

[dependencies.panic_unwind]



# Then build our custom facade. It (implicitly) requires the crates above to

# already be in the sysroot, so we need to set the `stage`.

[dependencies.std]

git = "https://github.com/rust3ds/ctru-rs"

stage = 1

```



### Patching sysroot crates



Xargo also supports the `patch` feature from Cargo. This allows you to force the use

of a custom crate throughout your sysroot's dependency tree. This can be especially

useful to force the use of a custom `libc` or `compiler_builtins` without having to

do intrusive changes to every transitive dependency.



``` toml

[patch.crates-io.libc]

path = "path/to/custom/libc"

```



Notice that you should not list patched crates as `[dependencies]`!

`[dependencies]` determines which crates are built in the first place; `[patch]`

lets you replace some of their (transitive) dependencies with your own choice.

Having a crate listed in both will likely lead to crate duplication.



### Check-only sysroot build



Xargo supports performing a 'check build' of the syroot

via the `xargo-check` command. This command is invoked exactly

like `xargo`, but will invoke `cargo check` instead of `cargo build`

when building the sysroot.



This is only useful for very specialized applicationsm like Miri.

The resulting libstd will *not* be useable in a normal build, since codegen

will not be performed. You should almost always run `xargo check` (note the space),

which will perform a normal sysroot build, followed by a 'check' build of *your application*



## Caveats / gotchas



- Xargo won't build a sysroot when used with stable or beta Rust. This is

  because `std` and other standard crates depend on unstable features so it's

  not possible to build the sysroot with stable or beta.



- `std` is built as rlib *and* dylib. The dylib needs a panic library and an

  allocator.  If you do not specify the `panic-unwind` feature, you have to set

  `panic = "abort"` in `Cargo.toml`.



- To build without the `jemalloc` feature include the following in `Xargo.toml`:



  ``` toml

  [dependencies.std]

  features = ["force_alloc_system"]

  ```



  What this flag means is that every program compiled with this libstd can only use the system

  allocator. If your program tries to set its own allocator, compilation will fail because now two

  allocators are set (one by libstd, one by your program). For some further information on this

  issue, see

  [rust-lang/rust#43637](https://github.com/rust-lang/rust/issues/43637#issuecomment-320463578).



- It's recommended that the `--target` option is always used for `xargo`. This is because it must

  be provided even when compiling for the host platform due to the way cargo handles compiler

  plugins (e.g. `serde_derive`) and build scripts (`build.rs`). This also applies to how all of the

  dependant crates get compiled that use compiler plugins or build scripts. You can determine your

  host's target triple with `rustc -vV`. On *nix, the following rune will extract the triple:

  `rustc -vV | egrep '^host: ' | sed 's/^host: //'`.



- Remember that `core` and `std` will get implicitly linked to your crate but *all the other sysroot

  crates* will *not*. This means that if your Xargo.toml contains a crate like `alloc` then you will

  have to add a `extern crate alloc` *somewhere* in your dependency graph (either in your current

  crate or in some of its dependencies).



- Remember that rustc will always implicitly link `compiler_builtins` into your final binary, but

  won't make it available for `use` the same way `core` and `std` are. So if you need to manually

  call a `compiler_builtins` function, you will still need to manually add an

  `extern crate compiler_builtins` within your crate.



- Care must be taken not to end up with any "top-level" crates (`core`, `std`, `compiler-builtins`)

  twice in the sysroot. Doing so will cause cargo to error on build with a message like

  `multiple matching crates for core`. Duplicate crates in the sysroot generally occur when the same

  crate is built twice with different features as part of a multi-stage build.



## License



Licensed under either of



- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or

  http://www.apache.org/licenses/LICENSE-2.0)

- MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)



at your option.



### Contribution



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.