How do I specify the linker path in Rust?

Question

I'm trying to link a Rust program with libsoundio. I'm using Windows and there's a GCC binary download available. I can link it like this if I put it in the same folder as my project:

#[link(name = ":libsoundio-1.1.0/i686/libsoundio.a")]
#[link(name = "ole32")]
extern {
    fn soundio_version_string() -> *const c_char;
}

But I really want to specify #[link(name = "libsoundio")] or even #[link(name = "soundio")], and then provide a linker path somewhere else.

Where can I specify that path?

I tried the rustc-link-search suggestion as follows:

#[link(name = "libsoundio")]
#[link(name = "ole32")]
extern {
    fn soundio_version_string() -> *const c_char;
}

And in .cargo/config:

[target.i686-pc-windows-gnu.libsoundio]
rustc-link-search = ["libsoundio-1.1.0/i686"]
rustc-link-lib = ["libsoundio.a"]

[target.x86_64-pc-windows-gnu.libsoundio]
rustc-link-search = ["libsoundio-1.1.0/x86_64"]
rustc-link-lib = ["libsoundio.a"]

But it still only passes "-l" "libsoundio" to gcc and fails with the same ld: cannot find -llibsoundio. Am I missing something really obvious? The docs seem to suggest this should work.

Answer 1

As stated in the documentation for a build script:

All the lines printed to stdout by a build script [... starting] with cargo: is interpreted directly by Cargo [...] rustc-link-search indicates the specified value should be passed to the compiler as a -L flag.

In your Cargo.toml:

[package]
name = "link-example"
version = "0.1.0"
authors = ["An Devloper <an.devloper@example.com>"]
build = "build.rs"

And your build.rs:

fn main() {
    println!(r"cargo:rustc-link-search=C:\Rust\linka\libsoundio-1.1.0\i686");
}

Note that your build script can use all the power of Rust and can output different values depending on target platform (e.g. 32- and 64-bit).

Finally, your code:

extern crate libc;

use libc::c_char;
use std::ffi::CStr;

#[link(name = "soundio")]
extern {
    fn soundio_version_string() -> *const c_char;
}

fn main() {
    let v = unsafe { CStr::from_ptr(soundio_version_string()) };
    println!("{:?}", v);
}

The proof is in the pudding:

$ cargo run
    Finished debug [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target\debug\linka.exe`
"1.0.3"

---

Ideally, you will create a soundio-sys package, using the convention for *-sys packages. That simply has a build script that links to the appropriate libraries and exposes the C methods. It will use the Cargo links key to uniquely identify the native library and prevent linking to it multiple times. Other libraries can then include this new crate and not worry about those linking details.

Answer 2

Another possible way is setting the RUSTFLAGS like:

RUSTFLAGS='-L my/lib/location' cargo build # or cargo run

I don't know if this is the most organized and recommended approach, but it worked for my simple project.

Answer 3

I found something that works OK: you can specify links in your Cargo.toml:

[package]
links = "libsoundio"
build = "build.rs"

This specifies that the project links to libsoundio. Now you can specify the search path and library name in the .cargo/config file:

[target.i686-pc-windows-gnu.libsoundio]
rustc-link-search = ["libsoundio-1.1.0/i686"]
rustc-link-lib = [":libsoundio.a"]

[target.x86_64-pc-windows-gnu.libsoundio]
rustc-link-search = ["libsoundio-1.1.0/x86_64"]
rustc-link-lib = [":libsoundio.a"]

(The : prefix tells GCC to use the actual filename and not to do all its idiotic lib-prepending and extension magic.)

You also need to create an empty build.rs:

fn main() {}

This file is never run, because the values in .cargo/config override its output, but for some reason Cargo still requires it - any time you use links = you have to have build =, even if it isn't used.

Finally in main.rs:

#[link(name = "libsoundio")]
#[link(name = "ole32")]
extern {
    fn soundio_version_string() -> *const c_char;
}