Linking Rust application with a dynamic library not in the runtime linker search path

Question

I have a shared library that I'd like to dynamically link into several separate binary Cargo applications. I include its location in the linker using the -- -L /path/to/dir format and the application compiles correctly with the significant decrease in binary size I expect. However, when checking the generated binary using ldd, I get a message saying that the library couldn't be found:

casey@Gilthar-II:~/bot4/backtester/target/release$ ldd backtester 
    linux-vdso.so.1 =>  (0x00007ffc642f7000)
    libalgobot_util.so => not found

If I add the library to the /lib/x86_64-linux-gnu directory, the application runs without issue.

Is there a way to get Rust to look for .so files in the same directory as the binary or in a directory like lib in the binary's directory to be loaded at runtime? If that's not possible, is there a way to at least get Rust to insert the absolute path of the library it was linked with?

I've tried setting rpath = true with no effect.

Answer 1

Here's a complete solution...

I created a C library exporting a simple addition function. I also created a Cargo project to use this function.

/scratch
├── executable
│   ├── Cargo.lock
│   ├── Cargo.toml
│   ├── build.rs
│   ├── src
│   │   ├── main.rs
└── library
    ├── awesome_math.c
    └── libawesome_math.so

awesome_math.c

#include <stdint.h>

uint8_t from_the_library(uint8_t a, uint8_t b) {
  return a + b;
}

The library was compiled as gcc -g -shared awesome_math.c -o libawesome_math.so.

src.rs

extern crate libc;

extern {
    fn from_the_library(a: libc::uint8_t, b: libc::uint8_t) -> libc::uint8_t;
}

fn main() {
    unsafe {
        println!("Adding: {}", from_the_library(1, 2));
    }
}

build.rs

fn main() {
    println!("cargo:rustc-link-lib=dylib=awesome_math");
    println!("cargo:rustc-link-search=native=/scratch/library");
}

Cargo.toml

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

[dependencies]
libc = "*"

[profile.dev]
rpath = true

Doing all of this exhibits the same problem that you experienced. This is called a Minimal, Complete, and Verifiable Example and you should provide one when asking a question. If this were provided, this answer might have been created 12 hours earlier.

Investigating further, I asked the Rust compiler to print out the linker args it was going to use:

cargo rustc -- -Z print-link-args

This printed out a bunch of stuff, but the two important lines were

"-Wl,-rpath,$ORIGIN/../../../../root/.multirust/toolchains/stable-2016-11-08-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib"
"-Wl,-rpath,/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib"

These are directives to the linker to add specific values to the rpath of the finished binary. Missing is any reference to the dynamic library that we are linking to. In retrospect, this probably makes sense, as how would the compiler know that we want to include it in the rpath?

A workaround is to add another directive to the linker. There are interesting options (like $ORIGIN), but for simplicity, we will just use an absolute path:

cargo rustc -- -C link-args="-Wl,-rpath,/scratch/library/"

And the resulting binary prints the right thing for ldd and runs without setting LD_LIBRARY_PATH:

$ ldd target/debug/executable | grep awesome
    libawesome_math.so => /scratch/library/libawesome_math.so (0x00007fe859085000)
$ ./target/debug/executable
Adding: 3

---

Turning to making it relative, we can use $ORIGIN:

cargo rustc -- -C link-args='-Wl,-rpath,$ORIGIN/../../../library/'

Be careful to escape $ORIGIN properly for your shell, and remember that the path is relative to the executable, not the current working directory.

Answer 2

Adding to what Shepmaster said (apparently I don't have enough reputation to comment): I'm not sure when this feature was added, but as of at least Rust 1.20, you can achieve the same effect by setting the environment variable RUSTFLAGS:

$ RUSTFLAGS="-C link-args=-Wl,-rpath,/the/lib/path" cargo build

This can be more convenient than the cargo rustc option if, for instance, you're using build scripts that just invoke cargo build.