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

Here's a Minimal, Reproducible Example that exhibits the same problem that you experienced. I created a C library exporting a simple addition function. I also created a Cargo project to use this function.

dynlink/
├── executable
│   ├── build.rs
│   ├── Cargo.lock
│   ├── Cargo.toml
│   └── 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/main.rs

extern {
    fn from_the_library(a: u8, b: u8) -> u8;
}

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=/home/shep/rust/dynlink/library");
}

Cargo.toml

[package]
name = "executable"
version = "0.1.0"
edition = "2021"

[profile.dev]
rpath = true

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

cargo rustc -- --print link-args

This printed out a bunch of stuff, but an important line was:

"-Wl,-rpath,$ORIGIN/../../../../../../.rustup/toolchains/nightly-aarch64-unknown-linux-gnu/lib/rustlib/aarch64-unknown-linux-gnu/lib"

This is a directive 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,/home/shep/rust/dynlink/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 => /home/shep/rust/dynlink/library/libawesome_math.so (0x0000ffffb1e56000)
    
$ ./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.

See also:

• How can I specify linker flags/arguments in a build script?

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.